MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Electric Fan Override


I am planning on fitting a Fan override switch based on the override V8 diagram at

https://www.mgb-stuff.org.uk/s_fans.htm

For ease the schematic is also below – I hope it is OK to reproduce it here from Hammer and Spanner.

A couple of questions:

What rating switch do I need – 25 A should be enough? I assume my redundant overdrive switch is not up to it? I will add a fuse into the fan feed form the relay as well probably 15-amp blade?

Anything to do when I wire up the override switch with tell-tale – I am planning on using an illuminated toggle and already have a couple fitted for other purposes, one lights up and the other does not, and for the life of me I cannot work out why.

I currently only have 3 cables into the relay (6 RA type) – green for power in, black with brown for the thermostat switch and black with green trace to power the fans, so essentially no brown in.

Changing to this 4-cable layout although attractive will be for another day - one step at a time!



Mike Dixon

Hi Mike
Just about any switch will do the job for an override if you wire it as per the diagram you've put up--It's switching the low side of the relay which would normally draw something like a quarter of an amp.
Your redundant overdrive switch would handle the job easily but you wouldn't have a telltale unless you were to wire one in- AND this brings up another point, even with it wired as per the diagram the light will operate every time the fan(s) come on in either override or auto mode--the only way you can stop that happening is to have a double throw switch with the earthing for the relay on one throw and the power for the telltale on the other OR have a diode in the line from the switch to the relay to stop the thermo switch feeding back.
willy
William Revit

No problem reproducing the diagram, an Willy is correct on both counts.

I have a switch with built-in tell-tale and that is useful as I can see that the thermostatic switch has operated the relay when the temp gauge starts going up in hot traffic. Well, I say 'operated', but all it shows is that an earth has been sent to the relay, and even if it does operate it doesn't confirm that the relay is sending power to the fans ... or that the fans are actually spinning! You need the temp gauge for that :o)

Incidentally that diagram shows a four-terminal relay that mine came to me with and not the original factory three-terminal. I had thought that was a PO mod to take the load of the fans off the green circuit fuse - which is already overloaded with the HRW (I have a fused relay off the brown circuit for that), but have since come across another one done in exactly the same way so it may have been a dealer mod.

It also shows the relay powered from the accessories circuit instead of the ignition circuit, so on a hot start the fans cut out during cranking.

I would be interested to know the resistance of your three terminal relay between W1 and C2. It would need the black/orange (not black/brown) to be removed from W1 but if you don't fancy doing that no worries.
paulh4

Incidentally when the four terminal relay was substituted for the three-terminal and a brown feed picked up from the fusebox to power the fans it wasn't fused, so I added one as shown in the diagram.
paulh4

It seems I have a 4 terminal relay wired as a 3!

Apologies for the rough sketch but it hopefully shows how it is wired. I only have one black wire with trace though - it is a bit faded/grubby so could be brown or orange trace. It is the one connected to the otter switch.

I assume if I remove the bridge wire W2 to C2, I could put a feed from Brown to W2 to match the diagram above

Is the easiest way of doing this a piggyback spade divider off the fuse box?

I may have a look at the resistance but do not want to damage anything! Volts and Continuity is my comfort zone



Mike Dixon

It's a fairly common practice to wire a 4 post relay as a 3 by bridging the two power posts. Nothing wrong with that-----Leave well alone.
All you need for an override switch is to piggy back onto your otter switch wire (W1 in your pic) and run a wire from there through a switch to earth. IF you want an indicator light then you will also need a power supply to the light and then connect the earth side of the light to the otterswitch side of your override switch.

To be honest i can't see the need for an override switch. To me the best/simplest fan is the std mechanical one, no relays, no switches, no wiring ,no problems, nothing to go wrong.
William Revit

The drawback with leaving it is that the load of the fan comes off the green circuit and can overload it.

The drawback with removing the link, leaving the green on W2 and providing a brown from the fusebox to C2 instead is that the fan is then unfused.

Is your car a factory V8 or a 4-cylinder? As you mentioned 'black with brown' which on a V8 would be black/orange with the orange dirtying to brown then I assumed factory V8 which always had electric fans and I wouldn't change that.

I do agree that prior to the factory electric fan(s) for the 1977 model year the 4-cylinder shouldn't need anything other than the original mechanical fan, especially in this country. These cars ran in desert states around the world from inception and didn't have cooling issues. Those fans didn't use a fan relay but they did have an ignition relay (which did the same job in taking the load off the main fusebox) and a separate in-line fuse (later a thermal circuit-breaker) for the fan circuit.
paulh4

My 1980 B has a thermo operated switch installed in the rad header tank. All that does is complete the circuit to feed 12 volts to the fan. This is I gather standard as the car left the factory.
I fitted a MG FAN switch to match the exiting switch for the internal fan into the facia. There was a spare position!
This switch is wired across the existing thermo switch using the same size wire as connected to the fan. When operated it just bypasses the thermo switch and supplies 12volts to the fan.
If the OVRERIDE is NOT operated the Thermo switch operates normally as the 2 are in effect in parallel.
I just keep an eye on the TEMP gauge. 20 years operation zero problems. Simple is best!
The standard yellow bladed fan makes enough noise thus I didn't bother with warning lights. When the ignition is off the fan doesn't run so no chance of running the battery flat.
The pic is in the Tankwa Karoo on our way back to Natal Midlands cfrom the MG Indaba at Bot Rivier in the Cape. Best shade we could find Temp was 35deg C. Car was cool , we were not!


Ray Bester

Yes, with 4-cylinder 77 and later the switch connects power from it's own fused spur off the ignition relay to the fan motor directly, no relay or tell-tale.

It's not been confirmed yet whether the OP has one of those or a V8, which is different.
paulh4

It is for a factory V8. Black with a yellow trace is as close as I can find - close enough!

I am tempted to leave well alone and stick with a 3 wire relay , but I would add an inline fuse if I did connect the brown.

I will have quick look tomorrow to find some power for the switch so I can have it illuminated, just as another reminder I have it on. There should be plenty of things behind the dash I can tap into.

Thanks for all of the input - really helpful
Mike Dixon

Ahh-a v8 The factory fans usually work quite well.
Just in case the fans have been out for some reason, it might pay to check that the fans have been fitted to the motors the right way round---They will fit either way round and the grub screws are easier to get at the WRONG way round so just might pay to check.
They will work either way round but pump air better the correct way which is with the concave side of the blades facing the rear of the car and the rounded convex side to the front-
I do like a good v8
willy
William Revit

The instrument voltage regulator is a convenient place to piggy-back off as it is on the lower edge of the bulkhead under the wiper motor and there is (or was originally ...) a spare spade on both connections, use the 'B' terminal (green wire) not the light-green/green on the 'I' terminal.

I got caught out when I moved the fan relay from the ignition circuit to the accessories, with the tell-tale still on the ignition circuit it cause some odd symptoms!
paulh4

Good point about the fans. I am in the process of changing one of the motors, just waiting for DPD to deliver it. It took me a couple of minutes to realise I had to remove it in situ to get it out.

I will get the override switch wired in once the new motor is in and working.

Mike Dixon

I had to change a motor but could not remove the fan so had to remove the two together. Maybe the one I did is easier than the other, and being an RB the oil cooler is below the apron not above.
paulh4

This thread was discussed between 05/03/2025 and 11/03/2025

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now