Coolant temp gauge suddenly reading low.

[Album56 0]

My G60 has suddenly got a low reading temp gauge, has always read normal 90c when moving & rises to approx 108c in traffic & fan cuts in ok, now gauge reads 70c or less but engine temp seems normal & fan cuts in if I stand still for long Main suspect is the black sensor so have tested it & it reads approx 150ohms when hot. Does anybody know if this is normal?

Testing the wires see earth ok on brown wire & approx 10v battery on yellow/green with ignition switch on.

I have a spare set of clocks so tried a different gauge but its the same so its not the gauge, fuel gauge etc are working fine.

Any help with black sensor test much appreciated.

[davidwort 0]

Thermostat stuck open?

[Album56 0]

Its a fairly new thermostat & heater is working fine, so I'm pretty sure the engine temp is normal as the oil temp reads 90-100c on the MFA but the gauge is reading low. Its either a fault in the loom or the black sensor I guess...

[davidwort 0]

Testing the sensor is pretty easy, you need a meter with resistance measuring and run the engine from cold, record the resistance every 30 secs or so until the radiator fan kids in, then compare the values you have to the range graph for that sensor, I think the Bentley manual has the coolant sensor graph if not I found one for the digifant 8v fairly easily trawling the web. I've found even vw sensors vary, enough to affect the gauge, but as long as its within the allowed range it should read ok. Iirc resistance increases with temp on these sensors, so faulty wiring usually increases resistance and the gauge then reads high, shorting the loom pins should give you a maxed out reading on your gauge.

[Album56 0]

Still tracking this fault down, as a retired communications engineer I have a good basic knowledge of electronics & particularly wiring, continuity testing etc.

I have a spare G60 cluster which I have dismantled to examine the circuit board & note that the fuel & temp gauges share a common 10v +ve battery supply & the other side goes via the loom to their respective variable resistance earths.

The fuel gauge works ok so will look into the loom (yellow/green wire) to the black sensor & the (brown wire) earth.

Does anybody know where the brown wire from the sensor actually earths? could be one of the brown wires that terminate on the head cooling flange stud. I'll try a good known earth via a 110ohm resistor to the yellow green.

I'll update if & when I fix it because I know how useful this forum is when things go wrong.

[Album56 0]

Dash was out a couple of months ago for a new heater matrix but everything was working ok after that job but more recently I have had a new alpine CDE-173BT stereo fitted by halfords, stereo works great even thro oem corrado speakers & they even connected the auto dim when the lights are switched on!!.

I wonder if all this messing about behind the dash has caused this problem, I'm pretty sure the black sensor is ok as it tests as per the Bentley manual. I'll have to temporarily piece out the whole of the yellow green wire from the sensor to the multi pin binnacle plug to see if that fixes it.

[Album56 0]

Looks like its the black sensor after all, after much proving continuity of wires & substituting the complete cluster I re-tested the sensor & it was 9k ohms with a cold engine. New sensor on order....

[Album56 0]

Black sensor replaced & temp gauge working again. Thanks guys

[Jim 2]

Good work - and always appreciate it when people update the thread with their resolution to a problem rather than leaving it unanswered :)

[Jim Bowen 1]

Might try another Black one in mine. Think I only looked at Blue. Might explain the popping exhaust.