My electric radiator cooling fan on my SP250 continues to run when the cars engine is switched off, Is this OK or should I feed it through the ignition switch.

Steve.

Hello Steve,

On the classic cars I have and have owned, I've always allowed the fan to run after the ignition is switched off - it helps to move heat soak within the engine bay and continues to cool the coolant in the radiator albeit not being circulated.

I will be doing the same for the Daimler, once I've installed the electric fan.

It's a personal choice, as I am sure you will find out !!

Regards.

Richard.

My electric fan has been wired both ways. When my car engine was restored I asked for the manual override switch and was happy with that, but when I had the car body restored the restorer said the manual switch was not recommended and didn't fit it. Since then the fan does continue to run for about four or five minutes after the ignition is switched off, which is a waste of battery juice, but I put up with it.
I can't really understand why the fan needs to run on after the ignition is off as the water pump isn't working to move localised boiling coolant in the engine to the radiator and cooling the radiator won't have any effect on that. Cooling the engine bay with the fan can't make much difference in UK climate but I can understand it in the tropics or Australia.

Steve. The only point that I can add concerns the capability of the ignition switch.

I understand from MG websites that running too much current through an ignition switch is liable to lead to failure (including breakdown) and overheating (possibly even fire (!!!)).

The solution is to ensure that any high current circuits (notably radiator fans and headlights, but possibly including heater fan, horn and auxiliary lamps) which one may want to be only operative when the ignition is on) are powered through a relay or relays which are only switched by the ignition.

This is what I shall do with my radiator fan, heater fan and headlamps. Why bother with headlamps? I’ll let you work that one out...

Steve,

Since the thermostat switch used to control the fan is controlled by temps remaining in the cooling system, the time to get to shut off temp can vary immensely from hot to cool days. Hot days and the battery is tested greatly. Some modern cars have timed circuits to control the fans after shutoff for a set period of time. Worth investigating, I have not.
Always use relays in hi current circuits, its what they were designed for.
I also have a shutoff panel switch in my fan circuit to limit time on or off.

Assuming you have a modern fan system comprising fan, electronic controller and switching relay, the fan should be connected with adequately rated cable directly across the battery via the relay. The controller can connect to the relay with low current cabling and you then have the choice of connecting the supply to the controller either via the permanently live outlet from the fuse box (also supplies the horn) that will allow the fan to run independently of the ignition, or via the ignition switched outlet (that supplies the rest of the electrics) .

Allowing the fan to run after turning the engine off will help to lower the under-bonnet temperature but will make no significant difference to the temperature in the block since the water pump is no longer running.

Personally I prefer to wire mine via the ignition since the current draw is high - up to 30 A. Should I be stationary in traffic so long that I turn the engine off and the fan were to continue running, starting again could be difficult until the fan cuts out and allows adequate current to the starter.

I have experienced that scenario as Nick suggests whereby, the engine would not start when I inadvertently stalled it whilst the fan was running.
Therefore, I have wired mine through the ignition switch.

Dave.

Ah ha mystery solved, just like Nick and Dave I had trouble in the recent hot weather with starting my SP250 when hot.

Should have sussed it, my fan is wired via a separate fuse, live with the ignition off. Useful when cold to keep the engine warm. But recently, the battery which is an 9 year old Bosch has become lazy.

No problem cold with no fan, but when hot difficult to start. When the car is stationery with the engine switched off for a short period, I tend to let the fan run and it was still running when I tried to start the car. Having read this, switch off the fan and the car starts instantly.

So the best option is yes, wire through the ignition with a relay, but fit a manual switch also, in my humble opinion, which I shall be doing shortly. Syd

P.S. will be fitting a new Bosch battery soon.

Failure to start even though the starter is cranking normally is often a symptom of a tired battery. The problem is 'coil robbing' whereby the voltage drop arising from the starter motor current draw severely reduces the current available through the coil. A common symptom is the engine attempting to fire just as the starter motor is released.

Any high current draw item - such as electric fan or headlights - that are running before starting is attempted is only going to exacerbate the problem. Even a good battery can suffer the problem if the current draw of the accessory (e.g electric fan) is particularly high.

Thank you everyone for your thoughts on switching, The power of the forum is wonderful.
I have just carried out a current test on my fan which is an Aeroline and it runs at 7 Amps.
To sum up I think I will run a relay coil feed through the ignition switch and feed the relay contacts from a permanent 12v source and use my existing overide switch as a backup if the thermostat fails, then add an isolating switch to the relay coil feed in case of low battery voltage to enable a hot start.
That should cover it !

Steve.