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Oil gauge gets case of jitters

| Saturday, June 29, 2013, 12:01 a.m.

Q: My oil pressure gauge has begun to get the jitters. It seems to work normally for a while then jumps around like crazy when I speed up from a stop. The truck is a 1998 GMC half-ton. Could this be an engine problem or just the gauge?

— Alex Fuller

A: I'd start with a check of oil level and condition. Low oil level could cause the gauge to intermittently drop out, perhaps on turns or acceleration. Also, it doesn't hurt to check for dilution or contamination. If color and thickness seem OK, we'll move on to the gauge.

Vehicles that have an oil pressure gauge employ a sending unit, threaded into a port on the engine. Yours is thumb-size and black. It's located toward the top and rear of the engine, near the base of the distributor, and has a single tan wire attached to it. The sending unit contains an electromechanical mechanism that varies in electrical resistance in response to oil pressure. Sending units can be flaky and sometimes leak oil.

When an oil, temperature or oil pressure gauge fails to work at all, a neat trick is to disconnect the sending unit wire and look for the gauge response — many will read zero. Next, grounding the appropriate wire at the sending unit typically causes the gauge to read high/full, or hot — probably a bit past the upper limit, though some gauges respond in the opposite direction. If the gauge shows these two characteristics, the fault lies within the sending unit.

If your oil pressure gauge is the only one of its siblings to act erratically, my money is on the sending unit or perhaps its electrical connection. Verifying the cause of a jittery electrical signal is a bit tricky to explain and do, making replacement of the sender a very high-odds $50 gamble.

Electric door jam

Q: All of the functions in the driver's side door of my 2004 GMC pickup have stopped working, but sometimes will come back, but only with the door open. Any ideas before I take it in for repair?

— Tom Lindquist

A: Tom, I have a hunch your truck may be suffering from a broken wire in the bundle that enters the door from the A-pillar. With the key in the run position (engine off) and door open, try depressing and holding a window or door lock switch as you gently wiggle/massage the black rubber boot, which is about 1.5 inches in diameter and located just below the upper door hinge. If normal operation occurs, even briefly, this tells us there's trouble inside the boot.

Disconnecting/pulling the boot back from the pillar will allow a peek at the wire bundle within. Look in particular for a broken orange or black wire. If a wire repair is needed, splice in a 6-inch length of wire, looped, to lessen flexing and strain on the repaired connections. Solder and shrink-wrap — no crimp connectors here!

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.

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