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How Does A Temperature Sender Unit Work?

Abo Bob

By Abo Bob,
in Engines & Forced Induction

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Abo Bob Grand Master

Abo Bob

Posted
Posted

Just trying to work out what's wrong with the temp gauge in the R31 and trying to eliminate the sender unit from the equation.

I've always been pretty hopeless with the old multimeter but I can work out how to make it show 12 volts between + and - terminals of the battery so that setting should do me.

With the engine off and the sender disconnected I touched the red wire to the + of the battery and the black one to the sender unit where the wire connecting it goes normally. The voltage was quite erratic between about 0.3 volts and 1.3 volts.

Next I started it up and plugged the sender in, red to + and black shoved in the connector to the sender. I got a fairly steady 5 volts. Engine was warm by the way.

So what is it supposed to read? How do I check it? Like the first way or the second way or another way?

I think the gauge is stuffed but I want to eliminate the sender before I pull the dash apart.

Cheers.

MattSR Community Regular

MattSR

Posted
Posted

Its a temperature depedant resistor. its resistance in ohms varies depending on what temperature it sees.

The correct way to test one is to remove the connector to it and test the resistance of the sensor. It should read a nominal value (ie for your particular car it might be 100ohms @ 25 degrees) that you will need to look up in a service manual. You need to use the ohms (resistance) setting of your multimeter.

If the sender is screwed it will usually be open circuit (no reading at all) or short circuit (very low resistance, pnly a few ohms)

You will always see 5 volts when its running as thats the voltage coming from the ECU to the sensor.

Measuring the voltage between the battery and the back of the sensor is a meaningless reading BTW.

Cheers,

Matt

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