oil pressure gauge problem

[antonellomopar]

Hello group
strange things are happening to my oil pressure gauge @ 71 Challenger RT.
When I redid the dash, I checked if gauges were working, and everything checked out ok. Needles, when grounded, went to max and then came back to zero. Installed the dash in the car, put new sending units, and started the car with the oil pressure sending unit removed, and a oil pressure gauge conncted directly, and showing good pressure.
Installed the new Standard sending unit, and needle moved just one millimeter...and dint move any higher. Changed the sending unit wiht another new one, same problem. Removed a sending unit from my 71 Cuda that I knew it was working, and still same issue. Installed the first sending unit, and it worked perfectly. Grounded the grey wire that went to the sending unit, and needle went to the top.
So, what is happening and how can I fix the gauge? The water temp and gas tank gauges are working correctly.
Please help...thank you

[usraptor]

My guess would be a ground issue.  Is the gauge itself grounded properly?  I'd run a temporary ground wire from the gauge housing (not the post) to a known good ground and see what happens. 

[Bullitt-]

  I wonder if there could paint buildup in the sending unit threads preventing a good ground.
I know this has been an issue with temperature sending units when sealing tape was used.

[Blowout]

I'm having the same issue on another car of mine.  When I do the test to the sending unit it responds like it should. I've had my gauge tested and came back prefect.  I've checked all my grounds on the back of the cluster, good.  Only thing I can think of, is the tape on the threads, as just mentioned.

[antonellomopar]

Tried to improve the ground on the dash frame and dash bezel, but nothing changed. Will try to clean the threads, but I suspect it is not the problem. Would really hate to install an extra oil pressure gauge under the dash.

[73_Cuda_4_Me]

Try clamping a separate ground wire from the sending unit to the battery ground and see if the gauge starts reading correctly. If it does, it's sending unit to block grounding issue or an engine to battery ground issue....

[Chryco Psycho]

the circuit doesn't ground anywhere at the dash , it grounds through the sender , so it has to be a connection problem between the sender & block or the block & battery .

[Blowout]

Anything recommended on the threads to the block or bare is best? Just snug it up and hope for no leaks?

[RUNCHARGER]

Engine to body ground is important on a 71, as I recall (from 45 years ago) the ground is at the back of the right cylinder head to the firewall on a 71 which is different than a 70. Just something to check.

[dodj]

Tried to improve the ground on the dash frame and dash bezel, but nothing changed. Will try to clean the threads, but I suspect it is not the problem. Would really hate to install an extra oil pressure gauge under the dash.

This would do nothing to help your problem with the oil gauge. With ohmmeter read from sending unit case to sending unit connection. If it's infinite, you have a sender problem. If it's something like 75 ohms (I think) it's good.
If it's resistance is good, check that you have 5vdc on the lead to the sender. if you don't, find the break in the wire somewhere. If you do, it's a ground issue. Either block to battery or sender to block. Most likely sender to block.

[73_Cuda_4_Me]

I used a little copper coat to seal my oil and temp sender threads...

[antonellomopar]

I dont have an ohm meter...but used a test light to check for ground. Engine is grounded, and sender is grounded to the engine. Tried a direct cable from the sender to the gauge , bypassing the stock wiring and bulkhead, and still nothing.

[anlauto]

Maybe you have no oil pressure ? Is the engine making loud knocking noises ? 

Just kidding...

[antonellomopar]

I had a mechanical oil pressure gauge conncted before starting the engine....no noises...
Will connect a gauge tomorrow...just in case...

[dodj]

If you have tested and are 100% sure that ground is not your issue, then your problem must be in getting 5v to the sender from the gauge. I would first redo the test you did when to rebuilt the dash. Ground the sender wire at the sender and verify the gauge and wire again. If it doesn't move you may have a wire/connector problem or maybe the gauge has failed.