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MG MGB Technical - Question about electric rad fan

How often is the electric cooling fan meant to be on?
1978 MGB with the single fan.
As soon as the engine reaches "normal" temperature, the fan cuts in and seems to be on continually untill I switch off.
The temp guage needle sits about halfway all the time.
Just wondering if this is normal or would the thermo switch be operating prematurely?
AdrianS

If you look closely at your temp gauge, you'll see two little white dots above the white line across your gauge. The fan should switch on when the needle reaches the right-hand mark and remain on until the needle falls back to the left-hand mark.
Geoff Everitt

Thats assuming the gauge is calibrated or reading accurately.
Didn't know if there was a way of testing the thermo switch. Was thinking about flushing the cooling system first as don't know when it was last done.
AdrianS

It shouldn't come on until about half-way from N to H, and go off again slightly lower than that, which happens to be by the two little dots, but that isn't what they are for. Either you gauge is down *and* your engine is running hot, or your sensor is too sensitive, maybe it is the wrong one. I have a Burlen switch on my V8 when the Otter failed, and that comes on slightly lower than as above, but often stays on until the temp gauge goes right back to N. You need to check the temperature of the coolant/rad header tank, and if you find it is at or about the thermostat temperature of 82C then it it is the sensor that is wrong.

You don't have an aftermarket fan and sensor do you? Most peple who bother to fit these set them to come on as you describe, even though the 'normal' range for engine temperature goes right up to the white or red section by the H.
Paul Hunt 2010

Standard set up.

When I bought the car the front bearing on the existing fan was shot so I assume it had be running excessively. Didn't want the replacement to go the same way prematurely!

Engine and car run well and the temp needle normally sits just below half way on the gauge and doesn't fluctuate.

I assume if the gauge is ok then it probably will be the sensor. Just wondered if these have a habit of playing up?

A friend told me his late MGB tends to run with the needle just above half way but he hasn't got a clue he says when his fan is working!!
AdrianS

Normally sensors either get erratic or just fail to operate the fan, becoming over-sensitive is unusual, I'd say, hence my comment about an aftermarket sensor. The system wasn't designed to run them continuously, so I think it is inevitable that it will wear out sooner than if it were switching on and off, and normally it would only switch on when in stopped or slow-moving in traffic.

Cars do vary for all sorts of reasons, especially when 30 year old and in varying condition and state of tune. Just above half-way running in free air is fine, and mostly you can't tell when the fan is running except by the gauge moving up and down, which it should do. Because my V8 has twin fans I can hear them when stopped, I also have a manual override switch (which I never use) with internal illumination, which tells me when the sensor has switched on. But even that isn't really needed, if the gauge goes above mid-way between N and H in 'normal' conditions then it's time to keep a close eye on it to see if it keeps going any further, and investigate if it does so.
Paul Hunt 2010

This thread was discussed between 20/04/2010 and 22/04/2010

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