E30 Zone

Can anyone explain to me how the viscous fan works on the M20 engine? Does it just start to kick in when the engine gets to temperature or does it wait for a signal from a sender unit to kick in? The reason I ask is that my temperature gauge sits anywhere between half and three quarters for any types of driving. I recon this is running too hot and suspect the fan. The bonnet does not get hot, the heater works fine, there are no signs of head gasket problems although the hoses do get quite firm when up to temperature but there is no coolant loss through the expansion tank. The top rad hose gets hot and the bottom is warm so that probably means the rad is ok.
The fan is just as easy to turn by hand when engine off and hot as it is when engine off and cold. Also, unlike my E34 535 there is no â€Ëawind' noise from the fan cutting in when the engine is hot.

Could the temperature gauge just be at fault or is it the fan or could there be a sender unit not triggering the fan to cut in. I notice there is some kind of sender in the thermostat housing. Is this the temperature sender or something else.
Any help on this would be much appreciated as I don't want to push then engine too much in this state! Just for the record it's a 2 litre, 1990 with just over 100K on the clock.

Cheers

The viscous fan is completely self contained and controls itself. No input from any seperate sensors.
As long as the temp gauge needle is away from both the blue and red zones, then the temperature is fine. The middle black section only covers a few degrees.
If the temperature doesn't climb into the red when idling or stuck in traffic, then I suspect there is nothing wrong with the viscous coupling, or anything seriously wrong with anything else.
Although your gauge is within the limits set by BMW, it does appear to be reading a little higher than most.
You could try a new sensor (brown one on the thermostat housing) and if that doesn't make any difference, fit a new thermostat.
As with all E30s, check that the coolant pressure cap has been replaced under the 1997 recall!

I recently had a temp gauge that moved all over the place once the engine was warm, which could be a brass nut on the back of the gauge.
I never got that far to check it as it came right when I refitted the engine earth strap because the starter motor wouldn't turn recently although the gauge was playing up for quite awhile, may be worth a try.

Thanks for the quick reply guys, most helpful. How would I check if the cap has been replaced, ring BMW? I've got no paper work saying it's been replaced!

hellblazer wrote:Thanks for the quick reply guys, most helpful. How would I check if the cap has been replaced, ring BMW? I've got no paper work saying it's been replaced!

Dealer can tell from the VIN if it's been replaced, but that doesn't mean for certain that the replacement is still on the car. The ones supplied by my local dealer have a yellow disc on the bottom, but there are others around.
If it hasn't been replaced, your dealer should give you a nice new one FOC.

mattG has a good point with the brass nut! Normally a loose nut results in the gauge jumping about, but I did have one where the result was a constant, but slightly high reading.

my temp guage on my 320i is healed over into the red from cold then after 5 mins of driving to work (20min journey) it jumps into life flicking bout all over the place for a few seconds then settles at the right temp always reads around the just before middle mark or a touch over when sat in traffic for 10 mins. this healed over then flicking bout fault..... ive never looked into cus it doesnt bother me. got other things further up my "to do" list
whats this brass nut thing? earth lead to the head?

Nut is a small brass nut on the back of the instrument cluster, directly behind the temp gauge. There is a second one directly behind the fuel gauge, which, when loose, can cause similar effects on that.
The nuts connect one of the three connections to each gauge to the PCB.

The car had a new radiator just before I sold it 2-3 years ago.