GM ATO Style Fuse Block
Why write a page about a lowly fuse block? Because it's a smart upgrade if you're rewiring your vehicle and it's easy to do, that's why! In all of my late 1960's and 1970's era GM vehicles I have, and all of the late 1970's and early 1980's GM vehicles I've carefully examined in the junkyards I frequent, the fuse blocks are all identical in how they mount to the firewall. That means you can physically swap in a later model fuse block with easier to replace ATO fuses. These fuse blocks are also built to handle larger capacity electrical systems with a whopping 19 fused circuit locations available, two of which can accept plug in circuit breakers. There are also integrated locations for the turn signal and hazard flashers, as well as a way to "clip on" an optional warning chime module on the top edge. They also have a number of places to "plug in" extra accessories from the front of the fuse block using a simple "piggyback" style connector - this makes plugging in extra accessories wiring harnesses quite easy. Glass fuse style fuse blocks typically took only 9 fuses, and only had a place for the hazard flasher - leaving the turn signal flasher to get mounted somewhere, usually else hard to get at. The warning buzzer also got put someplace else, along with circuit breakers for any high-drain accessories like power windows. Fuse capacities are also a bit higher with the ATO style fuse block - 25A to 30A fuses are common in some applications, whereas the glass fuses typically stopped at 20A. This means more "stuff" can be put on a single circuit before overloading it - providing you use the right gauge wire when wiring it up!
The ATO style fuse block uses different terminals on the back than the older glass fuse style fuse blocks which used simple right-angle Packard 56 style terminals. The terminals used on the ATO style fuse blocks can be either Pack Con I and Pack Con II style terminals depending on the fuse block cavity in question and the original application. There are up to four of them put together on a common "buss" to ensure that multiple cavities in the fuse bock get power from just one wire. This is used both on the feed side of the fuses as well as on the output side to provide the extra "plus in" locations to accept accessories. These are readily available in 1, 2, 3, and 4 terminal styles that will accept wire sizes up to 10 (!) gauge and the 4-gang terminals that accept 10 gauge wire are rated to handle a whopping 60A of current through the terminal - that's a lot of juice!
Related to this is the bulkhead connector that lives immediately behind the fuse block. As with the fuse block mounting, these all appear to physically interchange. There are various different configurations of plugs here with most cars using two different plugs, and the trucks usually using three. That last bit is important - by simply swapping to the truck style bulkhead connector, you get some additional wire locations and a second high-power feed for in-vehicle accessories (these use Packard 59 style terminals instead of the Packard 56 terminals in the rest of the bulkhead connector).
Doing the Swap
You need a donor ATO style fuse block from a junkyard - since I want the bulkhead connector too, I find a Chevy truck of the right era, clip the wires as far back as I can reach, unbolt the single bolt that holds the bulkhead connector in place from the engine compartment side, and unbolt the bulkhead connector and fuse block assembly from the passenger compartment side. Re-attach the engine compartment connector to the bulkhead connector so you don't lose the bolt (it's kind of important) and claim your prize at the counter. If you grab this while getting other stuff, typically the junkyard will only charge you a few bucks for it.
When you get it home, clean it up, and strip the fuse panel of all fuses, breakers, and flashers. Save the fuses and such for later if they look to be in good shape - fuses either work or they don't - no sense buying new ones if it's not needed. Separate the bulkhead connector by unbolting it, and set about removing the terminals from the fuse block and the bulkhead connector. Be careful as the multi-terminal ones on the fuse block are often hard to remove and will require some patience, or multiple small screwdrivers or other suitable removal tools. They will come out if you gently work at them - all of mine did, and some of them quite literally fell out of the fuse block once I depressed the locking tangs.
Now, decide on your wiring choices - aka, what goes where - and then go get the right terminals for your needs. This is pretty open ended based on your needs. After that, you can commence wiring up the fuse block and bulkhead connector, and updating the wiring harnesses as needed. For many things on the bulkhead connector, this may be simply removing each terminal from the old bulkhead connector and installing it in the same desired in the new connector. There may be additional wires to run. For me, is part of why I started on this task - to run additional wires without having to drill holes in the firewall. Getting the ATO style fuse block was just icing on the cake - I was going to rewire the entire dash area anyway, and this solved a few problems with mounting additional circuit breakers and/or fuses for the other stuff I wanted to wire up. Anyways, after you get the wiring done, re-connect the bulkhead connector and fuse block (they clip together) and bolt them to the firewall from the passenger compartment side. Install the bulkhead connector on the engine compartment side - don't forget to use some sealing goo on the connector before you assemble it - and snug down the bolt. Don't gorilla the bolt down, just snug it up after the connectors and terminals are all fully seated. Double check that all terminals are properly seated and none backed out of the connector, and you're done.
Here are some pictures of the front of two fuse blocks I lifted out of different Chevy pickup trucks circa 1980.
Here's the back of one of those Chevy truck fuse blocks.
Here's a set of close-up pictures of one of the terminals from the fuse block. This is a double connector, the fuse block uses single, double, triple, and even quad connectors. This allows one input wire to feed multiple fuses, or even one fuse output to feed multiple output slots - the plug in extra slots on the front of the fuse block are handled this way.
Here are two schematic drawings of this type of GM fuse block. The first is as I scanned it from a 1978 Chevy Truck wiring manual. Specifically it comes from the diagram for the G bodies (the vans) - apparently the regular trucks did not switch to the ATO style fuse block until later - something to be aware of when hunting in the junkyards. The second is the same diagram with markup to make it clear where each fuse is, what it feeds, along with which end of the fuse is the feed and the output, and the separate extra "plug in" terminals for various add-on options. The red shapes indicate each fuse and it's associated output terminals. The red dots are the input to each fuse, the light green dots are the output from each fuse, and the blue dots are extra "plug in" terminals. The lone dark green dot is a fuse location that can accept a plug-in circuit breaker or a fuse, but still feeds it's power out the back of the fuse block and into the extra "plug in" terminal associated with that fuse location. The lone purple dot is a high-power output terminal that is intended to be used with a special circuit breaker and output wire assembly.
Here is another version of the same fuse block diagram, this time with markup to indicate what terminals are needed on the back side of the fuse block. They are color coded by the wire size they accept - red is 12-10 gauge, purple is 16-14 gauge, and blue is 20-18 gauge. The length of each colored bar indicates the number of terminals on a common "buss". These are based on the existing GM truck wiring diagrams I had available to look at, plus some intuition for filling in the extra/unused terminals. The idea here is to wire the back of the fuse block so all fuse locations have a power feed to them, and leave any unused/unneeded output locations available for later use.
Note that except in the two cases of extending an existing buss (for the gauges/turn signal bar and the stop/hazard/power accessory bar), I am specifically not indicating which terminals have battery, accessory, or ignition power - that is something you will need to decide for yourself based on your wiring needs. Battery circuits are live all the time, accessory circuits are live with the key in the "run" or "accessory" positions, and ignition circuits are live with the key in the "run" and "start" positions. I would assume most circuits will go to accessory power, the next largest batch to battery power, and the smallest set to ignition power. One interesting idea here is to wire up one extra fuse to be live in ignition and accessory to run, say, the radio in all three. It would require a couple of suitably sized diodes on the two power feeds - one from ignition power and one from accessory power - but it would work.
For purchasing purposes, the terminals break down like this:
* denotes part numbers provided by Garry Smith after reading this web page. He found some really detailed, though dense links to Delphi information at this link. Thanks, Garry!
Page last updated 03/29/2009 07:53:15 PM