Have I f***ed it?

[teecee90]

I think you have high resistance in the fan circuit and that's causing low voltage and low current at the fan. When you rev the engine it raises the voltage and current enough to start the fan turning but the voltage and current are still lower than they should be because of the high resistance so the fan is spinning slower than it should. I'd guess there's corrosion in a connector or wire.

To confirm it you could measure the voltage at the fan when the fan is spinning. The fan has to be spinning for the voltage test to be accurate because there has to be current flowing through the circuit. You should be seeing close to battery voltage on the fan connector when the fan is spinning but I think you'll see lower voltage because of high resistance. Basically in a good circuit all of the voltage should be dropped across the load which in this case is the fan but because there is added resistance in the circuit some of the voltage is dropped across the added resistance and what's left is dropped across the fan. If you find low voltage on the fan connector you can determine if the resistance is on the 12V wire or the ground wire by leaving one probe in the positive wire of the fan and touch the other probe to the positive of the battery. On a good circuit you should see close to 0V, if there's added resistance on the positive side of the circuit you will see some voltage on the meter so you know then that the problem is on the positive side. You can then do the same test on the ground side of the fan. One probe in the ground wire of the fan, touch the other probe to negative battery terminal. On a good circuit you should see close to 0V. If you see voltage, there's resistance on the ground side of the circuit. Again the fan has to be spinning for it to be a valid test.

Thanks for the advice - I've just ordered some back probes for my multi-meter and will give this tests a try. Cheers