1975 Suburban - Wiring Changes
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Wiring Changes

I've installed a few non-stock items or otherwise modified the wiring to be done differently. I can't stand a hack wiring job and I always try to do it as "right" as possible. That often includes a lot of research and extra effort to find the correct connectors. I also try to avoid splicing if at all possible, and this means I often re-use the plastic wiring connectors that were there originally, but run new wires with new connections made at the terminals and re-routing things as needed.

  •  Optional Tachometer
  •  Driving Lights
  •  "Hot Start" solenoid relocation kit
  •  Engine harness relocation, extension, and reconfiguring
  •  Electric Choke

Stuff I need to add in wiring for:

  •  Starter Solenoid Relay (for added voltage boost when hot)
  •  HEI Power Relay (for maximum voltage to ignition)
  •  Low Vacuum cut-out switch for AC Compressor
  •  Fog Lights
  •  TCC Lockup Wiring
    • Power to Vacuum Switches
    • Low Vacuum Sensor Return
    • High Vacuum Sensor Return
    • 4th Gear Lockup Feed
    • Any Gear Lockup Feed
    • Indicator Light
  •  Trans Temp Sensor
  •  Engine Temp Warning Light
  •  Oil Pressure Warning Light
  •  Choke Power Through Oil Pressure Switch
  •  Under Hood Light

Here are some pictures of the main connector at the firewall for reference. Note that on the engine side there are three separate connectors that clip together into one larger connector which is then plugged into the one screwed to the firewall from the inside. The large bolt in the center is used to hold it in place so it does not vibrate loose. The fuse block is part of this assembly and sits on four small plastic spacers that hold it about an inch away from the wiring plug on the firewall. There is a foam gasket that seals the plug at the firewall to prevent water leakage into the passenger compartment.

1975SuburbanFirewallConnector01.jpg (1512389 bytes) 1975SuburbanFirewallConnector02.jpg (1441169 bytes) 1975SuburbanFirewallConnector03.jpg (1337486 bytes) 1975SuburbanFirewallConnector04.jpg (1407142 bytes) 1975SuburbanFirewallConnector05.jpg (1442615 bytes)

Here is a close-up picture of the main connector at the firewall for the engine harness, labeled for what each original wire does. Note that my truck was not equipped with the oil pressure warning light - it has a mechanical oil pressure gauge instead - but I included it here for completeness. Each of the empty slots is available for use.

1975SuburbanFirewallConnectorLabeled.jpg (377256 bytes)

Here is a chart showing what wire colors go where in the firewall connectors in the original harness. The one on the left is the engine side, the one on the right is the passenger compartment side, and these are shows as if you were looking at the "mating" side of each connector, similar to the picture above. When working from the back of the connector (say, when removing existing wires or inserting new wires) you need to reverse the pictures. Think about it for a minute and it'll make sense, just triple check yourself before tearing into the connector to change stuff - it is somewhat easy to confuse yourself.

1975SuburbanFirewallConnectors-Original.jpg (44528 bytes)

Since I'm adding so many wires that need to go through the firewall, I decided to figure out how to add wires into the empty cavities in the main connector at the firewall. When I started this, I knew how to add them to the engine side from my previous work on Buick HEI conversions and learning about reusing plastic wiring plugs, but I wasn't sure about the passenger compartment side. I decided to remove the fuse block and connector from the firewall to see what the connectors looked like, and the good news is that it is a standard connector on the inside as well. That means that I just need to slip new wires and connectors into each empty cavity, reconnect everything, and I'm in good shape. The hard part is getting this all apart to work on it, so I might as well add all the possible wiring I can while I have it apart - I really don't want to do this again. The remaining wiring changes I have planned affect things elsewhere in the harness and don't need more wires to cross through the firewall, so this is pretty much all I have to be concerned with for now. As a result of what I learned while doing this work, I created a new page to document the various styles of automotive electrical connectors I learned about.

