Hi guys,

Among the issues in my new-to-me '88GT was an oil pressure meter which pegged as soon as the ignition switch went to the on position. It fell to 0 when the switch was turned off.

Since the meter had movement, it was very unlikely to be the meter....naturally the P.O. had supplied me with a replacement meter <sigh>.

It turns out that an open circuit leading to the meter (actually 90 ohms or more) results in the meter pegging.

So either there was an open in the wiring leading to the meter from the sender or the sender had failed open.

Since this is a high mileage car and the wiring generally seems to be unmolested, I went ahead and bought a new sender.

With a new sender in hand (I did not want to remove the old one until sure since it leaked no oil at all), unplugged the sender, turned the ignition to ON, and plugged the sender into the new unit.....and of course the meter stayed pegged. But it dawned on me it might need a ground through the threads on the sender, so I touched the threads of the new sender to the water fill cap, and the meter instantly dropped to zero. Case closed, the sender failed open. Remove and relplace sender.

If the meter had not dropped to zero, it would have been necessary to find the open or high resistance plug in the wiring harness....the data is all in the FSM, but I have not dug it out since I found the problem on first step.

If anyone needs the help I will be happy to walk them through the wiring isolation problem if neither the sender nor the meter are the problem.

Larry

Good logical troubleshooting.

Another useful approach would have been to disconnect the connector from the old oil pressure sender and connect a resistor between the appropriate connector terminal and ground. A 92 ohm resistor (92 ohms is a standard off-the-shelf value) should result in a full scale gauge reading, while zero ohms (a direct short) should produce a gauge reading of zero, and a resistor value in between (e.g. 47 ohms) should produce a mid range reading. If they do, then the problem is the sender. If not, then the problem is in the wiring or in the gauge itself. Divide and conquer.

[This message has been edited by Marvin McInnis (edited 05-02-2010).]

Absolutely right, Marvin. But my wiring is old and I would not fool with it anymore than required....and my fingers are a bit large to fit into the connector.....but even I can plug and unplug a connector, so I took the easy way out.