So...it looks like either there is another resistor in the circuit somewhere, or the wire connecting to the sending unit is a resistance wire. When I ground the wire, the gauge stays right in the middle of normal. Unless someone has previous experience on where to find the resistor, I'm thinking a new wire from the sending unit to the gauge will probably be the way to go. Any other opinions?
Get a Factory Ford Shop Manual (the one that covers the Body/Chassis) and there is a whole section to diagnosis, exact electric values and checking each gauge. I do know the sender is different (at least on my 1987) on the gauge equipped trucks from a vehicle that has the light only. (It uses the old gauge sender that goes back to a 1970 Mustang, I believe.) 250HD Custom
Thread resurrection!! I just saw this and wanted to add to it. I too have tried to do this mod, it worked on my old Thunderbird cluster no problem. Did the same thing to my 96 F-150 and... no worky. gauge stays at the center even with the factory switch installed (which when closed, engine running, reads 0.3 ohms to ground) so there is a resistor somewhere else. Yes I soldered the correct resistor, I am 110% sure since I soldered a jumper in, had a beer, then came back and checked it again to make sure I jumpered the correct component and verified with my ohmeter that the solder joints didn't have high impedance. Jumper read 0.1 ohm across it.
So... got to searching and found a couple people around the net that say there may be a jumper INTERNAL TO THE GAUGE! Grrrr... so what I'm fixin to do now is get the gauge mechanism out of another cluster I got from the junkyard and install it into my cluster. Actually I might trace out the PCB on the back of the cluster and if the oil circuitry is the same, I'll just plug the entire cluster in and try it. I dont actually want to use that junkyard cluster because it ain't got a tach and mine does. Will keep this thread updated, so anyone searching in the future can find the information...
I would look more to an internal mechanical stop than a resister. If it had an internal resister jumping the outside resister would still have caused it to read higher when tested.
I do not know how they could put an internal jumper in and make it work that way.
As you say this only seems to not work with the tach cluster. I have a 1992 tach cluster and the mod works fine but not in my 1995 with a tach. I do have a 1995 non tach cluster and on the bench the gauge will swing full scale.
This may also be true with the 1994 tach clusters as they seem to be the same as the 1995 clusters.
Subford, I didn't see any mechanical stops inside... the gauge needle was free to swing all the way over to the high end of the scale when I moved it by hand. I just did the swap, and it seems to work fine. That is, I still have the stock oil pressure switch, and once I did this swap I fired the engine up and the gauge pegged way over past the High mark. Tomorrow I'll swap in the new pressure transducer and make sure all is well. Here's a quick lil writeup with pics showing how to swap the gauge internals over to the older style that works with the resistor bypass trick. You'll need: - a gauge cluster from an older truck. Not sure what year cluster I have, I got it from the junkyard. MY truck is a 96. - Torx bits (Tamper-proof Torx not needed, but okay to use) - a fork - small common screwdriver (to release electrical connector clips) - new oil pressure transducer ("sending unit" in parts store talk) - your beverage of choice Alright this'll take maybe bout an hour if you work slow and take your time. First, take your cluster from the older truck and remove the Torx screws holding the clear lens and plastic bezel on. Put these aside, and carefully remove the oil pressure/water temp gauge section. Just pull it straight up and out, it's held in by it's terminals. Uploaded with ImageShack.us Now, take apart your instrument panel and remove your truck's instrument cluster. Remove the oil/water gauge section from it also. Ford advises that you not set the clusters face down... apparently this makes the liquid crystal stuff in the odometer LCD leak out and make black marks on the digital display. Got the gauges out? Good, now use the special tool shown below to carefully pry off the oil pressure gauge needles. Don't try to pry them off against the cluster, you have to lift them straight off or you'll bend the gauge shaft. It does take some force to remove them, don't be afraid to pull on them... just don't apply any sideways force to the shaft. If you've ever installed aftermarket gauge faces, you'll be familiar with this step. Be especially careful with the needle on the junkyard cluster, you need this part! Uploaded with ImageShack.us Once you get the needle off of both gauges, turn them over (careful not to mash the needle on the water temp gauge!) and remove the 2 Torx screws holding the gauge mechanism on. Note the locating pins on the back, that fit into holes in the gauge mechanism. You CAN put these on upside down, so make note which way is up. The terminals are in a triangle shape with the single pin at the top. Uploaded with ImageShack.us Alright, now before you swap the gauge mechanism (the silver can you just removed) from the junkyard cluster into your good cluster, notice that the newer one from your truck has plastic spacers or something where the screw holes are, and the one from the junkyard does not... at least this was the case for me, see pic below. Once you swap the gauge mechanism over to your cluster and try to put the screws back in, you'll see that the screws seem to bottom out. I went ahead and carefully tightened them til they were fully