66 Cuda Oil Gauge

-

Steve Clason

Active Member
Joined
Sep 8, 2018
Messages
35
Reaction score
15
Location
Piercetom, IN
My oil gauge does not register when the car is running. Here's what I've done thus far. My conclusion is bad gauge but I know there are experts that can confirm this.

1. with switch on accessory or run before start the gauge pegs high
2. when I start the car is drifts to pegged low
3. when I disconnect the sending unit wire while running or prior to start it makes no difference
4. when I ground the sending unit wire the meter pegs high
4a. the sending unit wire is not broken or grounded based on my ohmmeter testing.
5. a new sending unit made no difference
6. a new circuit board for the dash made no difference (it was needed due to poor connections for other gauges)
7. All other gauges read correctly so I presume the voltage regulator in the fuel gauge is fine. The voltage seems to be what is expected ~ 5v to the oil and temperature gauges.
8. I've read and tried various tests for the sending unit no change
9. I just had the top of the engine rebuilt (head redone, new valves and guides, etc. and that mechanic assures there is oil pressure.

So any other ideas or should I just get the gauge rebuilt at this point?
 
Did you use teflon tape or other sealant on the sending unit threads that would keep it from making a good ground? Test with an ohmmeter from the body of the sender to ground.
 
Get a resistor close to 27 ohms which I believe is half scale. Disconnect the wire to the sender and attach resistor to it other end to ground gauge should read half way.
 
Did the guage and sender ever work correctly?

Is the replacement sender the same model number as the other one?

Please post a photo of the sender.

Are you sure your sender is for a guage and not a light?
If it was a light sender it would be shorted when engine is off and open when there was oil pressure.

This would give you some of the symptoms you describe.


Quick test... With sender wire disconnected and engine off what ohms do you read between sender and block?

Same setup with engine running what ohm reading do you get.




1. with switch on accessory or run before start the gauge pegs high
Assuming the guage works the same as temp. That would indicate a short in the wire between sender and guage

2. when I start the car is drifts to pegged low
Very odd


3. when I disconnect the sending unit wire while running or prior to start it makes no difference
Not sure what you mean here.

4. when I ground the sending unit wire the meter pegs high

That is what I would expect.
 
when I ground the sending unit wire the meter pegs high
Then the gauge is working. When the sending unit allows full current to flow through to a grounded block, that means more current flows though the gauge's bi-metal coil, and the needle will peg to high, Less current flowing through to the ground means less current flowing though the gauge's bi-metal coil, which means less needle movement. Sounds like a sending unit.
 
This condition reads exactly like a oil warning lamp sender in place of a oil pressure sender.
 
Thanks everyone! I will check to see if it's a sending unit or switch and post a photo in the morning. Thanks for the ideas!
 
Once again you guys nailed it. The unit is a switch not a unit for a gauge. The ohms between the tip and body of the sending unit are around 40 before starting the car. Start it and there is no ohm reading. Once again I forgot a rule. Never assume the part on a restoration is the correct one to replace. Someone else could have put the wrong part there and you are just duplicating their efforts.

Thanks again! Here is a photo of what the unit looks like and the Standard box part number.

IMG_20200827_114013.jpg
 
So one additional question, since the threads are smaller on a gauge sending unit than a switch, is the correct fix to use an adapter?
803-008-2T.jpg
 
You are suggesting 2 different size holes in engine blocks for oil senders. That doesn't seem right to me. If you do need a bushing, so be it. You want nearly zero resistance between the body of a sender and engine block, and neg' battery post. Most of the new senders I've purchased already have a very light coating of sealant on the threads. A brass bushing in cast iron threads should get the same. A little brake cleaner will remove oil from threads in the block. Teflon tape could interrupt the ground path.
 
OK. It should go in the back of the oil pump and filter housing.
upload_2020-8-28_9-10-50.png


Should be 1/8 NPTF but maybe some years had a larger thread. That I don't know.
 
Yes, they used an adaptor on a slant 6 sender with an oil pressure gauge.
 
-
Back
Top