Not counting the Oil Pressure Warning Light slot, there are six free slots in the engine harness connector. The TCC wiring alone needs six slots, and I'm not real keen on taking all of them up for just the TCC wiring since it goes to the transmission and a couple of vacuum switches in the engine compartment. I would like to move the tach and choke wiring into this connector if possible, so that's two slots out of the six. The Trans Temp Sensor, Trans Temp Warning Light, and Engine Temp Warning Light are three more slots out of the six, leaving one left for other uses. I'll also add another wire for an electrical Oil Pressure Gauge Sensor so I can convert the existing mechanical gauge over to a later model electric gauge - oil lines in the passenger compartment are something I do not like. That brings me to six. But I also want to add wires for a future Oil Temp Sensor and Oil Temp Warning Light, so that's two more for a total of eight new wires and six open slots. To fix this, I'll move wire #30 (tan) - the Fuel Level Sensor - to the Body Harness Connector to make up for one connection - it's really part of the body wiring anyway. I can also reclaim both of the existing wires that go to the factory amp gauge - I want to convert to a later model voltmeter, so why bother keeping two wires for the amp gauge when I re-do the engine harness? I'll pass these through the firewall separately and have them go to their original locations separately from the new engine harness. Wire #105A (black) originally went down to the battery connection at the starter and now goes to the battery connection at the solenoid. Wire #106 (black with white stripe) goes to the main junction block on the firewall. I'll preserve the existing terminals and just use a two-wire connector to join them together at the firewall, and then extend the original wire to the battery side of the solenoid as needed - it's not going to be there forever, so I'm not too worried about that one splice.

For the body harness connector, in addition to moving the Fuel Level Sensor into one of the three open slots, I'll also add wires for a Low Fuel Warning Light and a Rear End Temp Sensor. The Fuel Level Sensor wire goes down the passenger's side frame rail to the back of the truck, and I will re-route it to run down the driver's side frame rail when I run the other two wires down there.

For the front Lighting harness connector, I'll put the driving and fog light relay trigger wires into two of the three empty slots in the main connector at the firewall. I'll stick an unused wire into the remaining slot for later use in case it's needed.

It turns out the oil pressure light isn't in my harness at all, so I'll have to add it both inside and outside. No biggie, it just needs to be done. That means there are a total of 13 new wires that will be added, 1 to be moved between connectors, and 2 to be removed and placed in a completely separate connector. Got all that? Good. There's going to be a test later. :-)

  •  Engine Harness Connector Additions
    • Oil Pressure Sensor (Using factory location for non-gauge equipped trucks) - 20 Dark Blue
    • Oil Pressure Warning Light - 20 Light Blue
    • Oil Temp Sensor - 20 Dark Green
    • Oil Temp Warning Light - 20 Light Green
    • Trans Temp Sensor - 20 White
    • Trans Temp Warning Light - 20 Grey
    • Water Temp Warning Light - 20 Yellow
    • Tachometer - 18 Brown
    • Choke - 18 Pink
  •  Engine Harness Connector Removals
    • Fuel Level Sensor (Moved to Body Harness Connector) - 18 Tan
    • Amp Gauge Battery Feed (Will pass through firewall separately until removed) - 16 Black
    • Amp Gauge Alternator Feed (Will pass through firewall separately until removed) - 16 Black with White Stripe
  •  Front Lighting Harness Connector Additions
    • Driving Lights - 18 Purple
    • Fog Lights - 18 Yellow
    • Spare Wire For Later Use - 18 White
  •  Body Harness Connector Additions
    • Fuel Level Sensor (Moved from Engine Harness Connector) - 18 Tan
    • Low Fuel Warning Light - 20 Grey
    • Rear End Temp Sensor - 20 Dark Blue
  •  Extra Wires To Pass Through Firewall Separately
    • Rear End Temp Light - 20 Light Blue
    • TCC Power from Before Brake Switch - 18 Red
    • TCC Power from After Brake Switch - 18 Pink
    • TCC Power from After Vacuum Switches - 18 Yellow
    • TCC Power for 4th Gear Lockup - 18 Pink
    • TCC Power for Any Gear Lockup - 18 Orange
    • TCC Indicator Light Ground - 18 Dark Green
    • Trailer Brake Power - 10 Red
    • Trailer Brake Control - 10 Blue
    • Left O2 Sensor - 20 Light Green
    • Left O2 Sensor Ground - 20 Black
    • Right O2 Sensor - 20 Dark Green
    • Right O2 Sensor Ground - 20 Black
    • O2 Sensor Heater Power - 18 White
    • O2 Sensor Heater Ground - 18 White
    • Amp Gauge Battery Feed (Will pass through firewall separately until removed) - 16 Black
    • Amp Gauge Alternator Feed (Will pass through firewall separately until removed) - 16 Black with White Stripe

Here are three more graphics showing what gets done to the wiring at the bulkhead connector on the firewall. The first graphic shows the the changes to the existing wiring - moving the Fuel Lever Sensor wire from the engine harness connector to the rear body harness connector. The second chart shows removing both of the amp gauge wires from the engine harness connector. The third graphic shows the additions of the new wires and what color wire goes where - refer to the list above to figure out what each one is for. In each graphic, the changed boxes are outlined in red to make it more obvious. Also, note that in the first chart that both changed slots in the engine harness connector are shown in white because they are now empty. The same goes for the two removals in the second chart. In the third chart, there are two white wires used - one in the engine harness connector and one in the front lighting harness connector. In this case the boxes highlighted in red will have a wire inserted into them.

