Help Troubleshoot an Electric Oil Pressure Gauge

[Bob Chadwick]

I've been having problems with rapid oil pressure fluctuations. I put in a mechanical gauge and have confirmed that the problem is the gauge and not the motor. While the electrical gauge would flutter between 60 and 80 psi intermittently, the mechanical gauge would stay rock steady at 50 psi. So it would appear the motor is not the problem.

The sending unit is new, the gauge was out and checked, the gauge circuit board was replaced and the plug that connects to the sending unit is new. None of this changed anything. The oil pump was also replaced.

The way I understand the system to work is that power is supplied to the gauge from a switched fuse and that a wire then runs from the gauge to the sending unit where it grounds through the sending unit. I don't understand how the sending unit uses the ground to create the pressure reading.

Checking a gauge that is not working is supposed to be as simple as grounding the wire attached to it. If it pegs out, these means a bad sending unit. Because the pressure always seems to rapidly fluctuate up and back to the original reading it seems to me that either the wire is grounding or the sending unit has an issue with the resistor that is allowing full ground.

Any thoughts? Is it possible to check a sending unit. I read somewhere that it should have a range of resistance. Of course, when it is intermittent it is always harder to figure out.

 

[JohnZ]

The sending unit contains a variable resistor operated by a diaphragm which senses pressure and moves the wiper on the resistor. One end of the internal resistor gets 12 volts, and the wiper which moves along its windings is grounded to the sender body (which is grounded to the block); as pressure changes, the wiper connected to the diaphragm moves, and changes the resistance in the circuit between the gauge sender terminal and ground, which changes the gauge reading.

The only reference I have at the moment shows that GM electric oil pressure senders have one ohm resistance at zero pressure, and 80-88 ohms resistance at max pressure reading. Using the same test logic used with temp and fuel senders (which also use variable resistance to ground to operate the gauge), if you disconnect the wire at the sender (infinite resistance), the oil pressure gauge should peg at max, and if you ground the sender wire to the block (zero resistance), it should peg at zero. That will check the gauge and the circuit to the sender.

You can do a gross check on the sender by disconnecting the wire and connecting a digital VOM set on ohms between the terminal on the sender and ground; with the engine off, it should show one ohm; with the engine running, it should vary up to 88 ohms as rpm increases.

 

[Bob Chadwick]

Thanks John.

I've got one of those multi-meter's that was given to me. I guess I'll just need to go figure out how to use it. :L

 

[jdp6000]

I bought a new oil pressure sensor.They are cheap. Anyone know how you get a wrench on the one on the car...its real tight?

Jim

 

[Bob Chadwick]

Got one already. Mine is down under the car. Yours sounds like it is beside the distributor.

 

[jdp6000]

Correct, its right beside the distributor. any idea on how to remove it?

Jim

 

[vette-dude]

Correct, its right beside the distributor. any idea on how to remove it?

Jim

They make a special deep socket to remove/install these oil senders. Should be available at any discount Auto parts store. Personally, I have use a 7/8 deep socket or (in some cases where room will permit) a small off set plier to remove. Installation is what you need to worry about. You don't want to crack the plastic seal. The sockets are cheap and I finally bought one and now never have a need for it!! And no, I won't sell it as you never know when it will be needed (just like half the tools in my garage)!!

 

[jdp6000]

Your only 8 hours away from me. Can ya drive up an get it off for me? Eh? Just kidding. I'll see if the the local garage has one of these sockets. They must have built the engine around this sensor.

Jim

 

[Bob Chadwick]

When I put the mechanical gauge in temporarily I looked at the spot on the front of the engine and the one near the distributor. Guess where I tapped in.

 

[vette-dude]

When I put the mechanical gauge in temporarily I looked at the spot on the front of the engine and the one near the distributor. Guess where I tapped in.

Taking the easy way out, Huh!!! :W Did you get the gage problem fixed??

Randy

 

[Bob Chadwick]

Taking the easy way out, Huh!!! :W Did you get the gage problem fixed??

Randy

Certainly.

The car is in the shop now for trailing arms and bearings. I'll take a look at it when I get it back.

