Cooling fan run on runs on and on

[Wullie 1]

Got wakened this morning about 3 am with the car alarm doing it's thing. Tried to reset it but it just kept going off so I opened the bonnet to disconnect the battery (not easy in just a pair of kegs with an alarm two inches from your ear, in the dark) and noticed that the cooling fan was running. Got everything shut up and went back to bed. This morning had a look and the battery was pretty much dead, for some reason this sets off the alarm, happened a couple of times before when I had a battery drain.

 

Charged up the battery this morning and everything seemed OK, fan didn't run when it was reconnected. However, it also didn't run when I shorted the cable from the run on sensor to the head. It's a new sensor and a quick check showed it to be working OK.

 

Is there aything else that controls the run on?? Or anyone have a clue as to what may be happening. Apparently the sight of me half naked quite upset the neighbours.

[davidwort 0]

I'm not sure, but I reckon the radiator temp switch will also run the fan with the ignition off if the temp is high enough, perhaps that is dodgy and shorting at low temp?

[Wullie 1]

Good shout, I'll go and stand on my head in the engine compartment, fully dressed of course and see what I can find there.

[davidwort 0]

the 1.8 doesn't have the engine bay fan control relays like the later 16v, I'm not sure if there is an equivalent inside the car on the fusebox, another route to investigate anyway...

[Wullie 1]

Seems to have been the temp switch at the botton of the radiator. It was covered in light corrosion inside where the terminal spades are. Gave it a good clean out and lo and behols, the run on lead starts the fan when touched to the head and the fan stops when it's removed and replaced on the sensor. Excellent call David. Now for the next bit.