Topic: Oil/water gauge

[Bosco]

I have done a search but can't seem to find this condition. Both gauges are stuck. They have not move, needles are just left of centre.

Any ideas before I pull the gauge out? I am also waiting for the manual in the mail.
I have read to pull the wire or ground the wire from sender and the gauge should spike one way or the other, but what if the gauges never move?

This on on a 1976 With a 400 which I just bought.
Thanks for any ideas.

[Macdave]

I can not see both going south at the same time unless they have been out and setting somewhere to get them totally messed up. Sounds more like a bad ground to the cluster or the main connector has an issue.

[Bosco]

Thanks for the reply.
I guess I will just wait for the manual so I have the wiring diagrams before I pull the cluster out.

[Angelo]

Once you do pull them you can bench test them.

[KJMason]

Once you do pull them you can bench test them.

Angelo, do you know the correct voltages for the bench test?  I mean what voltages should be for different PSI?

[Angelo]

It is not voltage, it is ohms that it measures. Everything on the car is 12 volts. The gauges measure resistance, the temp gauge will send 12 volts of current with a specific amount of resistance, the gauge then reads this resistance with the needle in the gauge. I hope this quick explanation makes sense. You can ground the wire and the gauge will peg one way or the other for example.

Here is my write up how to bench test the fuel gauge: http://www.modsandrods.tv/2012/03/29/bench-testing-fuel-gas-gauge/ Oil pressure sweeps in a different direction than the temperature gauge, otherwise it's the same procedure to test the oil/temp gauges.

Oil Gauge
1 Ohms = 1psi
45 Ohms = 40psi
88 Ohms = 80psi
If the wire is disconnected to the oil pressure sender, there will be nothing to read, oil pressure would measure excessive pressure (over 80). Ground the wire and the gauge will show no pressure.

Water Temp Gauge
46 Ohms = 260 degrees
68 Ohms = 220 degrees
350 Ohms = 100 degrees
Note: 80/81 cars have a different ohm range
If the wire is disconnected to the water temp sender, there will be nothing to measure  which would show as a very cold engine. Ground the wire and the gauge will show as overheated.

Fuel Level Gauge
90 Ohms = Full tank
45 Ohms = 1/2 full
0 Ohms = empty tank
If the wire is disconnected to the fuel sender, there will be nothing to measure and the gauge will read beyond full. Ground the wire and the gauge will read empty.

You can think of a disconnection as providing infinity amount of ohm, and a wire that is grounded to have zero ohms.

Read the link to my site, you need at least a 500 ohm potentiometer to test the accuracy of the gauges. If you just want to test that the gauges work, you can use the ground/disconnected method which will test the extremes explained above.

I will warn you though that I suspect either a fuse or a bad power/ground to your gauge cluster. Chances of both not working are low. If someone swapped the motor, the gauges would at least read extremes because they are not connected, which is why I would eliminate that suspicion.

Factory gauges, specifically the temp gauge, are inaccurate from my testing. I would not rely on the factory temp gauge. The fuel and oil pressure gauges are acceptable. Either way I added new oil pressure and water temp gauges (in addition to the factory ones) in my map pocket.

[KJMason]

Thanks Angelo.  I'm not the OP but have been trying to troubleshoot my oil pressure gauge and the resistance explanation clears things up.  At idle, my gauge will read about 40 psi but when I go over 1000-1500rpm it quickly pegs all the way past 80psi.  It doesn't just jump, it moves up smoothly (but quickly) and is obviously inaccurate.  I've swapped out the sender and the gauge and it's still doing the same thing so I must have a bad wire, connection, or ground somewhere which is raising the overall resistance in the circuit.  It just occurred to me that a quick check of this would be to simply turn on the ignition and if I get a positive PSI reading without the engine running then it's high resistance in the circuit.  I'm guessing that it's probably the printed circuit behind the gauges as it has some shade tree repairs already done on it.

Edit:  Actually, now that I think about it this, it could be that a bad ground is causing other problems.  My temp gauge seems to read low (typically about 150) and higher resistance should equal lower temps.  Also, I know my tach reads low (compared to my timing light/tachometer).  Have you ever seen a bad ground cause a tach to read low?

[Angelo]

Even a frayed wire holding on by a thread can add resistance. Resistance where it's not designed will usually cause heat and possible fire, definitely something you want to look further into.

You are looking at the oil pressure when the engine is warmed up right? My car also has about 60-65psi cold at idle and quickly goes up to 70-75psi on the gauge when cold. Hot though it is more like 30psi with 10+ for every 1,000 rpm.

My AutoMeter gauge, i'm actually at 55-60psi cold and 25psi warm at idle.

[KJMason]

Yeah, it's with the engine warm so definitely not right.

[KJMason]

Angelo, I don't think you saw my earlier edit. Do you think a poor ground or bad wire would cause these other symptoms below?


"Actually, now that I think about it this, it could be that a bad ground is causing other problems.  My temp gauge seems to read low (typically about 150) and higher resistance should equal lower temps.  Also, I know my tach reads low (compared to my timing light/tachometer).  Have you ever seen a bad ground cause a tach to read low?"

[Angelo]

lIf everything in the cluster is not working right then it could be a ground or a bad printed circuit.