Oil pressure light won't go off?

[Tommymc]

My daughter is away at school with our 96 Legacy wagon. The 2.2L engine has aprox 150k on it. She called and reported that the oil light came on....the oil was a little low but not enough to trip the light under normal circumstances. She has topped off the oil and let the car idle for a minute or two, the light is still on.

 

The only thing I can think of is that the oil pump must be shot.....or the oil pressure sensor. Any thoughts on this? I've told her not to drive it until we get the problem resolved. I'm thinking that we need to tow it to a garage (she has AAA) Thanks for any insight.

[Fairtax4me]

Depending on if the engine is making noise (lifters ticking) I'd check the wiring for the sender first.

 

It is common for the back plates on the oil pumps to come loose and bleed the pressure off. But the only way to find out is to take it off.

[OB99W]

[...] I've told her not to drive it until we get the problem resolved. [...]

Not driving until you determine what's happening is a prudent move. The low-pressure switch changes state at only 2 PSI (normal oil pressure is >14 PSI at idle and >43 PSI at 5,000 RPM), so something is either significantly wrong or there is a wiring/sender problem. Oil pressure below 2 PSI could cause engine damage fairly rapidly, but it also would typically announce itself with some engine noise, as previously mentioned.

 

If the light remains on when the wire at the sender is disconnected, it would indicate a short of that wire to ground. If the warning light goes off when the sender is disconnected, then either the sender is bad or the pressure is actually very low. An oil pressure gauge can be attached in order to verify actual pressure.

[frag]

 

If the light remains on when the wire at the sender is disconnected, it would indicate a short of that wire to ground. If the warning light goes off when the sender is disconnected, then either the sender is bad or the pressure is actually very low. An oil pressure gauge can be attached in order to verify actual pressure.

 

Are you sure of that OB99? When I replaced the intake manifold (and all that comes with it) after having replaced an exhaust valve, I misconnected the oil pressure sender wire and the light was coming on when the wire was off the connection.

96 Legacy also.

[OB99W]

Are you sure of that OB99? When I replaced the intake manifold (and all that comes with it) after having replaced an exhaust valve, I misconnected the oil pressure sender wire and the light was coming on when the wire was off the connection.

96 Legacy also.

The sender/light circuit is very basic. One side of the light connects to +12v, the other goes to the sender. When there is no pressure, the sender completes the connection to ground, causing the light to come on. (That's exactly what happens when you turn the ignition switch to the "ON" position before cranking, allowing the light to be checked.) Oil pressure above 2 PSI opens the sender contact, breaking the ground connection and extinguishing the light.

 

If the sender wire is truly disconnected, it can't be grounded via the sender contacts, and that should cause the light to remain off. However, if the wire is "misconnected" and the connector touches something that's grounded (or gets pinched between some grounded parts), that would cause the light to be lit any time the ignition switch is on. Perhaps that's what you experienced.

[frag]

Makes sense.

Thanks.

[nipper]

Start with the sender and work your way up to the complicated (expensive) items. in the mean time dont use the car. if the sender doesnt do it buy a cheap mechanical oil pressure gauge and see what it says.

[Tommymc]

Thanks for the info. Since the car is in Rochester NY and I'm in VT, I have no way to check the sender myself. Unless anybody can recommend a good mechanic out there, I'm going to check the Car Talk Mechanx files and find somebody nearby. She has AAA so towing should be free.