One interesting point to note is that the slot for the red wire and the final empty slot in the engine harness are both slightly larger than the rest of the slots in the connector. That's why the spacing on things is a bit weird looking. The red wire is a 10 gauge wire and carries all of the power going from the main junction block on the firewall to the rest of the vehicle. The other slot is the same "slightly larger" size, so I can only assume that in some models this slot was used for another high power connection going inside the vehicle. After some poking around in other catalogs, I believe these two are "Packard 59" style connectors - the same as used on sealed beam headlights. The rest of the connectors are "Packard 56" style connectors. I have not confirmed this yet, but it seems right from what I have learned.

1975SuburbanFirewallConnectors-Changes.jpg (45189 bytes) 1975SuburbanFirewallConnectors-Removals.jpg (45310 bytes) 1975SuburbanFirewallConnectors-Additions.jpg (47162 bytes)

On the engine compartment side, the new wires will simply become part of each harness that is already there and get routed to their proper places that way - I can either terminate the wires correctly now or leave enough extra wire to complete the job later. If I can complete a given section of the harness, then I will, but wire is pretty cheap, so I can afford to leave enough there to complete the job later. I will be re-routing the harness anyway and changing some of the existing wiring, so this is an easy time to do all this work.

On the passenger compartment side, I decided to break the 13 new wires down into groups based on what they go to and terminate them in individual connectors in the under-dash area. That way I don't have to complete the wiring right away and I can leave the connectors there for easy hookup later on. The lighting control and under-dash gauge pod connectors need to be somewhere over by the radio. The power connector for the choke can go directly into the fuse block - in the back if I can find the right connectors to use. The main gauge pod connector should be somewhere around the steering column - for this one I just have to be sure it ends up high enough up that it doesn't hang or fall down and interfere with the driver's feet. In several cases, there are additional wires listed in each connector for the interior wiring. These are the extra power and ground wires needed in each group of wires. They are included in this list so I make sure I have enough slots in each connector.

