89 w 250 cummins has low oil pressure. new sending unit no help. what should i check next? oil light comes on at low idle goes out if you speed engine up a little.
thanks chan.

 

Hook up a mechanical gauge to the engine and verify the reading. More than likely it is time for some engine work. The low oil light is a pressure switch, so it can't give a false reading.

 

My gauge read low too.... but with a mechanical gauge I got 50psi.

 

The sending units have a habit of "sooting up" from dirty oil. The nature of a diesel engine. The soot causes the transmitter to not read correctly giving you a low reading and effects the warning light. These old trucks are also subject to electrical resistance problems that can also effect the reading. If you haven't replaced your sender, try that first.

 

replaced oil pressure switch low oil pressure light comes on at low idle with engine hot. it may be the oil pressure regulator stuck open. i will check that next. thanks for the info. i will post how that works out.
thanks

 

I screwed a 2" liquid-filled mech. gauge on a 1/8" NPT nipple into the top of the top of the filter adapter. I check the real oil pressure whenever I want. (er, at least when the truck is stationary ).

 

I did the same pressure was 60 psi cool and 45 psi hot the guage still wrong with new sending unit. oil pressure regulator was good.
thanks
chan

 

thanks for the info,new guy here . Doing body and paint on 1993 really like the old truck it was in fairly good shape when i go it . first guy ordered it from factory.

 

Like cougar said, it could be a resistance problem. Does the low oil pressure light come on at an idle when cold and hot? If so it is an electrical issue, beyond the pressure sensor. According to your mech gauges the actual oil pressure is correct. The high resistance in the connectors and wires is giving the PCM a false reading.

 

Ironically, my truck started this, but it doesn't have the low oil pressure light/buzzer. Can you agree that it's the gauge or wires again? It will drop to the rest, and come back, and drop randomly no rpm shake or movement related. Where would you hook up a mechanical oil press gauge and what is the tech manual specification for low idle and high idle oil pressure?