Temperature Gauge Haywire

[KevinB]

I fitted a water temp gauge a while ago, bought on eBay and it's been working fine. Suddenly today I started the engine and the gauge immediately went to max against the stop, even though the engine was cold?

Temperature Sensor

PXL_20260822_152627294.PORTRAIT~2.jpg (1.23 MiB) Viewed 332 times

Something has failed presumably, but I now realise I don't know how the sender works? Probably an easy question for someone does.

Thanks in advance

[svenedin]

Hello Kevin. Your sender will use a thermistor which is a special type of resistor that varies in resistance very precisely according to its temperature. In fact it is probably a negative coefficient thermistor that has a high resistance when cold but the resistance drops as it gets hotter. Suggest you check all connections are tight and clean at both sender and gauge end.

You could remove the sender wire from the sender and test the resistance between the connection terminal on the sender and earth. This should change between engine cold and engine hot. If it doesn't the sender has failed internally.

Stephen

[Bill_qaz]

The sender provides a resistive earth from the gauge via the engine block. Hotter the coolant usually lower the resistance so gauge moves further. With ign on and gauge gone to max when cold, your fault, pull the wire off the sensor, if the gauge now falls back to cold the sender has failed. If no change then the gauge has likely failed or the sender wire is going to earth somewhere.

[geoberni]

Sender is a temperature sensitive resistance, the hotter it gets the lower the resistance and therefore the increased current flow moves the needle.
A short to earth will send the gauge full scale.
Pull the cable off the sensor to see if the gauge fails to move.
If it still goes full scale then you have a short elsewhere on the wiring.


I've just seen that Stephen has replied but I think he has it back to front.
The temp gauge needs a low resistance at the sender to read higher. If it didn't, then simply disconnecting it would send it full scale.

It's like with the fuel gauges, the early bouncy type operate differently to the dampened ones and their fuel tank senders work in the opposite sense.

[KevinB]

Thank you Stephen, Bill and geoberni. I shall work through what you have suggested.

[svenedin]

Sender is a temperature sensitive resistance, the hotter it gets the lower the resistance and therefore the increased current flow moves the needle.
A short to earth will send the gauge full scale.
Pull the cable off the sensor to see if the gauge fails to move.
If it still goes full scale then you have a short elsewhere on the wiring.


I've just seen that Stephen has replied but I think he has it back to front.
The temp gauge needs a low resistance at the sender to read higher. If it didn't, then simply disconnecting it would send it full scale.

It's like with the fuel gauges, the early bouncy type operate differently to the dampened ones and their fuel tank senders work in the opposite sense.

I did have it back to front back but I corrected myself before I saw what you wrote!

Stephen

[geoberni]

Sender is a temperature sensitive resistance, the hotter it gets the lower the resistance and therefore the increased current flow moves the needle.
A short to earth will send the gauge full scale.
Pull the cable off the sensor to see if the gauge fails to move.
If it still goes full scale then you have a short elsewhere on the wiring.


I've just seen that Stephen has replied but I think he has it back to front.
The temp gauge needs a low resistance at the sender to read higher. If it didn't, then simply disconnecting it would send it full scale.

It's like with the fuel gauges, the early bouncy type operate differently to the dampened ones and their fuel tank senders work in the opposite sense.

I did have it back to front back but I corrected myself before I saw what you wrote!

Stephen

[Bill_qaz]

I said what he said and you said what I said
3 versions of the same thing one of them should fixit

[KevinB]

I said what he said and you said what I said
3 versions of the same thing one of them should fixit

Thanks Bill, I'm away for a couple of days, but will get on it shortly.