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Temperature Gauge Haywire
Posted: Sat Aug 22, 2026 4:37 pm
by 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 336 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
Re: Temperature Gauge Haywire
Posted: Sat Aug 22, 2026 5:02 pm
by 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
Re: Temperature Gauge Haywire
Posted: Sat Aug 22, 2026 5:09 pm
by 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.
Re: Temperature Gauge Haywire
Posted: Sat Aug 22, 2026 5:18 pm
by 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.
Re: Temperature Gauge Haywire
Posted: Sat Aug 22, 2026 5:25 pm
by KevinB
Thank you Stephen, Bill and geoberni. I shall work through what you have suggested.
Re: Temperature Gauge Haywire
Posted: Sat Aug 22, 2026 5:31 pm
by svenedin
geoberni wrote: ↑Sat Aug 22, 2026 5:18 pm
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
Re: Temperature Gauge Haywire
Posted: Sat Aug 22, 2026 6:35 pm
by geoberni
svenedin wrote: ↑Sat Aug 22, 2026 5:31 pm
geoberni wrote: ↑Sat Aug 22, 2026 5:18 pm
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
Re: Temperature Gauge Haywire
Posted: Sun Aug 23, 2026 5:38 pm
by Bill_qaz
I said what he said and you said what I said
3 versions of the same thing
one of them should fixit
Re: Temperature Gauge Haywire
Posted: Sun Aug 23, 2026 7:22 pm
by KevinB
Bill_qaz wrote: ↑Sun Aug 23, 2026 5:38 pm
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.