Faulty Temp sender unit?

Joined: 30 Oct 2002 Location: Brisbane

Hi Guys

I have a '85 FJ73 landcruiser

Shortly after starting the car, and driving all of 10 mins the temp gauge hit the top and stayed there.

I stopped and checked the radiator and it was warm but not boiling hot.

Would this be normal behavior for a faulty temp sender unit?

Cheers
Matt

Joined: 11 Jan 2004 Location: Adelaide

Completely wrong characteristics (mismatched to the gauge) could cause something like that. I suppose a dud could do similar. It used to work OK?

Joined: 15 Jan 2008 Location: Cooma NSW

The temperature gauge is actually a milliampmeter. As the temperature in the sender unit increases, the resistance drops and you get more current through the meter - hence the gauge reads higher. The gauges are normally powered by a separate power supply - typically 5 Volts. A high reading means that the resistance is low - either by a stuffed sensor, or by a short in the wiring to the sensor. Or you could have a stuffed power supply (voltage too high).

The first thing I would check would be the resistance of the sender. Your manual will probably have the resistance in the specs. It varies with temperature, but if you check if when the engine is cold, it should probably read about 200 ohms, dropping to about 10 ohms when it is hot (85 Deg C). The actual values will vary with the type of car etc, but this gives some idea.

The take the lead off from the sender and see if the gauge moves. If it still moves, you have a short somewhere. Whilst is is off the sender, check the voltage - it is normally not 12 V but much less eg 5 V.

Joined: 30 Oct 2002 Location: Brisbane

-Scott- wrote:

Completely wrong characteristics (mismatched to the gauge) could cause something like that. I suppose a dud could do similar. It used to work OK?

it worked perfectly yesterday and for the last 5 years I replaced it once when I replaced the thermostat housing (as the old one was stuck in)

Joined: 30 Oct 2002 Location: Brisbane

I took a stab in the dark and replaced the sensor.

It seems to work fine, engine was warmish, swapped it over, and it reads normal temp.

I think the old one just died a nasty death rather quickly.

Thanks for your help

Matt