 

[lone73]

Certainly.

The car is in the shop now for trailing arms and bearings. I'll take a look at it when I get it back.

Bob,
So what was the fix? New sensor? I have the same prob on mine.
thanks
.
.
.

 

[gec]

Mines pegged at max...just one other electrical problem I have'nt got around to yet, does that mean the sender is bad or the ground?

 

[nyernga]

Thanks John.

I've got one of those multi-meter's that was given to me. I guess I'll just need to go figure out how to use it. :L

C'mon Bob....I used that thing for 2 years. Call me if you need help deciphering it...

Len

 

[Bob Chadwick]

Bob,
So what was the fix? New sensor? I have the same prob on mine.
thanks
.
.
.

At this point I'm not sure. I replaced or rebuilt everything and need to do some exploring. When I get to it, I'll make sure I post what I find out.

C'mon Bob....I used that thing for 2 years. Call me if you need help deciphering it...

Len

I'll take you up on that Len.

 

[Mr. Chuck]

Your problem looks very much like a case of marginal contact at one or more electrical connection points. I would check each wire crimp for signs of being loose or burnt (blacken or baked look) also check each connector to see how tight or loose it feel when you plug them together, the blade type of connector weaken with age and become lose. Intermment connections may get worst as the engine gets hoter. A common fix is to either replace the connector if it is really bad or just bend the two female contacts closer together a small amount or carefully bend the male / blade side slightly so it is no longer straight. This in affect improves the contact within the connector. Gage flutter can also be caused if the connector gets full of oil from oil leaks from valve cover, etc. If so a good gunking may help to resolve your gage reading problem.
Note: If you gunk / clean your engine be sure to cover alternator and any other open / vented electrical accessory & of course remember to remove covers before engine start-up.

 

[Bob Chadwick]

I would check each wire crimp for signs of being loose or burnt (blacken or baked look)

One of the things tht I did was replace the connector down at the sending unit and a length of wire. The insulation was burned on it. I'm going to take another look at this and probably repace an additional section. Thanks for the advice.

 

[Mr. Chuck]

As JohnZ indicated a pegged reading means a "Open Circuit" or a defective sending unit diapram / wiper (contained within the sealed unit); Either a stuck wiper at maximum resistance or a open circuit within the sender. To repeat the quick test suggession of temporary connecting the sending unit wire to ground (car frame / any bare metal within reach) should result in the gage reading "Zero". Electrical wiring in older cars are prime canidates for degraded or loose or otherwise bad connections at any point of electrical connection. My experience is that unless you take a hard, close look at each wiring connection point; you can go right passed the cause of your problem. A lession learned years ago when working on Navy aircraft electrical systems; go slow and start at the end of the circuit where environmental conditions are the worst (at the sender). Write down what you did and your findings. Don't assume anything is good until you check it, "new" doesn't always mean good. Most senders / sensors make ground through it's metal threaded body, so a simple thing as the presence of teflon tape on the threads to stop a possible leak, may be the source of high resistance in the ground side of the circuit. Using a ohm meter on the time 1 scale (check meter Zero by touching the two meter test leads together). With the engine off, and sender wiring disconnected, measure the resistance from the sender connection pin to ground (a bare metal connection on the block or other such point on the frame). With the ignition key on, meter should read Zero ohms, then repeat the ohm meter reading test with the engine running (idle). Remember, never connect ohm meter to any source of electrical power; a sure way to smoke a good meter. If you are a experienced in electrical trouble shooting, then there is no reason to go into the fore going trouble shooting procedure listed above. A good rule of thumb when checking circuit resistance is to always check for the presence of DC power with the meter set up to measure DC Voltage first. Then measure sender resistance with the engine running, ohm meter should read a resistance greater than Zero. Higher the engine speed, higher the resistance reading, until a maximum is reach, that represents a maximum oil pressure produced by the oil pump.

 

[Bob Chadwick]

Thanks for the detailed info. This is one of those things that's on my list when I get the car back from the paint shop.

 

[Ken Anderson]

Did you replace the gauge? Common to fail.