Voltage spike in sensor, when engine running

[mikebikeboy]

Hi everyone,

I have a Volvo Penta MD17C. When the ignition is turned on the water temperature sensor and gauge function correctly. As soon as I start the engine and the alternator is charging the batteries however, the voltage readout from the water temperature sensor is about 3v too high (in the 5-6v range), causing the gauge to read 0deg.

After the engine warms up and I turn it off, but ignition still in 'on' position, the water temperature sensor's voltage returns to a normal reading of 2-3v and the gauge gives an accurate water temperature readout, something like 160deg.

My question is what could be causing the voltage readout from the sensor to rise, only when the engine is running? I've cleaned all the ground connections so I'm fairly sure it's not a grounding problem. The only thing I have not done yet is remove the sensor and clean that up. It's the old single-wire sensor type where the body of the sensor is the ground.

Any insights appreciated.

[smac999]

are you measuring the voltage at the back of the gauge? measure the voltage to a better ground point as well. main neg bus or battery post. and also measure at the sensor. to engine block while running and compare all 3 readings.

[mikebikeboy]

Thanks @smac999,

I took your advice, checked the voltage at both gauge and sensor, and also made sure my ground was good. Both readings were the same. Voltage is within normal range while engine is off. When engine is running, voltage climbs from 2-3v to 5-6v which makes gauge give a reading of 0. Once engine is warmed up, I turn it off and then voltage returns to normal range (2-3v) and the gauge gives what seems to be an accurate reading (165 degrees or so)

The only difference when the engine is running vs not running is the alternator is charging the batteries. But I don't see how this would affect the voltage reading from the sensor.

[jamhass]

Originally Posted by mikebikeboy

Thanks @smac999,

The only difference when the engine is running vs not running is the alternator is charging the batteries. But I don't see how this would affect the voltage reading from the sensor.

That makes a very large difference, depending partly on the state of charge of the batteries. Charging voltages can be 2 or 3 volts above the non-charging voltages. Well discharged batteries might read 11.5 v, while charging voltages can be over 14 v.