In an attempt to solve an oil pressure problem, I'd like to know how the oil pressure sending unit works. Can you help?
Tom Wood responds:
There are two types of oil pressure sending units. A few actually have a pressure tube that runs from the engine to the gauge, but these are very rare and I haven't seen one in years.
Most have an electrical connection and a wire running from the top of the sending unit to the gauge. The sending unit itself is a small rheostat actuated by oil pressure. As the oil pressure increases, it pushes on a bellows inside the sending unit, which in turn makes additional contact on the rheostat, allowing more electrical current to pass.
Now the gauge is actually a small, calibrated voltmeter. A small amount of 12-volt current is fed to the gauge and from there to the sending unit, so the rheostat could be said to be on the ground side of the circuit. When the engine is off, oil pressure falls to zero, the circuit is broken so that no current passes, and the gauge reads zero. As oil pressure builds, the rheostat allows more current to pass and the gauge has an increased reading.
Some problems you may encounter are:
- The sender is bad—if the bellows break inside, the electrical portion will be entirely coated with oil and fail. They are inexpensive and easy to replace.
- Wiring or connections are bad anywhere in the system (positive to the gauge or ground wire from gauge to sender).
- Sending unit not making good electrical connection to engine block (you obviously don't want to insulate the threads with Teflon tape in this connection because the threads are the electrical connection to ground—bad connection, bad reading).
- Ground wire from battery to engine block in poor condition—very common and mysterious cause of many engine electrical problems.
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