Quote:
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.
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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.