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MG MGF Technical - Fan monitoring

I would like to see when the Rad and engine bay fans are running. I thought of installing a couple of leds in the dash area but i'm not sure how to wire these up to the fan motors. Any electronics out there that could help?
is the diagram.

A simple LED in line with a resistor of app. 680 Ohm may help, connected parallel to the fan. I suppose.

But the parallel connection to the fans or to the relay output will only show whether they get powered or not. Power to the fan will not mean that it really works.
Currency measurement required.

Any othe thoughts ?

Dieter Koennecke

I can hear both fans in my car - well, feel and hear the radiator fan, through the pedals (it only ever runs when stuck in traffic for a time, so no other stuff to get in the way), and hear the engine bay fan.

Ed Clarke

Actually quite a difficult problem:
1. How to get wires from fans to dash and make it look OK
2. Volts measurement on fans just shows they SHOULD be working
3. Current measurement is difficult because no current means not working but too much current can also mean not working.
4. Also, load on motor and thus fan speed may well be dependent on speed of car.
Answer is.... forget it!
T Green

T -

Too much current should cause fuse to go, so should = no current.

Load on motor - there must be a threshold somewhere though;

Wire routing - from rad shouldn't be a problem (obviously depending where one wants to put the LED or whatever). Isn't the engine bay fan relay operated? Where's the realy? Might be closer to fromt of car (I'm not going to check now - too late). The route of the bonnet release is a possible rear-to-front routing though, anyway.

Fan operation test - REALLY tricky. Say the gearing of the fan failed - the only point to measure would be the spinning of the blades of the actual fan itself - or some kind of anemometer? Getting a bit silly now though :-)

Ed Clarke

I was concerned that my engine-bay fan *never* came on (the radiator fan does, and is audible) - so I tested it by unplugging the sensor and plugging a 4k7 (1k would be better) pot into the harness. The engine-bay fan came on at about 250ohms, which appears to be the correct resistance for the VVC (85degC). I didn't test the 130degC upper limit.
So I'm fairly happy that everything works as it should - I did consider some monitoring electronics, but I don't want to change the loom / connectors unnecessarily. You could always monitor the e-bay temp - Maplin sell a remote sensing digital thermometer for about 15 quid, which I used for a time.


Assuming the fan has its own fuse then both current and voltage measurement are possible.

Some time ago I had a Ford Sierra Ghia (I know!) This monitored all its lamps and their fuses. You would have needed to measure the fuse voltage drop for current measurement (about 0.1V), the voltage across the fuse for blown fuse detection and the fact that there are volts at the fuse but no current for fan failure detection. Not really complicated but would need a dozen or so components on a printed circuit.

Sounds like a school holiday project for some of our more foul mouthed contributors.


Why not use a digital thermometer behind the radiator or on (or in?) the water return pipe. Assuming correct function of the thermostat, then a hot radiator with no cooling air will register hot.

I notice from the Demon Theeves catalogue that you can buy split water/oil temperature read outs in the same size as the oil temp guage, so installation of a good looking alternative is possible (if a little expensive). Alternatively, you could mount LEDs behind the condom flap if you have a mk1 MGF.

Rob Bell

A couple of thoughts:

It should be possible to detect current flow (and fans take a fair bit) by winding a few turns of its feedwire around a "reed relay", which will close due to the magnetic field that the current induces.

LED's can be connected to 12V with a series resistor like Dieter said (I thought it was 4k7 for 12V - can't remember though - I DO know they glow very brightly very briefly without one :-) )

I had wondered about mounting "tombstone" LEDs (the flat ones) in front of the white instrument surround from a bracket underneath...


Reed relay - neat solution.

A typical reed relay needs 50 - 100 amp turns. Assuming the fan takes at least 5A then it would need about 20 turns to guarantee operation. Two LED's side by side could show volts at the fan and current being taken.

But are the fans so unreliable? A temperature alarm buzzer is probably a more worthwhile mod


Thank you Brian :-)

I guess you really need a combination of the two: a temperature sensor that indicates that the fan SHOULD be running, combined with a monitor that says it isn't!


The fans are pretty reliable, but I have heard that the fuse does go from time to time. In this instance the LEDs would fail to illuminate at all- and you'd have over heating.

I reckon that a temperature warning would be better.
Rob Bell

Thankyou all for your advise/comments, a lot of ideas to play around with. I will keep you informed about the resulting mod.

This thread was discussed between 01/08/2001 and 02/08/2001

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This thread is from the archive. The Live MG MGF Technical BBS is active now.