seated against the gauge cluster, and no damage happened. If you don't feel comfortable with this, then use washers instead. Go ahead and swap the mechanism form the junkyard cluster over to your truck's cluster now. The pic below shows the difference in thickness near the screw holes... my factory gauge at the bottom, "new" gauge at the top. Also you can see a difference in the internal workings of each, this is why the newer gauge won't work with just a sending unit swap/resistor bypass. Uploaded with ImageShack.us Looking at the needle from your parts cluster and comparing it to the one you just took out of your truck, you'll see that the shaft is smaller on the older gauge. You'll need to use this needle when you put your cluster back together. See pic below for comparison, and don't put your needle on yet... Uploaded with ImageShack.us Go out to your truck and carefully plug the cluster harnesses in. Just the 2 large rectangular ones will do, no need to plug in the small square one that connects to the PSOM. Switch on the ignition but don't start the engine... you want zero oil pressure at this point. Now, take the needle that came from your junkyard cluster, and lightly press it onto the shaft of the oil gauge... Uploaded with ImageShack.us Nudge it left or right to wherever you want it to read when you have no oil pressure. I put mine just to the left of the red area. When it's where you want it, push it straight onto the shaft but don't bottom it out against the gauge face. Switch off your ignition, remove the cluster and put it back together. Take it back to your workbench and solder a jumper across the resistor in the signal circuit. I know I just advised to you to not turn the cluster face down, but doing so for a couple minutes shouldn't hurt it. If you're worried about it, remove the speedometer and set it face up somewhere safe while you do this. Now, I don't have a picture of this since I already soldered mine last week, but there's only one resistor for the oil signal circuit and if you look close it's obvious which one it is. It's a 20 ohm resistor, near the OIL SIG terminal. You can remove it and solder in a jumper, or solder a jumper around it. It does NOT matter electrically. Optionally, if you have an ohmmeter it's a good idea to measure across your two solder joints to make sure you got a good connection. If so, you'll read close to 0.00 ohms... mine is around 0.3 ohms, which is fine. Put it back together, and also since you have the cluster out now is a good time to replace any blown bulbs, or remove any warning lights you don't want. Now it's just a matter of reinstalling the cluster and instrument panel parts. If you didn't change your oil pressure switch out, then go ahead and do that when you're done inside the cab. I don't have a pic of the completed install since I ain't changed mine yet either... that's tomorrow morning's project. If y'all run into any problems let me know and I'll try and help. If I run into any problems after I change the sensor, I'll keep everyone posted on here.
Last edited by dixie460; 12-03-2011 at 08:06 PM.
Reason: Clarified some stuff
Well, I replaced the idiot light pressure switch with the real deal today, and it works fine. I thought I mightve had a problem at first since my oil pressure was movin up and down with the engine RPM at a steady speed... pressure was between 40 and 50 on the stock gauge (right smack dab in the center is 40PSI on these factory gauges after the swap and if you put the needle on like I did) so I put a mechanical gauge on there too (temporarily) and sure enough, it's needle was fluttering too... looks like I might have a lil wear in my oil pump. Max pressure I was able to get out of it even at 4000 RPM was around 55 PSI. Plenty enough for this motor, I don't spin her up much past maybe 3000 RPM unless I have to. Glad I did the swap, now I know what's really going on. Wouldn't have found that with the factory setup :-) Eddiec1564, you're right. They might have done this to the vans too... I took a closer look at that factory gauge and just like you said there's only half the windings there, which would explain why the needle couldn't move past the halfway point when driven magnetically, but moved fine when pushed by hand. Wonder why they still had the resistor on the back of the cluster then?? Ah well, who cares. Problem solved regardless
Hows your oil pressure when at a hot idle? On my 89 F250 its LOW and wandering around 15psi. At 1100rpm and up its around 45psi under load, when I let off the gas, it will drift up another 5 to 10 psi to about 55psi(wore out rod bearings). My engine is wore out and that will cause a drifting oil pressure reading. I did the mod to my 89 and it works good till the sending unit started the usual oil leak and filled up, so right now I have a mechanical guage on it.
I just took a non-tach cluster apart out of a 1995 F150 and the oil gauge had both winding like the one you took out of a 199? truck.
I also checked it on the bench and the needle moved across the full scale using a resister sub box.
All of the drawing numbers on the cluster and module clusters had numbers that started with F4.
Also the Temp gauge winding were the same as the oil gauge and read the same across the scale with resister sub box.
I did notice that the oil and temp gauges had a resister pushed on to the signal and power pins. The resister for the oil gauge was 115 Ohms and the temp gauge was 95 Ohms. I guess these resisters supply current to make up a voltage divider between the resistance of the sender and the coil of the gauge for gauge calibration. These resistors are not shown on any of the cluster wiring diagrams from Ford.