Oil pressure gauge.

[Doughnuts]

Happy new year every body.

I have been having an issue with my oil pressure gauge in my 91 c1500. The gauge typically sits at 60. Sometimes though it will sit at its correct location but that is only for brief moments. Now I am know that its not actually running with 60+ pounds of oil pressure at idle, so it must be that gauges fault or the sender units fault. So I have been doing some research on the sender unit and it seems there is two of them, one behind the distributor and one above the oil filter. Now from what I've read the one behind the distributor works with the ECU and has noting to do with the gauge, and the one above the oil filter is used only for the gauge. So I climb down under my truck and behold there is none. Now my question is this, How do I get my Stock oil pressure gauge working again? And how do I find out what it is plugged in to.

Thanks.

[speedygonzales]

It's behind the distributor. Test the gauge before you replace anything.

[Doughnuts]

How would I go about checking the gauge? Its the Moonie style instrument panel.

[Mr_Rich]

It's a little hard to see from this pic. Pull the lead off the sender and ground it through a 90 ohm test lead. That should peg the gauge if it's responsive.

[speedygonzales]

Find out the operating range of the sending unit then duplicate the condition it would be in artificially, if the pressure is where you think it is.

For example, let's say for this example, the sending unit ranges in resistance from 10Ω to 100Ω and at 0PSI it's 10Ω and at 100PSI the resistance is 100Ω. Then extrapolate the resistance at the pressure you're looking for.

If we pick 30PSI and want to know the resistance (I can't stress enough for you to know the actual method and range of the sending unit) we calculate 27Ω. So you get ahold of some resistors at radio shack to add up to 27Ω and connect it in the cable in place of the sending unit. Then observe the gauge reading. If all else is good, but the gauge reads incorrectly, you know the problem is the gauge and not the sending unit.

Some folks are lazy and don't want to go through all that work so they get the sending unit and if that fixes it, they stop there. If it doesn't fix the problem, they get the gauge. But here's where they are really not doing themselves any good.

Let's suppose the numbers I used are correct above. That is 27Ω at 30PSI of oil pressure. You didn't test the gauge, you bought a sending unit and it didn't fix the problem, so you just went and bought a gauge. Now you put the new gauge in and your reading at idle is say 18PSI. Do you have any confidence in that reading?

Now let's suppose that due to manufacturing variances, tolerance or what ever, the gauge at 27Ω actually shows 20PSI and not 30PSI. You would have known that up front, you would be more knowledgeable about the operation of the system (gauges, oil pressure and sending unit) and you would not have wasted money buying unnecessary parts.

[Doughnuts]

Alright so putting it in most basic terms take the oil pressure output to the gauge and run a 12 volt to it through a series of resistors to replicate what the oil sensor would do? So this would check the gauge in the cluster how would I go about checking the sensor its self? Thanks for the input.

[ray_mcavoy]

Originally Posted by Doughnuts

Alright so putting it in most basic terms take the oil pressure output to the gauge and run a 12 volt to it through a series of resistors to replicate what the oil sensor would do? So this would check the gauge in the cluster how would I go about checking the sensor its self? Thanks for the input.

Close. You do use the test resistor(s) to replicate what the oil pressure sender does. But you don't connect them to 12V. Connect them to ground instead.

In other words, temporarily unplug the wire(s) from the sending unit. Attach one end of your test resistor to the signal wire that you unplugged from the sending unit. If the sending unit has more than 1 wire (and I think it does for '91) you'll have to identify the signal wire (usually tan and might have a white stripe). Connect the other end of the test resistor to a good ground. Then turn the key on and observe the gauge movement.

To expand on what speedygonzales posted earlier, the GM oil pressure sending units typically range from 0Ω at 0PSI up to about 90Ω at the max gauge pressure (often 60PSI). So directly grounding the sending unit wire (0Ω) should make the gauge go to zero. Hooking in a temporary 45Ω test resistor should cause the gauge to read close to 30PSI. And hooking in a temporary 90Ω test resistor should make it read 60PSI.

To check out the sending unit you'll need a multimeter set to the resistance / Ohms scale. You'll also need some way to apply a known amount of pressure to the sending unit. That could be done by leaving it attached to the engine and temporarily plumbing in a second test gauge. Or by removing the sending unit and connecting it to a compressed air source with a gauge & variable pressure regulator. With known pressure being applied, hook one of the meter's leads to ground (sending unit threads or ground terminal if it has one) and hook the other lead to the sending unit's signal terminal. The reading on the meter should correspond to the pressure applied. Example, 30PSI should give a reading near 45Ω.