Temp gauge not working properly

Mon Jul 28, 2008 12:27 pm

I had a problem with my rev counter and temp gauge not working at all. The batteries in the service indicator unit were corroded, so I replaced the front half of the instrument cluster that holds the SI unit, and now the rev counter works fine, as does the fuel economy gauge, but the temp gauge just moves from off the scale to the bottom of the cold mark and then stays there.

There is some input to the gauge as it does move to the bottom of the cold mark, but I'm wondering whether the fault could be with the gauge itself, or with the input from the engine. Where does the gauge temp input come from? Is it one of the coloured plugs at the back of the thermostat housing (M20B25 engine)?

The engine runs ok, so its obviously not affecting the temp signal between ecu and engine, just between engine and temp gauge. If the thermostat was stuck open I would expect it to run cold, but not that cold. Even if I leave it running while stationary the temp doesn't go up which it would if it was just the thermostat.

Any ideas?

Mon Jul 28, 2008 1:37 pm

Temp gauge will move up to full cold when powered up, without a sensor input.
Sensor is the brown one on the thermostat elbow.
Take apart the 20 pin engine loom plug and socket on the bulkhead just next to the fusebox. Identify pin 4 on the side that goes to the fusebox (brown/purple wire, if you flip back the rubber cover), switch on the ignition, and use a piece of wire to short it to earth (bonnet rear bracket). Gauge should go to full hot.
If it does, reassemble that lot, pull the brown connector off the sensor, and short the single pin in the connector to earth.

Mon Jul 28, 2008 4:15 pm

Will replacing the sensor help, or is it more likely to be a fault with the connector/wiring?

If shorting the connector to earth is just to get around a broken temp sensor, I'd rather replace the sensor (assuming they're not hideously expensive).

Mon Jul 28, 2008 4:42 pm

Brown sensor from GSF @ £7.64 inc vat may be worth ding the blue sensor aswell priced @ about £8.00

Mon Jul 28, 2008 5:51 pm

Chase007 wrote:Brown sensor from GSF @ £7.64 inc vat may be worth ding the blue sensor aswell priced @ about £8.00

Are the gsf ones ok?

I come from a VW background, and the gsf temp sensors for VWs are known for being not as good as the ones from VW.

If a sensor from BMW is not that much more I'd prefer to get get one of those.

Thanks for the price check though.

Mon Jul 28, 2008 7:07 pm

Brianmoooore wrote:Temp gauge will move up to full cold when powered up, without a sensor input.
Sensor is the brown one on the thermostat elbow.
Take apart the 20 pin engine loom plug and socket on the bulkhead just next to the fusebox. Identify pin 4 on the side that goes to the fusebox (brown/purple wire, if you flip back the rubber cover), switch on the ignition, and use a piece of wire to short it to earth (bonnet rear bracket). Gauge should go to full hot.
If it does, reassemble that lot, pull the brown connector off the sensor, and short the single pin in the connector to earth.