NOTE: This page has been written using information gathered from various sources. I have not yet tried this on my vehicle to verify it actually works, but since I'm 99% sure it will work and because this has valuable background information on this issue, I wanted to get this page posted right away.
In many '80s and '90s Ford vehicles that come equipped with an oil pressure gauge, the gauge wiring and sender came from the factory in such a away that it isn't a "true" oil pressure gauge. There are many websites out there that explain this unexpected and deceptive piece of engineering from Ford, and if you Google for things like "ford oil pressure gauge real convert" or "f150 oil pressure gauge real convert" where you can find many more unhappy car and truck owners trying to figure this out - and if possible, fix it. Ford even had the audacity to release a service bulletin that explained to dealers how to retrofit this dysfunctional system to earlier vehicles if customers came in complaining about low oil pressure. Instead of simply explaining to the customer how to properly read the gauge or (*gasp*) making sure the engines were actually working right, Ford told it's dealers to make the gauge "lie" and claim that anything over ~4.5 PSI of oil pressure was shown as in the middle of the "normal" range on the gauge face. TSB #88-5-14 is one example of this for the 1987-1988 F series trucks and Broncos. If you need to wonder why Ford has been so hard pressed to make money recently, you need not look much further than stunts like this. I can't imagine how screwed up their corporate engineering and marketing culture has to get to even consider something like this, but they did, and boy do they have problems these days. But, I digress...
If you are not sure if this will work for you, read your factory manuals carefully, search around online, and find out the scoop on YOUR vehicle before doing any modifications to your electrical system. I am not responsible for any mishaps or accidents that may happen to, with, for, or unto your vehicle, yourself, or any nearby property or persons as a result of you trying to apply the information I have posted here. This is all strictly FYI to other people curious about how stuff works. If you do decide to work on your vehicle using the information I have provided, use common sense at all times and stay safe, be smart, and keep your stuff working - well, at least as well it was working when you started. Don't ruin perfectly good stuff (including your own body) because you don't know what you are doing. If in doubt, don't do it - understand what you are doing first, and experiment on parts you are not afraid to destroy (which hopefully does not include your body). Use the proper safety equipment, including and especially your brain, at all times. Always look both ways before crossing the street. Don't run with scissors. The lawyers made me do it. (You get the idea...)
Overview of the Problem - and of a Cure, at Least for Some
If you have a "idiot light gauge" like this in your vehicle, what can you do? Well, you could live with it and feel like a chump. You could install an aftermarket gauge on/under/around your dash and leave the in dash gauge to do nothing or leave it as-is. Or, you could actually fix the Ford gauge system to work like it should. In my case, I ran into this when working on a 1989 F150, and the situation annoyed me enough to research it to see if I could fix it - and I have found a solution to the problem. The factory engineering here was really quite simple, at least in the earlier years it was applied. Later years may have this engineering built right into the electronics that drive the instrument cluster, but in my case, the gauge is a rather simple affair and can be corrected without too much trouble. To turn the gauge into a "visual idiot light", the factory did two things. First, they changed the oil pressure sender (used with a real gauge) out for an oil pressure switch (used with an idiot light). Second, they added a fixed resistance into the sender circuit (a simple 20 ohm resistor seems to be what they used in all of the cases I could find) so that the gauge would read at about half-scale any time the oil pressure switch was active. The way to undo their "fix" is simply to reverse it - swap out the sender for a real oil pressure gauge sender, and bypass the fixed resistance so that the gauge reads normally. The interesting thing about these instrument clusters is that the gauge is still a real gauge, which is what enables you to make it work correctly. Other folks have written pages on how to solve this problem in the Thunderbirds, and some have indicated you can solve it on the Mustangs as well, so I'm inclined to believe it will work on any vehicle where you can find and bypass the fixed resistor (20 ohm) in the circuit.
