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Sensor Voltage Shenanigans

1996 Geo Metro 1.0L.

Been on this one for months- starts fine but can’t give any throttle before it warms up, or it stalls out.

Checked the voltage at the Coolant Temperature Sensor by backprobing.

It has three wires:

Light Green/Black, which is common and goes to the MAP, TPS, and IAT. Gray/White, which is the signal to the ECM.

Yellow/White, which is not shown on wiring diagram, but I assume is the input from the ECM.

LtGr/Bk is ground- no voltage.

Gy/Wt (signal to ECM) reads 2.65V when stone cold, decreases with rising engine temp and finally goes to 0.5V when hot, at which point the cooling fan kicks on and takes it back up to around 0.65V.

At anything over 1.0V, the engine will stall if the throttle is opened even slightly.

The book says the resistance should DECREASE with rising temperature, which would cause the signal voltage to INCREASE. The voltage steadily DECREASES.

What does not seem reasonable is the voltage on the Yellow/White wire which I believe should be the standard 5V input from the ECM, but reads 9.2V.



I hope this is the gremlin I have been looking for since April, but I don’t have a clue how or where it could get 9.2V.

Maybe something a little over 12v, yes- but not 9.

And how can my sensor work BACKWARDS?

“The book says the resistance should DECREASE with rising temperature, which would cause the signal voltage to INCREASE.”

The first part of this statement is true, most temp sensors [thermistors] have a negative temperature coefficient of resistance, a fancy of saying ‘DECREASE with rising temperature’.
However, the second part of the statement doesn’t necessarily follow. Depends on the circuit layout.
If the thermistor is connected between output [signal line to ECM] and ground then the output voltage will decrease as temp increases.
If the thermistor is connected between output and the supply line [Yellow/White] then the output voltage will increase as temp decreases.

All the ones I’ve seen are wired the first way.

Are you sure the supply is supposed to be 5V? Typically the 12-14V from the battery is reduced with a linear voltage regulator. They can short out and put out a higher voltage, but regulation is lost.

Check the Yellow/White line with the engine stopped, ignition on vs running. That will vary the battery voltage quite a bit. If it holds to 9.2V in both cases it means it’s being regulated. 9V is a frequently used supply voltage.