Need Oil Pressure Gauge Education

[Tim Denning]

My 1986 D150 has an oil pressure gauge that has been bypassed and replaced by an oil fed mechanical unit. I would like to remove the aftermarket item and return to the original gauge in the dash. I've bench checked the gauge and it is working so I'm assuming that either the wiring or the sending unit failed.

I know where the sending unit is supposed to be, however, there appears to be an oil pressure switch in addition to the aftermarket oil feed line that was installed (again, I assume in place of the original sending unit). I'm thinking the original oil pressure sending unit went there, however, I can't find the connection wire to it (I believe it's supposed to be white).

Question: Was it normal to have both a switch and a sending unit together in the the truck or am I missing something? The wiring under the hood has been pretty heavily hacked and I'm slowly trying to return it to it's original functionality.

 

[Tim Denning]

Just wanted to give an update.

I replaced the oil pressure gauge with a new sending unit. The dash gauge worked but seemed to expose another problem.

I discovered that all of the 5 volt sensors were reading extremely low (This is probably why the gauge was changed to aftermarket in the first place). In addition to the low oil pressure, the water temp gauge had almost no reading at all even when the truck was warm and the fuel gauge was reading close to empty after 10 gallons was added.

I replaced the voltage divider behind the dash with an adjustable switching buck converter and set the output to 6.5 volts. All the gauges seem to be reading relatively accurately now (based on the original aftermarket oil gauge that was reading 60psi cold and about 30 psi hot). I won't know for sure how accurate the fuel gauge is until I do a fill up (and run out of gas). I could also take the fuel sender wire off but I don't know how much trouble that would be to get to under the bed.

Based on the condition of the wiring, my guess is that due to oxidation, there is increased resistance in the sensor wires causing the readings to be lower than normal. A true fix would be to replace the wires and I may end up getting a new harness, (I didn't see an exact replacement for a 1986 D150 so it will probably have to be generic), but for now, all is well.

 

[crazzywolfie]

about the only gauge that has worked accureately over the years for me is the gas gauge. the rest are junk but oil pressure was 2nd followed by temp gauge which i rarely saw move and very little when it did move.