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

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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.

[KevinB]

So I started the engine from what passes for 'cold' at the moment and the gauge immediately went full scale. Then pulled off the sender wire and the gauge went to cold. So it's a failed sender? Better order a new one. And hopefully of better quality ...

I will also measure the sender resistance as Stephen said, just to make sure I understand how it works.

Thank you all for explaining.

[svenedin]

So I started the engine from what passes for 'cold' at the moment and the gauge immediately went full scale. Then pulled off the sender wire and the gauge went to cold. So it's a failed sender? Better order a new one. And hopefully of better quality ...

I will also measure the sender resistance as Stephen said, just to make sure I understand how it works.

Thank you all for explaining.

It’s a failed sender or the wire you removed from the gauge is shorting to earth somewhere between the gauge and the sender.

Stephen

[Bill_qaz]

So I started the engine from what passes for 'cold' at the moment and the gauge immediately went full scale. Then pulled off the sender wire and the gauge went to cold. So it's a failed sender? Better order a new one. And hopefully of better quality ...

I will also measure the sender resistance as Stephen said, just to make sure I understand how it works.

Thank you all for explaining.

It’s a failed sender or the wire you removed from the gauge is shorting to earth somewhere between the gauge and the sender.

Stephen

Short to earth would make gauge go to hot not cold.
When hot the sender has lower resistance to ground so when disconnected it went to cold.

[KevinB]

I measured the resistance using a multimeter between the sender terminal and the battery earth post. It flickered a bit, but was basically zero. So new sender it is.

Can anyone suggest a supplier of a better quality sender maybe? Otherwise it's eBay.

[svenedin]

So I started the engine from what passes for 'cold' at the moment and the gauge immediately went full scale. Then pulled off the sender wire and the gauge went to cold. So it's a failed sender? Better order a new one. And hopefully of better quality ...

I will also measure the sender resistance as Stephen said, just to make sure I understand how it works.

Thank you all for explaining.

It’s a failed sender or the wire you removed from the gauge is shorting to earth somewhere between the gauge and the sender.

Stephen

Short to earth would make gauge go to hot not cold.
When hot the sender has lower resistance to ground so when disconnected it went to cold.

Yes that is what it did!

Stephen

[Bill_qaz]

It’s a failed sender or the wire you removed from the gauge is shorting to earth somewhere between the gauge and the sender.

Stephen

Short to earth would make gauge go to hot not cold.
When hot the sender has lower resistance to ground so when disconnected it went to cold.

Yes that is what it did!

Stephen

But Stephen when the op pulled the wire off the sensor the gauge went to cold so the wire can not be short to ground or the gauge would of remained reading hot. So its the sensor not the wire.
To ground:
OC = Gauge cold
SC = Gauge hot
Variable sensor resistance by temp = gauge reading

[KevinB]

Replaced the sender with a new one and the temperature gauge is now working again. No changes to the wiring needed.

So that old sender I bought new just a few months ago, failed quickly and after only modest use. Poor.
The new sender looks identical, so I'll see how long it lasts.
.

Temperature sender/sensor

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[Bill_qaz]

Glad you sorted it and learned from the diagnosis. Unfortunately the market seems flooded with poor quality replica parts and nos are getting harder to find.
I fitted a capillary type temp gauge so no electrical issues. Its lasted almost 4 years so far but even that I believe to be a copy not genuine Smiths despite the name on the guage

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[KevinB]

Thanks Bill.
Kevin