Please be aware that if you do not remove the fixed resistance from the wiring, then the gauge will seem to work, but will always read very low on the scale - as in, disturbingly low. That's actually how I found out about this - when I put a rebuilt engine from a 1987 F150 into a 1989 F150, the oil pressure gauge was reading very low all the time, which led me to investigate the factory gauge problem and verify the oil pressure with a mechanical oil pressure gauge. It turns out I had used a "real" oil pressure sender in a system with one of these "idiot light gauges". Luckily for you, the reader, I found out how to fix this and decided to post this page. Aren't you happy that I screwed up and stumbled onto this problem and the fix? Me too. :-)
For the curious, here's how Ford's little trick works. On a true gauge style setup, the gauge will show extreme low oil pressure (aka, pointer all the way to left) when no connection to ground is found on the sender wire. When a direct connection to ground is made on the sender wire, the gauge reads as extreme high oil pressure (aka, pointer all the way to the right). This is, in fact a quick and dirty way to test the gauge vs. the sender - disconnect the sender wire and observe the gauge, then quickly connect the sender wire to ground and observe the gauge. If it doesn't move full sweep, the gauge is toast. On an idiot light setup, the oil pressure switch is closed (direct connection to ground) when there is no oil pressure and open (no connection to ground) when there is oil pressure above whatever level the switch is preset for. Usually this is a very low value - around 5 PSI or so. The idiot light is wired from the ignition switch, though the light, and then out to the oil pressure switch - pretty much the same as the gauge, except with the light in place of the gauge. The gauge also has an extra ground connection on it, but you get the idea. Power -> gauge or light -> sender or switch -> ground to engine as needed. So what happens when you mix a oil pressure switch with the gauge? With the engine off (low or no oil pressure) the gauge properly reads low/no oil pressure. But with the engine running and oil pressure present, the gauge pegs to the far right, and would most likely burn out the innards of the gauge if left that way for very long. To add some resistance in the circuit to give a decent "middle of the gauge" reading and keep the magic smoke in the wires (if you don't get this joke, you haven't screwed up any wiring and witnessed a short circuit), Ford added just enough resistance into the circuit so that when the oil pressure switch was closed, the gauge reads mid way - the visual equivalent of around 40 PSI or so. That way most drivers are fooled because they think the gauge is reading right and telling them they have an "about in the middle" oil pressure reading, when in fact, it's just telling them that they don't have critically low oil pressure. If the oil pressure did drop, the oil pressure gauge would suddenly peg to the far left - the visual equivalent of the oil pressure idiot light coming on, but not quite as noticeable. Brilliant.
Please note that some of the later vehicles (apparently the 90's F-series trucks in particular, not sure what years exactly) have more complicated instrument clusters, and this little resistor bypass trick simply does not work on them. From my understanding, the instrument cluster is all solid state internally and this little piece of engineering insanity is programmed right into the electronics of the instrument cluster. Some bright person out there might have a solution to this case, but it wasn't my case (and thus I had no parts or manuals to compare against and/or work on), and I didn't find it out there in my Google searching, so I don't know about it. If you do know of a solution to these other electronic clustered vehicles, please pass it along - I'll add info about it here and give you credit for the details.
The oil pressure switch -> sender swap is an easy one that I'll leave most of the details to various other websites and your favorite local shop manual. Get a "real" oil pressure sender that matches the gauge in your dash (most Ford units up until the early 90's seem to all be pretty much the same), unscrew the original switch, and screw in the new sender. Put a bit of pipe thread sealing paste on the sender threads to help it seal, but not so much that the sender can't get a good ground. Teflon tape usually prevents a good ground and the paste seems to be preferred, though I've had good luck with both the tape and the paste. Just make sure it gets a good ground to the engine block and doesn't leak - no ground or a bad ground means the gauge won't work right, and leaks are, well, bad. You may need to find a special "extender" that goes into the block and lets the oil pressure sender sit away from the block. The sender is larger than the switch, and this is often needed. You install the extender piece into the block and get it tight, then you install the sender into the extender piece. The sender usually points up and sometimes slightly to the rear to give clearance for the power steering pump hoses. Don't let it touch the exhaust. This part should be pretty obvious when you go to put it on the engine, if not, compare against another Ford V8 in a vehicle with a real oil pressure gauge to see how it works. Old Mustangs are a good choice for this. You might be able to get the extender as a reproduction piece, or you might need to hit up the boneyards or someplace like Craigslist.
