Oil Pressure Gauge Running Backwards

[JohnW2112]

I have a 1975 that I purchased about one year ago. After a major interior restore, I finally got to drive it a few months ago. I noticed that the oil pressure gauge was running the wrong direction. Upon start up, the needle will go to the far right (110 or 80 depending on which numbers you read) and the warning light stays illuminated. When revving the engine, the needle will drop to as low as 40 depending on the rpms. The warning light does not go off until you shut off the motor and the gauge returns to 0.

It is working like I would expect, only working in reverse of how it should other than the warning light not going off while running. Not sure if this is a gauge issue, or one of the two sending units (light,gauge). I was able to start the car when I first bought it, but I did not notice this behavior at the time. The pressure seems to be there as the gauge moves while revving, so I don't think that is the issue.

Regarding upgrades, the only thing I have done with the engine is swap the original carb for a Weber 32/36 DFEV, which made it run very nice. I haven't messed with anything else in the engine compartment, although I did take out all the gauges to refinish the dash, but that process went pretty smooth.

Just hoping someone might have a thought before I purchased new senders to see if that is my issue.

[18Fiatsandcounting]

Wow, that is weird! Maybe you have discovered an alternate universe, where time and space collide...?

OK, time to get serious. The oil pressure gauge works by partially grounding the wire that goes to the sender, and when the oil pressure is low, the sender has a very high resistance and so the gauge reads low, and as oil pressure increases, the resistance of the sender unit drops towards zero and the gauge goes up. That's how it's supposed to work, and the low pressure light works the same way in that the sensor grounds that lead of the light when oil pressure is too low.

But, it's not all adding up for me, so try this: Disconnect both the leads for the light and the sending unit (making sure they don't touch a ground like the engine). Turn on the ignition but don't start the car. What does the gauge read, and what does the light do? The light should stay off, and the gauge should still read zero. Now start the car. The light and the gauge should both still be off or at zero. If they are, they it's likely you have a backwards sending unit and a backwards low oil pressure light switch. But it would be weird to have both of these be backwards...

Also, does your temperature gauge work OK? I can't imagine someone doing this, but if the leads for temperature and oil pressure somehow got mixed up.... Not sure what would happen, but weirdness is my guess.

-Bryan

[Nut124]

Bryan has some good info there.

I would do some quick and easy testing to try to make sure the right gauge is plugged into the right sending unit.

One at a time, unplug the pressure sender, pressure switch, temp sender and switch on head and observe the gauges to make sure the correct gauge responds.

[JohnW2112]

So, taking off both leads and turning the ignition on without starting, the warning light remained on, but the pressure stayed at zero. Plugging in just the light sensor, no change, light on and gauge at zero. Plugging in just the gauge sensor, light stays on and pressure tops out. I referred to two different wiring schematics that I have and the pressure wire is gray/black and the light wire is gray/yellow so the wiring appeared correct.

Temp gauge works so that isn't it (green/white wire connected to the temp sending unit).

Starting the car without either wire connected, light on, zero pressure. With just the pressure lead on and starting the car, light stays on and pressure tops out but will lower when revving. I'm guessing the light might stay on until the sensor tells it to turn off by depriving it of power, so I'm thinking of swapping out the light sensor for starters, it's pretty cheap. But that wouldn't explain the reverse pressure. Maybe swap them both. Although, I suppose it could be something internal with the gauge.

Thanks for the advice, they were great things to try!

[spider2081]

So, taking off both leads and turning the ignition on without starting, the warning light remained on,

My gut thought was to re check the gauge connections as that is where you worked last.

Most of the gauges Fiat used are a balanced bridge type movement. Power is applied to one side of the winding, ground is connected to the other and the sender is connected to the center. If the gauges ground connection is lost the gauges indications irrational. Some gauges the dash lights and the instrument share the same ground wire. Do the dash lights work in the gauge?

I think removing both wires from their senders was a great suggestion. Once the gray/yellow was removed the warning light should have gone out. The fact that it did not indicates some how the wire is shorted to ground. The wire diagram I'm looking at shows the gray/yellow from the low oil pressure switch connects to the ignition control relay and the low oil pressure warning light.
Does your car still have dual points? The relays are mounted under the hood on the driver side fender well.

[18Fiatsandcounting]

This is perplexing. Spider2081 has some good points, and I agree. In earlier Fiats, the low pressure switch simply grounds the warning light, and the +12v for the light comes from Fuse #9 (I), which is active whenever the ignition is on. Not much to go wrong there, but the 1975 model year has the additional complexity of the electric fuel pump relay which shuts off the pump if the oil pressure drops too low. However, since your engine runs, that wouldn't seem to be the issue.

You mentioned "major interior restoration" and taking out the gauges. I'm wondering now if something got connected wrong in the gauge wiring, or the ground for the oil pressure gauge isn't right. Fiat gauges will act goofy if they loose the case ground, but this wouldn't explain the warning light.

At this point, I'd pull out the instrument cluster and check the wires going to the oil pressure gauge. Make sure you've got the right connector attached, and (with a piece of wire or a clip lead) ground the gauge case by hand with the ignition turned on and see if that has an effect on the gauge reading. If the gauge goes from full scale back to zero, then the issue with the gauge is a bad ground. Also make sure the negative lead for the bulb socket isn't grounded all the time, as that should only go to ground when the switch on the engine completes that circuit. The positive for the warning light (yellow-black if I recall) should have 12V on it whenever the ignition is on.

-Bryan

[JohnW2112]

Thanks for all the great suggestions. I should have mentioned, as Spider2018 brought to my attention about points, I put in a PerTronic electronic ignition earlier in the year when I swapped out the carb. Not sure how that would affect the oil pressure, and honestly can't say which way the gauge worked when it had points as it barely run and that wasn't my area of focus. Hard to imagine why that would affect the oil gauge, but you never know.

I will take out the gauges when I have more time this weekend, gonna have to take off the steering wheel again. I will check the grounding and will say the dash light illumination does work (on all gauges). I will also take a look at the associated relays.

Thanks for all the great advice and I will update once I check into your ideas!

[spider2081]

I will check the grounding

Many of the cars interior ground are made at a grounding pod that is accessible above and slightly behind the ignition switch. It can be seen and connections checked with out removing anything, The hardest part is getting ones head into a position to see up behind the dash in that area. Good lighting helps.