  •  Reuse Tachometer Connector - Three Wires
    • Tachometer Signal (connect to bulkhead connector) - 18 Brown
    • Tachometer Power (connect at rear of fuse block to circuit 75, similar to circuit 139A) - 18 Pink
    • Ground (connect to a common ground point under dash) - 18 Black
  •  Additions to Main Gauge Pod Connector - One Wire
    • Oil Pressure Sensor (connect to bulkhead connector in factory location) - 20 Dark Blue
  •  New Main Gauge Pod Warning Light Connector - Six Wires
    • Oil Pressure Warning Light (connect to bulkhead connector) - 20 Light Blue
    • Water Temp Warning Light (connect to bulkhead connector) - 20 Yellow
    • Low Fuel Warning Light (connect to bulkhead connector) - 20 Grey
    • Alternator Warning Light (connect to circuit 25 bulkhead connector instead of circuit 130) - 20 Brown
    • Alternator Warning Light Power (connect at heater controls to circuit 50 where 50 and 50A connect, similar to how circuit 939 would be hooked up in a non-gauges harness) - 20 Orange
    • Instrument Cluster Power (connect at rear of fuse block to circuit 39A) - 18 Pink
  •  New Under-Dash Gauge Pod Sensors and Warning Light Connector - Six Wires
    • Trans Temp Sensor (connect to bulkhead connector) - 20 White
    • Trans Temp Warning Light (connect to bulkhead connector) - 20 Grey
    • Oil Temp Sensor (connect to bulkhead connector) - 20 Dark Green
    • Oil Temp Warning Light (connect to bulkhead connector) - 20 Light Green
    • Rear End Temp Sensor (connect to bulkhead connector) - 20 Dark Blue
    • Rear End Warning Light (connect to new rear harness firewall connector) - 20 Light Blue
  •  New Under-Dash Gauge Pod Power, Ground, and Lighting Connector - Three Wires
    • Instrument Cluster Power (connect at rear of fuse block to circuit 39A) - 18 Pink
    • Ground (connect to a common ground point under dash) - 18 Black
    • Lighting (connect at rear of fuse block to circuit 8A) - 20 Grey
  •  New Front Lighting Control Connector - Six Wires
    • Driving Lights Relay Trigger - 18 Purple
    • Fog Lights Relay Trigger - 18 Yellow
    • Low Beam Power (connect to circuit 12 near fuse block) - 18 Tan
    • High Beam Power (connect to circuit 11B near fuse block) - 18 Light Green
    • Ground (connect to a common ground point under dash) - 18 Black
    • Spare Wire For Later Use - 18 White
  •  Wires in Harness - Two Wires
    • Choke (connect from bulkhead connector to circuit 75 at rear of fuse block) - 18 Pink
    • Ground During Start (connect to each warning light circuit at any available connector; use inline diodes to prevent "cross-talk" between the various warning lights; add extra connectors as needed to easy later disassembly) - 18 Dark Green
  •  TCC Lockup Controls Connector - Six Wires
    • Power from Before Brake Switch (connect to new TCC firewall connector) - 18 Red
    • Power from After Brake Switch (connect to new TCC firewall connector) - 18 Pink
    • Power from After Vacuum Switches (connect to new TCC firewall connector) - 18 Yellow
    • Power for 4th Gear Lockup (connect to new TCC firewall connector) - 18 Pink
    • Power for Any Gear Lockup (connect to new TCC firewall connector) - 18 Orange
    • Indicator Light Ground (connect to new TCC firewall connector) - 18 Dark Green
  •  Trailer Brakes Connector - Four Wires
    • Main Power (connect to new rear harness firewall connector) - 12 Red
    • Trailer Brakes Control (connect to new rear harness firewall connector) - 12 Blue
    • Brake Signal (connect to circuit 17 at or near stop light switch) - 18 White
    • Ground (connect to a common ground point under dash) - 18 Black
  •  O2 Sensors Connector - Six Wires
    • Left O2 Sensor (connect to new O2 sensor firewall connector) - 20 Light Green
    • Left O2 Sensor Ground (connect to new O2 sensor firewall connector) - 20 Black
    • Right O2 Sensor (connect to new O2 sensor firewall connector) - 20 Dark Green
    • Right O2 Sensor Ground (connect to new O2 sensor firewall connector) - 20 Black
    • O2 Sensor Heater Power (connect to new O2 sensor firewall connector) - 18 White
    • O2 Sensor Heater Ground (connect to new O2 sensor firewall connector) - 18 White
  •  Cruise Control Indicator Light Connector - Two Wires
    • Indicator Light Ground (connect to indicator light terminal on transducer) - 20 Dark Blue
    • Indicator Light Power (connect to "always on" side of cruise control brake switch) - 20 Pink

In all of this wiring, we also need to take every warning light wire that is connected to a self-grounding sensor and make sure it is also hooked to the "GRD 1" terminal at the ignition switch via a 18 gauge Dark Green wire. This terminal is grounded only when the key is in the start position and wiring things this was ensures that all of the warning lights will come on when the key is in the start position. This allows the driver to notice if there is a burned out bulb or a problem with the wiring - if all the lights don't come on, there is a problem! This will be done by jumpering various wires together into a single wire that heads over to the ignition switch connector with in-line diodes to prevent one grounded sensor from turning on all of the indicator lights. Just make sure the diodes are pointing the right way! For the under-gauge dash pod and the the main gauge pod wires, the jumpering should be on the dash harness side of the connection - not the gauge pod side. The idea is that you want to be able to disconnect the gauge pods with one or two non-interchangeable connectors and have them come out easily. Once the connectors are disconnected, no wires should still be joining things together. When it comes time to reconnect things, the non-interchangeable connectors ensure that you can't hook things up incorrectly. Why run an extra wire through that connector that isn't needed?

An even better idea for the warning lights is to use the ground wire as a trigger and do a little clever wiring to have the lights stay on for a seconds or two after the key comes back out of the start position so the driver has a chance to see it and take note of any problems. I thought this up while starting my wife's Saturn while heading out on a parts run - it's a subtle but nice feature. Basically, you need a time delay circuit with a battery power feed and the ground goes through the ignition switch so it's only grounded in "start".

