Not Overheating but...........

[kitesurf_phil]

Not Overheating but...........on a couple of occasions, after climbing a long hill, I have switched off the ignition. On switching the ignition back on, just to open a window and not starting the engine, the electric fan in front of the rad kicks in. None of the cappilliary fins are damaged on the rad and all are clear of mud etc.
The fan sensor is oviously detecting that the engine is too hot, although the temp guage on the dash shows normal, therefore it is worrying that the fan won't operate until it gets an electrical feed via the ignition switch. Has anyone else noticed this?
The only place I can see any bent fins are on the ATX cooling portion of the rad. There is a 4" square area of fins, on the far left hand side, that are bent over and closed off. I am wondering if the heat being sensed is being transferred from the ATX cooler to the engine cooling system.
I am going to carefully straighten the bent fins to see if there is an improvement. If not I am thinking of wiring the fan so that it has a constant feed, via the sensor, so that it is independant of the ignition switch. That way it will run on for a minute or two after the engine has been stopped.
Opinions, please, from the more learned.

[dirtydog]

Mine does this all the time - its been doing it all day today for instance although Ive been towing. The gauge shows normal all the time. The second you start the engine it must cut off as then you have the viscous fan?

[stevetubbyturbo]

Mine is the same too if i give the Surf some stick the electric fan wont come on whilst the engine is running but if i turn off the enigne and a few minutes later switch the IGN on the electric fan cuts in for about 30 secs and then off.

I have a theroy why it does this and its something like this

when the engine is running the water pump is pumping water through the rad and then the cooler water goes back to the engine to cool it (common knollage) when the engine is switched off the water in the engine is still being heated and and the water in the rad is cooler, as the radiator is at a high point and heat rises the hot water from the engine slowly makes it way to the rad and pushes the cooler water to the engine but as there is no air flowing over the rad it heats up and triggers the thermo switch but as it connected to the IGN power supply it only cuts in when the IGN is on but i wouldnt think it would be too difficult to wire it up to a permenent supply

My theroy could be wrong but its only a guess

[Silversurfer]

Your theory sounds about right. When the engine is running the coolant is being moved rapidly around taking the heat with it. Once the engine is stopped there is no coolant movement so the coolant in the block/head coolant galleries will be in contact with the heat source for longer and absorb more heat without being able to lose it via the rad - it's not really a problem as it will eventually cool along with the engine. The sensor for your electric fan is obviously in a position where it can sense this extra heat which is enough to trigger it as you have found when you turn the ignition on a few mins later.

Beware if you fit a permanant feed to the fan - a friend of mine has a Kawasaki ZZR 1200 motorbike with a permanant supply to the fan via the temp sensor - unfortunately the sensor went faulty and flattened his battery. He had to disconnect the battery every time he parked the bike until he was able to get a new sensor under warranty.
How about wiring the temp sensor via a timer, same as say a turbo timer, then you won't have that problem but the fan will still be available for whatever time you set it for after you have switched the ignition off.