Oil Pressure Gauge swap


New Member
Nov 3, 2018
Anaheim, CA
Hello guys,
Can anyone help me with some info about the wiring for a 65 Mustang Instrument Cluster. I have a 65 Mustang Fastback with stock Instrument Cluster with the lights for Oil and Amps.
I am switching to a 66 IC with 5 Gauges. My question is about the wiring for the Oil Pressure light being used on the Oil pressure Gauge. How do you make that connection.
The oil pressure light has 2 wires, 1 for the sending unit and the other for the Voltage Regulator. The Oil Pressure Gauge has 2 posts. Not sure which post is what, positive and negative or what. I already changed out the sending unit the gauge one. I really appreciate the help and advice from this forum. Thank you.
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One side of the 66 gauge comes directly from the sending unit. It basically is a "floating" ground, with the ground being the supplied by the unit being bolted to the block. The other side is a voltage that is to be supplied from the little regulator found on the back side of the gauge assembly (or at least it should be there. That VR puts out less than12 V (5V? or so) and it also supplies voltage to the gas gauge and the water temp gauge (its daisy chained.) If the VR doesn't work correctly none of those gauges will work properly.

You can test the oil pressure gauge once its hooked up properly it should read zero (the L) with the key on and no wire on the sending unit. Then if you ground out the sending unit wire to the motor or body it should peg the meter. Both are checked with the key in the on position. When the key is off there is no power so the guage will read zero. If this works then the sending unit should function properly when hooked up and the car is running.
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Dennis, Can you help? I recently had all the gauges working. But I had to disconnect the cluster and re-install. Now the Oil Gauge does not peg the meter when the key is on. It did just a minute ago. I've tried to trouble shot it but not working. I tried using another brand new sending unit but same results. Is there a way the test the gauge on its on. Should there be a resistance across the leads. Dont know what happened, it was working perfectly, now its not. All other gauges and lights working fine. I also ran a brand new wire from the sending unit to the gauge and ran a separate ground wire to test. Same results. Does not peg the gauge. So now thinking its the gauge. It was brand new. So is there a way to test the gauge. Hope you can help. Thanks.
Its good that the temp and gas gauges work, which shows the VR is putting out voltage to the gauges. The illumination lights have no effect on the gauges and vice versa so they don't even need to be plugged in for testing. Just need to make sure the VR has its power wire connected (and the separate cluster housing ground is attached.) The cluster ground is needed for the VR to activate. Then its a matter of applying the ground from the sending unit to test.

The oil pressure gauge should read zero when the 5V VR power is applied to the appropriate side of the gauge and the motor is not running. There will be no ground signal from the sending unit until the motor builds oil pressure (or you ground the oil pressure wire with the key on.) It should peg the gauge when you ground the wire at the sending unit. That tests the single wire that goes back to the gauge.

You can duplicate the test at the dash by temporarily removing the wire that goes out to the motor and providing a temporary ground signal (perhaps using an alligator clip lead that comes from a body grounded screw) and turning the power on-it should peg when connected and go to zero when it is not. The gauge does not need to be bolted in the gauge cluster. Do not apply a straight 12V to either terminal-you must use the VR.

I would imagine that there would be some small resistance across the gauge leads when there is no power applied (with the wires disconnected.) It should never read high resistance (no continuity) across the 2 terminals That would definitely be bad. I would think that the resistance reading should be somewhat similar to those of the gas or the temp gauge.

It is also possible that the VR is weak and not able to provide enough power to activate all 3 gauges. I have witness that before. I wouldn't assume it right off the bat if the other gauges act normal. Make sure that the cluster housing is properly grounded when testing the gauge under power.

I'll see what I can find out resistance wise today, if the attic heat is not too brutal.
I measured the resistance of 3 original Ford oil pressure gauges and each was 13.0 (ohms plus or minus 0.1.) No wires were installed. Hopefully that will help. I also found that the temp gauges measured the same if you want another reference.

When reinstalling the wires make sure the red/white (or white/red wire) is closest to the speedo gear (or at least that is how I had one housing labled.)
WOW, thank you Dennis. That is a lot of great info. Going to go out to the garage and do those tests. Hopefully I can figure out whats going on. Thank you so very much. Are you local to Anaheim, Ca. Would love to buy you a beer to say thanks.
Dennis, I tested the gauge again. Gauge will peg when I ground the sender wire. I also found out the voltage wire that was going to the gauge was sending 12v. I spliced into VR wire going to the fuel gauge. That voltage fluctuates.
So now when I turn the key on the Fuel gauge goes to half fuel (pretty sure there's half a tank of gas) but the pressure gauge does not move. Thats because there is no oil pressure built up yet, the gauge will move once the car is started and builds pressure correct? Thanks
Correct. With the engine not running there is no pressure and thus no reading on the gauge. Also the needle will NOT travel up to what would be considered the "low" pressure mark with just the key on. It is waiting on an actual signal from the sending unit. The signal out of the sending unit is a variable resistance based upon having an actual pressure. Zero pressure equals maximum resistance. You should NEVER see the oil pressure gauge reach maximum on the gauge when the motor is running.