The original wiring for gauges in my truck does not have an alternator warning light. To rectify this, I need to wire a light between something that is connected to the "IGN 3" terminal on the ignition switch and terminal #25 on the passengers side of the engine harness connector - this is how the light was wired on non-gauge equipped trucks. The power source was actually circuit 50 where it feeds the heater blower control switch, which is fused and reasonably close to the dash cluster. It's a single connector, so I can easily create a stub to put in between and tie in the power wire there. At the bulkhead connector, I can simple remove the terminal #25, place it into a new two slot connector with the other slot being connected to the alternator side of the indicator light, and build a small "two into one" stub to hook up to the bulkhead connector. With any luck I can leave the existing resistor wire in place (wire #130) and basically wire the light in parallel with the resistor wire. Note that this indicator light must be an normal bulb - not an LED - to get the original/OEM effect of the bulb glowing proportionately to the amount the alternator is not putting out enough voltage.

I want to be able to add an indicator light to my cruise control system later, so I'm adding the stub wiring for it now. Basically I need to run a Dark Blue 20 gauge wire out to the cruise control transducer along the normal cruise control harness and put a single connector on the end. On the inside, this wire would go up the indicator light and be the ground side - for now I'll terminate it in a two wire connector. The other wire in the connector will be a Pink 20 gauge wire and it will be hooked up to the "always on" side of the brake switch used by the cruise control system.

I have a very large and very detailed wiring diagram available for download. I created it specifically for this project to work out all of the details. It's a BMP file and it's over 6Mb, but virtually every wire that matters for this work is in there.

Here's a couple of pictures of the completed wiring under the hood on the driver's side.

1975SuburbanNewWiring01.jpg (1326449 bytes) 1975SuburbanNewWiring02.jpg (1410602 bytes)

The lower alternator mounting bracket was busted in two (again), so I bought a replacement (it's chrome) and a brand new 94 amp alternator to bolt into place. Other than the fact that it's shiny clean and has a different fan, it's a direct bolt in replacement for the older unit. Just look down the catalog for an 80's Chevy truck with the optional 94A alternator - my local parts store had them in stock in several clocking styles (the relationship of the front and back case halves) for a very reasonable price.

1975SuburbanBrokenLowerAlternatorBracket.jpg (1463632 bytes) 1975SuburbanNewAlternator01.jpg (1463211 bytes) 1975SuburbanNewAlternator02.jpg (1459791 bytes) 1975SuburbanNewAlternator03.jpg (1408824 bytes)


While I was buying bits and pieces, I tried to keep track of things.

  •  Interesting Wiring Items I Bought
    • Diodes: 6 Needed
    • Wiring Terminals for Inside Transmission 18-16 gauge - Waytek #30037 (Box of 60)
    • Wire: 8 Gauge Red (100 Feet)
    • Wire: 10 Gauge Blue (100 Feet)
    • Wire: 12 Gauge Orange - Waytek #WG12-3-100 (100 Feet)
    • Wire: 12 Gauge Purple - Waytek #WG12-7-100 (100 Feet)
    • Wire: 16 Gauge Light Blue - Waytek #WG16-16-100 (100 Feet)
    • Wire: 16 Gauge Light Green - Waytek #WG16-15-100 (100 Feet)
    • Six Wire Female Connector - Waytek #38009 (7 Needed, Box of 50)
    • Six Wire Male Connector - Waytek # 38008(7 Needed, Box of 50)
    • Four Wire Female Connector - Waytek #38007 (4 Needed, Box of 50)
    • Four Wire Male Connector - Waytek #38006 (4 Needed, Box of 50)
    • Three Wire Female Connector - Waytek #38005 (1 Needed, Box of 50)
    • Three Wire Male Connector - Waytek #38004 (1 Needed, Box of 50)
    • One Wire Female Connector - Waytek #38001 (2 Needed, Box of 100)
    • One Wire Male Connector - Waytek #38000 (2 Needed, Box of 100)
    • Female Terminal 12 gauge - Waytek #32031 (Box of 100)
    • Male Terminal 12 gauge - Waytek #32030 (Box of 100)
    • Flag Style Terminals - Waytek #31017 (Box of 100)
    • Y-Terminal Adaptor - Waytek #31052 (Box of 100)
    • Piggyback Terminal Adaptor - Waytek #31051 (Box of 100)
    • Relay Base Terminal 16-14 gauge - Waytek #31073 (Box of 100)
    • Relay Base Connector - Waytek #75280 (Box of 25)
    • Relay SPDT - Waytek #75113 (Box of 25)
    • Circuit Breaker Without Bracket 20A - Waytek #46420 (6 Needed)
    • Circuit Breaker Mounting Bracket - Waytek #46466 (1 Needed, Box of 4)
    • Circuit Breaker Buss Bar 6 Gang - Waytek #46566 (1 Needed, Box of 5)


AC and Blower Wiring

While pulling out the wiring for the AC compressor and the blower relay, I found out that the high power connector to the blower relay had undergone a partial meltdown at some point in the past. It was junk, and I needed to transfer the wires to a new connector - if I could fine one. I spent a long time hunting and found a kind of close one intended for a dimmer switch. I removed the wiring pigtail from the plastic connector and trimmed off the excess plastic bits to get something that seemed to be right. I also put heatshrink tubing over the first ~6" of the two high power wires - the purple fan feed and the orange battery feed - going into this connector. The original insulation was pretty brittle and this should help keep in intact and weather-tight for a while longer. I would have replaced them, but I couldn't find a blower motor pigtail in the right size and color, and I didn't have any terminals big enough to accept wire that large.