The hard part to figure out is how to remove or bypass the fixed (usually 20 ohm) resistance in the wiring so the gauge works right with the new sender. The resistor is usually located in one of two places. Some early factory conversions and the "dealer installed" conversions have the resistor inline in the wiring right near the oil pressure switch/sender - it's easy to see it in the harness (as a bulge and/or stiff piece of wire with heat shrink tubing or tape over it) and just as easy to remove. Clipping out the resistor and replace it with a short piece of plain wire and you're done. Most of the factory installations of this system have the resistor on the back of the instrument cluster, as part of the flexible printed circuit board that is on the back of the instrument cluster. In my case, it was clear to see where the factory had modified the cluster circuit board to add the resistor, and on one of the spare clusters I had out of a 1990 Bronco it was even clearly labeled as a 20 ohm resistor. The non-tach instrument cluster was not labeled, but you could still find it pretty easily. Basically, you need to remove the instrument cluster from the vehicle, then turn it upside down and find the area on the back side of the cluster that is "behind" the oil pressure gauge. There will be three metal clips visible for each gauge and there will be three "traces" on the flexible printed circuit board - one leading away from each of the clips. Each one of these is a wire leading to the gauge, and the clips are how the gauge is electrically connected to the flexible printed circuit board. One is for the power source ("ignition" or "ign" on most diagrams), one or for the ground connection ("gnd", etc.), and the other is for the sender ("sig", "snd", etc.). The one we care about is the wire to the sender - looking at the gauge from the back, it should be the lower left of the three metal clips the gauge mounts into. Trace that wire (or a "trace", as it's known on a printed circuit board) back until you find the resistor, and then bypass it via your favorite method. You can solder a wire across the resistor leads, or if you're more handy with a soldering iron, right across the traces on the flexible printed circuit board. Don't screw this up - if you melt the printed circuit board or the instrument cluster housing, it's pretty much toast. If in doubt, find a buddy who is into electronics and have them solder it - bribe them with food and beer, but don't give them the beer until after they complete the work. Prepayment is a bad habit to get into, and besides, the beer can impair their ability to do the work. :-) Reinstall the cluster after it's been modified, and you should be able to see the difference as soon as you start the engine. In my case, I soldered a short piece of wire around the resistor - I found it easiest to solder to the existing solder connections vs. the traces on the circuit board. It's a bit sleazy, but it works.
Here's the back side of an instrument cluster out of a 1990 Bronco that has a tach in it. The oil pressure gauge in this view is the upper left gauge in the group of four on the right side of the instrument cluster. In the second view you can clearly see the resistor marked as "20 ohm" along with the ignition, ground, and signal wires labeled on the circuit board. Interestingly, you can also see the location where the original trace/wire lead was "broken" to allow for the 20 ohm resistor to be the only source of a connection to the gauge. Maybe some models (heavy duty) got blessed with a real working oil pressure gauge form the factory? Or maybe this was the cheapest engineering fix they could do to make this work? Who knows. You can fix the problem simply by connecting the two copper contacts next to the "SIG" label or by jumpering around the resistor itself as I did. The third picture is the same as the second, except that some key details are labeled to make it clearer to the non-electrically inclined out there.
Here's the back side of an instrument cluster out of a 1989 F150, this cluster does not have a tach. The oil pressure gauge in this view is in the upper left corner of the cluster. There are no labels on the traces, but it's easy enough to see the similarities between the tach-equipped cluster and this one, especially in the second view. The bottom left metal clip is the signal wire, and there is again a set of small copper contacts you can bridge to bypass the resistor and make the gauge work like a real gauge. The third picture is the same as the second, except that some key details are labeled to make it clearer to the non-electrically inclined out there.
NOTE: On each of these clusters, you can clearly see the standard resistor soldered to the flexible printed circuit board. If you decode the color bands on the resistor via one of the many online decoders you can find via Google (like this one), you'll see that the color code of "red black black gold brown" decodes to a 20.0 ohm resistor with +/- 1% tolerance and that this is a "5 band" resistor. If you are interested, you can get plenty of information on what he color bands mean via Google. It's a fascinating and simple way to encode a lot of information in an easy to read fashion, unless the reader is color blind or vision impaired.
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Page last updated 12/27/2011 10:23:21 AM