Erratic 300tdi temperature gauge

My temperature gauge packed up, before it had been perfect , needle exactly half way, I replaced the sender unit on the head (black one brought on ebay) and it worked fine , today however it stayed at about 2/3 and kept going up to nearly in the red and back down again within seconds , then I realised it went up more when I Switched the lights on , am I right in thinking it's a bad earth or just another eccentric thing LR's do !.
Your thoughts would be appreciated .

Sounds like the sender is goosed. Is it not earthed by screwing in to the head?

I would say almost certainly a bad earth... check the earth on the bulkhead.

+1 for bad earth. Had identical myself

Farmie said:

My temperature gauge packed up, before it had been perfect , needle exactly half way, I replaced the sender unit on the head (black one brought on ebay) and it worked fine , today however it stayed at about 2/3 and kept going up to nearly in the red and back down again within seconds , then I realised it went up more when I Switched the lights on , am I right in thinking it's a bad earth or just another eccentric thing LR's do !.
Your thoughts would be appreciated .

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I would say that your thinking that it is an earthing problem is correct, bad earthing equals resistance, resistance is what sends the gauge in the direction of high. Check earths from engine to chassis, body, and the instruments, problem is there somewhere, I'd bet.

Thanks, more work then.

Of course. An earth elsewhere (that's the closest smiley I could get to showing I've been stupid) I miss that massive range of smilies on the old site.

Gazbo said:

I would say that your thinking that it is an earthing problem is correct, bad earthing equals resistance, resistance is what sends the gauge in the direction of high. Check earths from engine to chassis, body, and the instruments, problem is there somewhere, I'd bet.

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Other way round actually, temp sender resistance decreases as temperature increases, that is why the gauge will read high when you short a sender out but read zero if you disconnect it. That said the gauge also needs a good supply as it is the gauge internals that measure the resistance of the sender and if that supply is erratic then you can get odd readings

The 2nd issue with mine as well as earth trouble was also the sender wire, I'm not sure if it was chaffing somewhere on the engine or whatever, but new sender wire. Cleaned all earth's and it's now perfect.

9 times out of 10, this is a result of a bad earth, normally it's the earth behind the bulkhead. Yes battery to chassis can cause this issue... but I would air towards the earth on bulkhead.

Kwakerman said:

Other way round actually, temp sender resistance decreases as temperature increases, that is why the gauge will read high when you short a sender out but read zero if you disconnect it. That said the gauge also needs a good supply as it is the gauge internals that measure the resistance of the sender and if that supply is erratic then you can get odd readings

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Yes, now you come to mention it,I do remember putting a Smiths electric temp gauge out of a '66 Mini Cooper S straight across a battery against good advice to the contrary, and it did go all the way high, fried it.

Landyfox said:

9 times out of 10, this is a result of a bad earth, normally it's the earth behind the bulkhead. Yes battery to chassis can cause this issue... but I would air towards the earth on bulkhead.

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Behind the Bulkhead? , can I see this from the engine bay or is it up under the dash ?.

Hi Farmie.... it's behind the dash binnicle or on some just to the right of the clocks on the bolts from the exterior windscreen hinge.

Thanks.

The gauge gets it's normal ground through the body of the gauge itself. Check that there is a good ground to the actual gauge (normally this happens as the gauge is pushed into a hole which is at ground). The temperature shown on the gauge is only possible because the gauge gets positive from a connection, variable ground from another connection which comes from the sender and full ground via the casing.

You could quickly check this by getting a jump lead and attaching one end to the handbrake lever bracket and touching the gauge body. If that fixes it just sort a connection.