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When I first tried to reassemble the wires into the connector, the wire connectors simple would not go into the new connector. This caused me much grief - until I realized that in my haste and the dark of night when I first attempted to do the work that I had inverted the plastic connector. Like a complete moron, I was attempting to insert the wire connectors from the front instead of from the back - oops! It was funny after I figured it out, but it sure wasn't funny when I was trying to put a square peg into a round hole. :-)

Part of the plan involves changing this wiring a bit to be connected to the main engine harness via a single wire, and for the on-engine wiring to the AC compressor to be a part of the main engine harness instead of being hung across the engine separately. Mostly a neatness thing, but it's important to me and it will make later servicing of the engine easier with fewer wires running about randomly.

I'll be adding a vacuum switch into the wiring for the compressor so that it cuts out when there is little to no vacuum, and that switch will be a part of the AC harness, not a part of the engine harness. There is already a low pressure cutout switch in the refrigerant line (for safety) and a low temperature cutout switch mounted to the evaporator core case right below the blower relay, with wiring running to each of them. I plan on mounting the vacuum switch in that general area with a small custom bracket. The manifold vacuum line to the vacuum canister goes right through that area too, so it will be easy to add in a "T" fitting and hook up the vacuum switch. The wiring needed a bit of work, but not too much. The switches all wire in series (they all have to be closed for the compressor to be able to kick on) so it was mostly a matter of deciding on how to route the wires neatly once I had the mounting for the vacuum switch figured out. The wire AC compressor trigger wire that came out through the blower harness had a simple single connector on the end that went to the low temperature switch, then there was another single connector on the wire that went over to the low pressure switch, then it went out to another single connector where the wire from the compressor itself hooked up. Since it was all single connectors, I just used the short bits of wire on the vacuum switch I got from the junkyard to wire the vacuum switch inline before the low temperature switch. It uses two more single connectors to make it a plug-in. I also wrapped up the wiring to the three cutout switches into a separate sub-harness so it would be easy to work on later if needed. It also allowed me to re-route the final AC compressor hookup wire to be back by the firewall so I could run the rest of it through the engine harness for neatness. I also decided to coil up the extra slack in the final section of wire instead of cutting it, just in case I decide to do something different, because the connector on the low pressure switch is a molded rubber piece and I can't remove the terminal from it and put more wire on it should I decide I need to later on.


Gauges and Warning Lights

If you read the list above, you're probably thinking "how many gauges is this guy planning on installing?!?!?" right about now. I freely admit that I like lots of gauges. I want to keep tabs on everything that is going on with my vehicles. Everything. Yes, you read correctly up there when I said I wanted wires for a rear end temperature gauge and warning light, so bear with my explanation of why I want this. A number of years ago, I blew the rear out due to overheating. Without realizing it, the pinion seal disintegrated while I was cruising down the highway and the rear promptly lost all of it's oil. With no oil left to lubricate and cool it, the rear diff got so hot it nearly welded the entire assembly solid. My only notice was a sudden increase in rear diff whine as the assemble was about to heat weld, then a loud band as it started to heat weld together and immediately broke the weld under the strain. The axle housing was quite literally smoking after I pulled over and the residual grease/oil/paint on the housing was in the process of burning off - it was that hot. A wasted afternoon, a tow home, a few days work to remove and re-install the axle, and $800 in axle rebuild fees later I was back on the road. The morale? A $45 gauge is cheap insurance - if I had a gauge and warning light, I would have noticed the temp rise rapidly as soon as I lost the oil seal and I would have pulled over, most likely saving the axle from any serious damage and limiting my repair costs to a tow home (AAA has great towing, BTW. :-) plus a new pinion seal.

