Earth issue thermometer gauge

Jonmlb748 · Nov 25, 2021

When I flick the light switch on the temperature gauge flies into the red zone and as soon as I turn it off it drops back to normal. I’m assuming it’s an earth issue. Any ideas where I start to look?

bluedog333 · Nov 25, 2021

Jonmlb748 said:
When I flick the light switch on the temperature gauge flies into the red zone and as soon as I turn it off it drops back to normal. I’m assuming it’s an earth issue. Any ideas where I start to look?

From memory, there is an common earth point inside the engine bay on the bulkhead, near to the brake servo. Give this a good clean, better still, pop another earth lead between it and the engine. This should eliminate the issue.
P.s The earth location may vary depending on the year of manufacture .

miktdish · Nov 26, 2021

The sensors resistance to earth drops as it gets hotter (its a thermistor). If it says you are hot when you apply power it kinda suggests that the resistance is very low/zero.
Either the wire is touching earth somewhere (the block, where it passes through the bulkhead etc) or the sensor has died.

To check, simply remove the wire from the sensor and the needle shouldn't move. if you touch the wire to ground (ie the engine etc) then the temp gauge should spring into life and show HOT
You haven't said what engine/car you have and there are a few versions of temp sensors, look up AMR1425 or AMR3321 oe ERR2081. thjey are all around the £5 delivered.
If the sensor is still saying HOT when its dissconnected, then you have a short to ground on the wire somewhere - or - the guauge is forked (rare).
If you disconnect the sensor wire from the guage you should be able to suss out the issue.

Corfewoodsman · Nov 26, 2021

Check the earth wire from the chassis to the engine.
I had exactly the same problem some years ago. This was the result of the wire making intermittent contact resulting not only in alarming temperature readings but also in perceptible dimming of the lights. The cable / connection had probably failed due to fatigue via relative movement of its ends.
A strip back to its core and refitting solved it immediately. Sadly, not before much anxiety and the unnecessary purchase of a thermostat!
Good luck with it anyway.

Jonmlb748 · Nov 26, 2021

miktdish said:
The sensors resistance to earth drops as it gets hotter (its a thermistor). If it says you are hot when you apply power it kinda suggests that the resistance is very low/zero.
Either the wire is touching earth somewhere (the block, where it passes through the bulkhead etc) or the sensor has died.

To check, simply remove the wire from the sensor and the needle shouldn't move. if you touch the wire to ground (ie the engine etc) then the temp gauge should spring into life and show HOT
You haven't said what engine/car you have and there are a few versions of temp sensors, look up AMR1425 or AMR3321 oe ERR2081. thjey are all around the £5 delivered.
If the sensor is still saying HOT when its dissconnected, then you have a short to ground on the wire somewhere - or - the guauge is forked (rare).
If you disconnect the sensor wire from the guage you should be able to suss out the issue.

Thanks guys, this is really helpful I’ll have a look in the morning. 300tdi by the way.

Jonmlb748 · Nov 27, 2021

Corfewoodsman said:
Check the earth wire from the chassis to the engine.
I had exactly the same problem some years ago. This was the result of the wire making intermittent contact resulting not only in alarming temperature readings but also in perceptible dimming of the lights. The cable / connection had probably failed due to fatigue via relative movement of its ends.
A strip back to its core and refitting solved it immediately. Sadly, not before much anxiety and the unnecessary purchase of a thermostat!
Good luck with it anyway.

Actually the dimming of headlights kind of makes sense ,also I don’t do much night time driving but I go through Several tunnels regularly and thought what’s going on with the headlights.

Felix_110 · Nov 28, 2021

I had a similar issue with my temp and headlights. Checked with Defender rebuilder here and he gave me the following advice: Add a ton of earths. So I added/replaced the following earths:
- Battery to bell housing
- Bell housing to chassis
- Battery to seat-box
- Engine to chassis (as close as possible to starter)
- Bulkhead to chassis

This fixed all my bad gauge readings, flickering lights and sometimes bad starting.

Earth issue thermometer gauge

Jonmlb748

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bluedog333

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miktdish

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Corfewoodsman

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Jonmlb748

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Jonmlb748

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Felix_110

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