Suspected Bad Earth on MK 1 Elise

[Walter Mclachlan]

My mates MK1 Elise has what we think a Bad Earth.
Problem arrose when he was travelling south in the daylight, as darkness fell he switched on his lights. After 20 mins he noticed that his temp gauge was reading very high. He stopped in order to cool the engine for a while, when he switched the engine on it was still high. As it wasnt far to his house he carried on.
Next day the car ran at its normal temp in the daylight. A week later he came up to my house at night and showed me while the car was running the temp gauge was steady but when he switched the lights on the temp gauge rose instantly. Lights of and temp gauge was normal. We then switched on the wipers and the temp gauge rose again, stuck car into reverce and low and behold the temp gauge rose as before.
Now I suspect there could be a bad earth but and this is where I may struggle. Does anybody or has anybody came across this before.
Any help welcome.

[tut]

Tis probably a bad earth, but the temp rise is spurious, with headlights on it will rise to around 112, put the fan on and it will start flashing and show 118. It does it on both my S1 Elises and you learn to ignore it. Turn everything off and it will drop down to 88 again.

If in doubt stop and check the expansion bottle, you will find it is not boiling. To check it go up in stages, sidelights, headlights, low, medium, and high fan, and you should see it rise each time. Robin will probably tell you how to cure it.

tut

[Gourlay83]

I have an S1 and mine use to do this (only a couple of degrees though) until i changed the battery. Now its perfect with or without lights on.

I haven't a clue what causes it - I always thought it was caused by the higher current draw on the battery, due to the lights being on.

Robin will probably give a more accurate explanation

Alan

[fd]

It's caused by the earth voltage of different parts of the car not being the same, because of voltage drop over relatively high resistance paths in the earth loop in the car . . . this causes the engine (specifically) to float at a higher earth potential than the battery earth which causes the sensor voltages from things in the engine loom to read incorrectly, which causes things like engine temp to read incorrectly . . . The higher the earth return current (lights/heater on etc) the higher the voltage drop and the more 'incorrect' the readings will be . . . simple . . .

Check

ECU/Main engine electronics earth, near the ECU (usually), lots of cables terminate here . . .
Main engine power earth (gearbox to chassis), very obvious
Main battery earth (battery to chassis), again very obvious

Anything that is not immaculately clean and corrosion free needs looked at, including all crimps and cables.

That should do to get you started . . .

Fd

[alicrozier]

. . . simple . . .

...when explained well!