The factory gauge cluster is a good start. It has the usual assortment of gauges for a 1975-era truck (Fuel Level, Water Temp, Oil Pressure, and Amps), plus I tracked down a factory tachometer setup. I also figured out how to do some clever mounting and get a vacuum gauge in the gauge cluster. The later trucks came with a Volt gauge instead of an Amp gauge, and they used an electric Oil Pressure gauge instead of the mechanical style one I currently have. Both of those would be a worthy upgrade, but that's somewhat of a separate issue to deal with later. That leaves only a few more gauges I would like to add.

I would also like to add warning lights to all of the gauges if at all possible. Again, this is cheap insurance. The only trick is the senders - especially the temp senders. VDO is nice and gives you the option of senders with built in warning contacts - check out their temp senders and their pressure senders and look at the "warning contact" column to find out where the warning contact kicks in at. Pretty sweet, huh? I think so. Except that VDO doesn't make a rear differential temp gauge that I can find and they have discontinued their transmission temperature gauge, at least in the Cockpit line. Such is life. That means I need separate "warning light" senders for each thing. Not much worry here - as long as I can figure out a factory "idiot light" style sender to use, they're a dime a dozen in salvage yards and readily available at the parts store for cheap. For temperature senders, the standard Buick 350 and 455 idiot light water temp sensor grounds at about 258F, plausibly right for water, oil, and transmission temperature. Cole Hersee also lists a number of senders in their catalog with different temperatures where they come on at. Oil pressure is easy - no pressure is bad and plenty of senders are out there to do whatever you need to do. Lastly, if external wiring for the warning light senders isn't desirable, Auto Meter used to make something called the Tri-Alert (part number 5347, apparently discontinued), which was a nifty box that hooks up to the existing senders and give you warning contacts for three gauges. It's a bit spendy, though, so I'm going the cheap-o route and adding separate wires in the harness for each gauge so I can have a warning light. Because of my gauge setup (OEM + Autometer), that means I need to have two senders for each gauge. Not the end of the world, but it is somewhat annoying to do in practice as it nearly doubles the number of senders - the alternator warning light is the only one that does not need a sender, but it does need special wiring in the harness to work right, see the notes in one of the previous sections on this page for details.

Adding new warning lights to the dash pod will be a bit of a challenge, but I think it can be done at home and look really professional. Check out the Fiber Optic Indicator Lights #L01N-T to get an idea of what I'm thinking of doing. The actual hole is only 1/16" and requires no bezel. The light source is a remotely mounted LED with a small fiber optic cable leading up to the dash face. This could easily be incorporated into the existing gauge faces with very professional results. Just drill a 1/16" hole in the gauge face and in a discreet location on the back or side of the gauge cluster, and thread the fiber optic cable into place. You can mount the light source either to the back of the gauge cluster or somewhere else further away as needed. The same thing goes for adding indicator lights alongside the aftermarket gauges in the gauge pod below the dash. Just route the fiber optic cable into a small hole right next to the gauge and mount the light source in a convenient location nearby. It would be even cooler if you could get inside the aftermarket gauge faces, but I have never been able to figure out how to do that, so I'll stick with a "alongside" mounting for now. Simple, small, easy. They are $43 for a set of three, but that's really not too bad for such a cool product. You can even specify color (red, blue, green, or amber) so that all the warning lights were red except for the low fuel warning light which would be amber. Pretty slick, huh?

  •  Water temp senders go into intake manifold
  •  Oil pressure senders go into engine block (where???) or into custom blocks on oil cooler lines
  •  Oil temp senders go into custom blocks on oil cooler lines
  •  Trans temp senders go into custom blocks on trans cooler lines
  •  Rear axle temp senders go into existing holes in the rear axle housing (likely the drain and fill holes)

The "custom blocks" I mentioned are something I plan on machining myself out of a block of mild steel about 3/4" thick by 1" wide by 2" long. I'll bore a 1/2" hole down the middle (slightly offset to one side) and put 1/2 NPT threads in each end. Then I'll put two 1/8" holes in the side that intersect with the 1/2" main bore and thread those with 1/8" NPT threads. Finally, since the main bore will be offset, I can drill mounting holes in the other side and easily mount this to any handy piece of sheet metal. I could even bore the inlet and outlet holes from the from and back and plus up one or both ends so that you could position this "over" a hole in the front radiator support sheet metal and have the "outlet" come out on the other side and hook right up to the cooler. With some sufficiently clever design work, I could make this rather easily on the drill press and use the right fittings and braided hose to finish off the setup nicely.

