oil pressure guage - test with a DC power source

[CyberMayhem]

Greetings, I've got an M929A2 with what I believe is faulty oil pressure guage. I noticed that the dial was pinned to the right and it stayed there even after turning power off. I tested pressure at the engine block with an external guage and the pressure appears to be normal. I bought a new pressure guage and am trying to simulate the resistence with a DC bench power supply and a resistor. DC power set to 24 volts and connected to the guage pin as documented in the vehicle manual. The DC power unit negative is connected to the guage pin with a 37 Ohm resistor which should move the needle but nothing happens. Per the manual, removing the resistor and connecting DC ground straight to the negative input should pin the guage to the right. Nothing happens in any of these tests and I'm finding it hard to believe that a brand new guage is defective. Am I going about this test properly?

 

[WillWagner]

Never done it like that. To test an electric gauge, I have always removed the wire at the sender and just grounded it. Pegs one way grounded and the other way with no wire shorted, the issue is the sender. I would disconnect the gauge in the dash now and see if it goes back down to zero. If it does, there is a wiring issue, a short to ground in the harness. If it stays pegged at max pressure, the gauge is failed. Install your new one and see how it goes. It will most likely take less time to do it that way than it has to mock up a circuit and bench test it!

 

[CyberMayhem]

Thanks for taking the time to reply. I'm stumped. I did what you called out as that was the exact procedue detailed in their maintenance manual. As an electrical engineer, that makes total sense but the guage remained pinned to the right. Swapped with a new guage; no reading at all. I definitely have 24 V and tested by running a hard ground. I bench tested the second unit with various resistors to simulate the 30-100 Ohm load delivered by the sending unit but it still looks bad. I've order a 3rd unit concluding that. was unlucky with the replacement. I'll confirm when it's resolved.