Each unit is a little different but with the motor running the gauge should advance (albeit slowly) into the good range. It should increase somewhat with higher RPM's, again albeit even slower. Being electric, it's only giving an idea that there is some oil pressure present with some variance that can't be seen on an oil light system. Only a mechanical gauge with a tube coming from the motor can give an actual and reliable reading that responds instantly to oil pressure changes.

I'd love to meet up for a cold one (and I sincerely appreciate the offer) but I reside on the the east coast.
Thanks Dennis, well if you ever make it to California (714)325-4466. Also, did I do the right thing by splicing into the VR wire to the fuel gage. I was using the same two wire that was connected to the oil light.

Glad to hear that you got it working.

I doubt if I'll get to your area again (ex was from Pasadena) but who knows since the newer wife has 3 kiddo's with grandkids in the far upper west coast.


So just to be clear. It was best to splice into Fuel Gauge VR wire and not use the Power wire that was used for the Oil Light that was giving 12v.
Was that the correct thing to do?

Hopefully tomorrow I will be able to take it out on the road.
Yes, the VR power wire is required for a gauge to function properly. Splicing was needed if there was no other taps for it.

The old oil light system used a full 12V for power and its old sending unit strictly as a ground. Direct 12V power is NOT compatible with the gauge. Tape up the old wire and forget about it.

Hopefully any prior attempts using 12V did not have any ill effects on the gauge. You should know how successful the oil gauge swap was within 10 seconds of starting the motor.

Good Luck!
Was able to run the car in the garage yesterday for about 1/2 hr. Everything seemed to work great except for the Oil gauge. The needle did not move at all.
The needle did move the other day when I would ground it, pegged. Could the Gauge or sending unit be bad. I have another brand new sending unit that I can swap out. Will try that later today when I get home from work. Do you think that the Gauge could be damaged by applying the full 12v. I don't want to take the cluster out again. Putting that Speedo cable back on is very hard. Thanks
Yes 12v applied directly to the gauge could damage it. Most electrical things will not survive when more than double the designed voltage is applied. We can't prove it one way or another without a replacement but we know that the resistance value taken earlier across its terminals indicates that it should be good.

Just for a reference, does your new sending unit look like a little canister that is at least an inch tall and at least and inch in diameter?


I can't test this theory until this weekend but believe one could remove the wire from the sending unit and get a resistance reading (to a good motor ground) across the sending unit first before and then when the motor is running. I have never measured it but expect it to be an open circuit (infinite resistance) with the motor off and a definite low resistance (nearly but not zero--wild guess maybe 70 ohms) when the motor is running. It might take a couple of seconds for the resistance to drop until its near steady when the motor is first started.
Hi Dennis, I measured both Sending Units. The unit out of the car measured about 800 ohms, the new unit measured about 8,000 ohms. Need to re-check them, something doesn't seem right. I put the new unit in and went for a drive. Needle did not move. Going to do more tests.
Hi Dennis, I measured both Sending Units. The unit out of the car measured about 800 ohms, the new unit measured about 8,000 ohms. Need to re-check them, something doesn't seem right. I put the new unit in and went for a drive. Needle did not move. Going to do more tests.

I did a resistance test of the functional sending unit on my car. From stud to the unit's case I get 5800 ohms. That may vary based upon the manufacturer and may not mean anything other than the unit is not shorted when off.

What may be more important is that with the wire off the sending unit I measured the resistance of that wire to a good engine ground. This effectively reads across the gauge and it was 22.2 ohms. Test that for sure (with key off) as I believe it is an accurate representation of approximately what you should have if your gauge is good.

I plan on doing an engine running resistance check across the sending unit in the next day or two. Since I have both the factory gauge and a good mechanical gauge installed at the same pressure port, I should be able to give you and idea of what sending unit resistance to expect with "measured" psi of oil pressure and how it registers on my OEM gauge.
I tried measuring the resistance of the 1st gauge that I had in the car ( the SU that was installed when I had 12v running to the Gauge.) It started at under 10 ohms and slowly increased. After about 3hrs it was at 7500 ohms and still slowly going up. I measured it out of the car on the bench. Tomorrow I will try to measure the 2nd SU that is currently installed on the motor. I will measure it also out of the car on the bench. I will also measure the SU wire.