Oil & Fuel pressure reading low

[agjolly]

Both of my pressure gauges (oil and fuel) are reading low (about half). We (my A&P and I) put a mechanical gauge in the circuit and verified the electrical gauge readings are low. We've checked wiring connections, power and ground. I've checked the sending units and not sure the resistance is correct. The sending units doesn't appear to be the original ones. I'm concerned that they may have been replaced with the wrong resistance units.

The fuel pressure gauge is 0-30psi

The oil pressure gauge is 0-100psi

Can someone tell me:

1) What should be the resistance ranges of the sending units?

2) What should be the voltage on the wire with it disconnected from the sending unit? I'm reading about 2 vdc.

This is a new plane to me and I have the service manuals ordered but haven't received them yet.

Any help or ideas are welcomed!

[Piloto]

It appears that the problem is on the supply voltage to the gauges. There is a common feed wire that connects them. According to the schematic it goes directly to the gauge CB +14V. Cycle this CB to see if it makes any difference.

 

José 

[agjolly]

Piloto,

The supply to the gauges is roughly 12vdc. The 2vdc is on the signal wire going to the sending unit when the wire is disconnected. With the wire connected to the sending unit it is lower than 2vdc.

I had already cycled the CB and didn't see any improvement. The voltage stayed roughly the same after cycling.

Thanks for the reply.