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At the engine instrument panel, there are usually alarms for low oil pressure and high coolant temperature, often with monitoring gauges. Although there are new technologies, most gauge transmitters (typically called senders) work on an electrical principle of varying resistance. The sender responds to a modification in temperature, pressure or fluid level by changing its resistance to an electrical current sent from the gauge.
This system relies on the principles defined by Ohm’s law, which states that as resistance decreases, electrical current must increase. The gauge is supplied with power from a switched source, typically from the ignition. The current flows through the gauge’s internal circuitry, out at the sender terminal and then through a conductor to the sender, which controls how much current can flow to the negative side, completing the circuit. Inside the gauge, as more current flows, the needle responds by moving toward its maximum position.
The oil-pressure sender consists of an internal diaphragm that moves a contact up and down a wound resistor. The fuel-level indicator uses a float to vary its resistance. In the temperature sender, the engine coolant affects resistance. This makes for a simple diagnostic test if a gauge appears to have failed. Remove the sender wire at the offending transmitter. With power to the gauge, the instrument should be at zero because there is infinite resistance. Briefly short the sender wire to a known, good ground. That action should prompt immediate gauge response to maximum.
If that happens, the circuit is intact, the gauge has full mechanical movement, and the sender likely is at fault and requires further testing. Ensure that any replacement matches the original to maintain accurate readings.
This article originally appeared in the September 2017 issue.