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MG MGA - Electric fan - which side of the ignition

My car has an electric fan with an thermostat and an over-ride switch. Usually during the time when I stop, open the garage to put the car away the thermostat will be calling for cooling and the fan comes on when I start the engine as I move the car inside. Obviously without the engine running the hottest water rises to the top!
I then find myself turning off the engine when the fan cooling is in demand. If the fan was on the other side of the ignition switch it would run irrespective of whether the engine was on or off. Any views as to whether this would be a better arrangement?
Graeme Williams

Electric fans are normally wired to the unswitched side of the 12 volt supply. The reason for this (as you have found out), is that once the car is stopped, there is no more air being forced through the radiator to carry away the heat and the coolant can actually heat sufficiently to boil. If the fan can come on after the ignition is shut off, the heat can be carried away by the fan. Cheers - Dave
DW DuBois

With most moderns you can hear the fan running as you walk away!
Graeme Williams

I run twin electric fans on my car and no engine driven fan. I connected them with a separate (fused) feed as I think that the extra amperage they draw would really stress an already overloaded ignition switch.

I can see the advantage of connecting it through the ign switch to stop the fans coming on after the engine has been switched off.
However, if I do decide to do this, I will use a relay to reduce the load through the switch.

To be honest, there is no real advantage in letting the fan continue to run after the engine has stopped as there is then no longer any coolant circulating through the radiator. So you are not really cooling the engine at all.

c firth

Picking up your point Colyn, since the sensor is in the top hose, without the water circulating the water the fan is cooling is not influencing the sensor. It could want to run for quite a long time!
Graeme Williams

Should be on a latching relay that is activated by power to the ignition (so switched power but only taking current for the relay) and running until the sensor tell the fan to stop because it is already cool. Then the relay should reset when as is no load any more.
dominic clancy

Thermo syphon cooling will take place even with the engine off, so fan run-on post shut down may be of some use, but thermo syphon is a slow process and while running the fan is a way to more quickly cool the entire system, it isn't generating any more heat anyway.

The reason for putting the sensor at the bottom of the rad is obvious, you want the fan to come on AFTER the water in the entire system has reached optimum running temp. Running the fan too soon can delay this.

Also, if the radiator is cooling sufficiently, the fan may not need to come on at all. The best place to gauge the rad's performance and decide whether it needs the fan's help is at the bottom, not the top.

There are dozens of discussions on all kinds of forums, namely the hot rod forums where we discuss building custom cars. New radiators will always have sensor ports at the bottom for this use.

Block temp gauge at the top, fan switch at the bottom.


I agree with MAndrus that whilst running the electric fan after the engine has stopped does cool the whole radiator and the engine and the engine compartment to a certain extent, the engine isnt generating any more heat when it stops running.

Also,consider that the standard engine driven fan always stops spinning when the engine stops and so MG were not really worried about the car overheating once the engine stopped

I fitted my thermostatic switch in the top hose mainly because it would have been just so awkward to get to and adjust in the bottom hose of an MGA.

Dominic, can I ask you about your relay suggestion, can you get a latching relay to unlatch when you disconnect the power? (Im not an expert here )
All the latching relays I have come across need a momentary switch impulse to make them latch and then a second impulse to unlatch them.

So in this situation I agree that when you switch off the ignition,your relay would stay latched to the on position, keeping the fan running until the thermostat switched it off.

But the next time you switch on the ignition the impulse would unlatch the relay switching off power to the fans thermostat so that it wouldnt run.

I think a standard relay would probably work better here.

(Sorry Graeme if this relay stuff doesnt help your help answer your question)

c firth

Colyn, you weren't dipping your thermostat sensor in boiling water yesterday?
Graeme Williams

This thread was discussed between 19/07/2015 and 20/07/2015

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