How to test oil-pressure gauge without motor

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This article was published 01/08/2014 (4081 days ago), so information in it may no longer be current.

Q. I am restoring a 1971 Plymouth Duster with a 340 wedge motor. I can’t get the oil-pressure gauge to work in the rally dash. I was told I need to install a Mopar sending unit for the gauge instead of the aftermarket one I have, and I have done that. The engine is not in the car right now, so is there any way of testing the gauge to see if it is working before I put the motor back in?

Also, what does a wedge motor mean to you?

A. Understanding how the gauge works makes it easier to diagnose the problem. The gas, oil and temperature gauges in this vintage of Plymouth products have a bi-metal bar inside the gauge with a heating element coil wrapped around the bi-metal bar. As current flows through the heating coil, the bi-metal bar bends and causes the gauge needle to move. The sending unit on the motor has an internal variable resistor and that resistance needs to match the gauge so it will respond correctly. Replacing the sending unit with a factory-type sender is a good start.

Also, the gauges are all fed power by a voltage regulator that is built into the instrument panel. If none of the gas, oil or temperature gauges work, the voltage regulator is the likely fault, and it can be replaced separately. In your vehicle, it appears only the oil gauge isn’t working, and it is possible to test it without putting the motor in the car.

First, check for voltage at the wire that connects to the engine sender unit, with the key on, battery connected and the sender not connected. You should have voltage at the wire if the circuit is complete through the oil gauge. Then you can momentarily touch the sender unit wire to a good ground and watch the oil-gauge needle. If the heating element coil is working, it should heat up, and the gauge needle will move.

Only leave the wire connected to ground for a couple seconds at a time. If you leave it connected longer, it can cause the heating element to get too hot and burn out.

If the gauge needle doesn’t move, test for voltage at the + terminal of the gauge in the dash. There should be voltage there. If not, the dash printed circuit board is faulty.

If there is voltage, the gauge is bad, but before replacing the gauge, check the circuit from the gauge to the oil sender unit for a complete connection. These older cars often get corrosion in the firewall wiring connectors that prevent a good connection.

Finally, to me a wedge motor referred to engines such as GM’s 409 and 348 and Chrysler’s 413 engines.

These engines have the top of the block machined at angle so the combustion chamber is formed in the wedge shape of the block when the piston is at top dead centre.

Your 340 engine doesn’t have an angle-machined block, and the combustion chamber is in the cylinder head, but the pistons do cause the gases to be compressed into a wedge shape in one part of the cylinder head. It would seem to me the wedge name was probably marketing to differentiate it from the Hemi engine with its hemispherical combustion chamber.

kerr.jim@sasktel.net