Xantia Temperature Gauge Antics!

[CitroJim]

On the Activa, the temperature gauge is behaving very strangely. In the daytime with lights off it behaves perfectly. Rock steady just below 90. Perfect.

Switch on sidelight and the gauge goes up. Put on headlights and it goes higher and on main beams it goes highter still. It slowly gets higher and higher the longer the lights are on As it goes higher so it becomes unstable and wanders betweem 100 and 110. Switch off all lights and all returns to normal. Last night was especially bad. It shot nearly to the red zone.

It is not overheating, the fans don't run under the above conditions and nor does the warning light come on so it is just the gauge mis-reading.

What on earth is going on? I'm stumped to even begin to explain this one

[Mandrake]

What on earth is going on?

Got it in one.

(if you didn't get the joke, it sounds like an earth fault)

Regards,
Simon

[AndersDK]

Ohms law :
resistance x amperage = voltage drop.

You have resistance in a common earth for the instrument and the headlamps. headlamps eats quite a few amps, produces a voltage drop in the earth, which is deducted from the +12V feed line.
As the temp gauge relies on higher reading by lower voltage, you then get a higher reading. If you short out the temp gauge sender, the gauge will read stuck high.

Simon did a much shorter explanation though

[CitroJim]

Thanks all

For the first time today I ran the aircon and the gauge did the same tricks then.

It all makes sense now

I am feeling a bit of a twit right now I'm a time-served electronics engineer and I just could not see it

I can be so thick sometimes. I'm going to ground for a bit

[Peter.N.]

Dont worry about it. I have recently retired as a TV engineer after 50 years in the trade and still get caught out by car electronics (and TVs for that matter)!