Radiator cooling fans

Since having my Chinese MGTF I have never heard the front rad fans cut in. The engine bay fan however was regularly cutting in however.
As I understand it both fans are controlled by the ECU, which is different to the last of the UK MGTFs.
The cars never overheated, so some would argue I'm worrying to much, but it concerns me that I can't test run the fans when they are in situ.
Just to be sure whilst I had the rad off and front end stripped I test ran both the rad cooling fans and they run fine.
I'm thinking of putting a switch in the car so I can manually start the fans, my only concern is could I damage the ECU by putting a 12volt supply to the fans i.e would this risk putting a back feed into the ECU.

My CEE electronics says fitting a diode between the ECU and the fan relay will prevent feed back to the ECU but I might be wrong, I took my CEE in 1983....

As said, use a diode to protect the ECU

1 Use a high current SCHOTTKY diode.

2 Use a diode with forward current approximatly 2x current needed to drive fans

3 Use a fuse

4 make sure switch can also handle the fan current

5 the fans will pull a lot more current than you think! Measure it or calculate it if you know the fan power.

However Shematic 2 is probably the more correct way to do it, though a little more complicated.

(Reason is a diode has a forward voltage drop of about 0.6V, so the voltage to the fan will not be 13.8V but more like 13.2V. A SCHOTTKY diode is a special diode which only has a forward voltage drop of 0.3V so is the prefared type of diode)

There is another way, you'll need 2 relays up to carrying the current required of the fan and an on-on switch (has no off position but diverts the power from the feed from one output terminal to the other).
Basically you have a live feed to the switch, when in position #1 it directs current to relay #1 which connects the ECU to the fans and relay #2 in not active.
When the switch is changed to position #2, it disengages relay #1 and instead directs current to relay #2 which connects a Fused live feed to the fans.

Wouldn't it be simpler to arrange a switch or relay that cut off the ECU feed as it switched in the direct 12 V feed.

EDIT sorry i hadn't seen that Cobber had got the first. However I don't think i needs to be as complicated as he describes

I have a twin fan radiator from a chinese car in my 2002 TF. I got Jon Norris to install a switch last cam belt change.
Don't know much about what he did but both fans are linked together. The fuse box fuse has been up rated to double the original, I have an extra relay and seperate fuse fitted too. I pop it on now and again in heavy traffic.
Am I correct thinking the extra fan is just there for the AC on a late TF? I don't have AC but two fans is better than one for cooling I imagine.

Thanks for the replies must admit the idea of the switch sounds the easiest, just need to run a couple of wire cores into the car. As long as it breaks before it makes it you understand my meaning.

davidej wrote: Wouldn't it be simpler to arrange a switch or relay that cut off the ECU feed as it switched in the direct 12 V feed.

EDIT sorry i hadn't seen that Cobber had got the first. However I don't think i needs to be as complicated as he describes

"Schematic 2" in my post.

Could do away with the relay, and just use a switch (1 pole 2 position), but this is bad practice, as requires high current wiring feeding to the switch position presumably in the dash. Would still work though!

The only time that the radiator fans have come on in my MGF is when the a/c is on, I am unaware of them coming on otherwise but then the car's temp gauge has never gone over the half way mark during my ownership; there has been no reason for the fans to come on.

It could be that the radiator fans will not come one with the ignition turned off (had a VW that worked this way), unlike the engine fan which has come one many times just after the car has been parked.

mowog73 wrote: I am unaware of them coming on otherwise but then the car's temp gauge has never gone over the half way mark during my ownership; there has been no reason for the fans to come on.

On a hot day, I would often hear the front fan running on mine when stuck in traffic (single fan, no AC). Again never saw the temp gaige go over slightly above half way, where it always sat. I think the front fan does cut when the ignition is off, as water pump not running!

While moving at reasonable speed, airflow from forward motion will exceed flow from fan significantly, so fans should never operate except in slow moving traffic.

Engine bay fan used to tun a lot, easily noticeable when parked.

The gauge is not linear, the middle position represents quite a wide range of temps! Use ODB2 to monitor temp, and see how the gauge responds.

mowog73 wrote: ...….I am unaware of them coming on otherwise but then the car's temp gauge has never gone over the half way mark during my ownership; there has been no reason for the fans to come on. ………..

The "normal" position for the temperature gauge is just under half way. This covers between 88' and 110'c. The cooling fan should come on at 96' and slow speed and fast at 102' going back to slow at 96' and off at 92'. The only way to see these temps is reading live OBD data from the ECU. You won't hear slow speed and will only see the temp gauge go over normal is if the coolant is way low.

mowog73 wrote: ...….It could be that the radiator fans will not come one with the ignition turned off (had a VW that worked this way), unlike the engine fan which has come one many times just after the car has been parked. ………..

The ECU is live for 10 minutes after ignition off monitoring sensor outputs so in theory it could run the radiator cooling fan if it senses temperatures rising dangerously. In later cars the ECU also controls the engine bay cooling fan.