Back to the subject of the factory gauge cluster, I happen to have a 1978 "gauges without tach" cluster laying in the garage for reference and a 1978 wiring diagram for comparison. Things are close enough to be plausible with new wiring added for the oil pressure gauge and some other new stuff like a seat belt warning light. The printed circuit on the 1978 cluster is labeled with part #25044897 "PED-2" which is the correct factory part number for the printed circuit board. Doing some research in the Classic Industries catalog was very educational as they sell the printed circuit boards for most all of that era Chevy trucks and they also list the factory part numbers for each item along with what years and styles each one fits. Oddly enough, this appears to be electrically identical to the tach version, except that it has a circular section removed right where the fuel gauge hookups would be - perhaps so a clock could be installed? Seems nutty for the factory to hack up a perfectly good part just to make two part numbers they had to make and stock, but they do weird stuff like that sometimes, much to the frustration of folks like me 30 years later. With any luck, I could use the printed circuit from a later tach unit on my earlier housing and get the voltmeter and electric oil pressure gauge, but I would have to do some custom hacking on the oil pressure gauge area to make holes for the gauge clips. Finding a later model tach gauge pod would be ideal, but it could be pricey and hard to find. We'll see how that goes. I will almost certainly have to re-do some of the wiring in the connector that goes to the gauge cluster, and I'm already hunting for the right connectors to do the job right.

For reference, here are the printed circuit board part numbers I could find. These should help identify a cluster you might find at a swap meet so you know if it will work for you.

Part Number Make Year Application
6290457 Chevy and GMC 1967-1972 With Warning Lights
6290458 Chevy and GMC 1967-1972 With Gauges
6290459 Chevy and GMC 1967-1972 With Gauges and Tachometer
8905934 Chevy 1973-1975 With Warning Lights
8905936 Chevy 1973-1975 With Gauges and Tachometer
8917778 Chevy 1976-1982 With Warning Lights
25015092 Chevy 1977 With Gauges
25015093 Chevy 1977 With Gauges and Tachometer
25017325 Chevy 1978-1980 With Gauges
25017327 Chevy 1978-1980 With Gauges and Tachometer
25044897 Chevy 1981-1987 With Gauges
25044898 Chevy 1981-1986 With Gauges and Tachometer
8911133 GMC 1975-1977 With Warning Lights
8905935 GMC 1975-1977 With Gauges
8917407 GMC 1975-1977 With Gauges and Tachometer
8917408 GMC 1975-1977 With Gauges, Tachometer, and AC
8917775 GMC 1976 With Gauges
8917776 GMC 1976 With Gauges and Tachometer
25017328 GMC 1978-1980 With Gauges, Tachometer, and AC
25017329 GMC 1978-1980 With Gauges and AC
25017330 GMC 1978-1980 With Gauges
25017326 GMC 1978-1980 With Gauges and Tachometer
25044900 GMC 1981-1983 With Gauges and AC
25044901 GMC 1981-1983 With Gauges
25044896 GMC 1981-1983 With Gauges and Tachometer
25044899 GMC 1981-1983 With Gauges, Tachometer, and AC
25077363 GMC 1984-1987 With Gauges
25077362 GMC 1984-1987 With Gauges and AC
25077360 GMC 1984-1987 With Gauges and Tachometer
25077361 GMC 1984-1987 With Gauges, Tachometer, and AC
CT20800 All 1993-1974 Tachometer Harness
G2072 Chevy 1973-1975 With Gauges (Original GM Part)
G2073 Chevy 1976-1987 With Warning Lights (Original GM Part)
G2074 Chevy 1978-1987 With Gauges (Original GM Part)
G2075 GM 1988-1991 Blazer, Jimmy, Suburban (Original GM Part)

While you're poking around the Classic Industries catalog, go look up part K0067 - it's the proper non-adhesive harness tape used by GM and it's $19.95. I guess it's expected that a restoration-oriented place like this would stock the right harness tape. Eastwood also sells this, search for "wiring harness wrapping tape" and get the vinyl one.


TCC Wiring

The TCC wiring I have chosen is relatively straightforward, but a bit intricate. It has two vacuum switches, uses the same brake switch as the cruise control, an indicator light, a SPDT center-off toggle switch, and has a total of three wires going down to the transmission. I will implement this as a separate "mini-harness" with power and ground connections, and one connector inside the passenger compartment to disconnect the entire thing, plus a connector at the firewall to disconnect things there for ease of servicing later on. I have a separate tech tip page devoted to this topic.

Comments? Kudos? Got some parts you'd like to buy/sell/barter/swap? Nasty comments about my web page so far? You can email Mike or Debbie.

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Page last updated 01/02/2009 01:51:39 PM