[V6-12v] Radiator Fans not switching in correctly...

[The CyberPoet]

This isn't my area of expertise, but I do know that on my V6's (at  
least based on my experiences), the following seems generally true:

(A) There are two electrically-driven fans, the primary one triggers  
on temp of the radiator temp sensor, and the second of which triggers  
based on air conditioning use (as well as kicking on at a higher  
thermal level threshold for the radiator/engine).

(B) The wiring connectors for the sensors is often the first thing to  
go bad, because of exposure in my experience. Cleaning the contacts  
thoroughly & dielectric grease is often the best place to start.
Sidenote: I've found a dielectric grease called Nolox that contains  
zinc particles in the suspension which forces a fresh cut through  
surface corrosion on connectors every time the plugs are mated works  
better than traditional vasoline-like dielectric grease if there is  
already a problem with connectivity. There are Brit-market  
equivalents (I selected Nolox because all the other options were UK- 
based and not readily available here). This is not a total solution  
for bad connectivity, but given a connector that already had to be  
cleaned of surface corrosion once, I'll grab the nolox just to  
improve the situation in the future (such as plugging/unplugging tail  
light connectors on the road where I don't carry a full toolkit with  
sandpaper, etc. with me)...

Now, having said that, are you SURE the radiator is getting the hot  
fluids in volume? A partially-blocked or slow-opening/inop thermostat  
valve at the block might be preventing the radiator from getting the  
hot fluids in volume, and thus the sensors for the radiator aren't  
reading it as too hot (just a thought). Taking a kitchen probe or  
laser/infra-red temp probe to the radiator and hoses should tell you  
definitively (a hand would probably tell you too, but I won't suggest  
you potentially burn yourself to find out).

Good Luck!
=-= Marc Glasgow

On Aug 28, 2006, at 6:36 AM, James Whitehouse wrote:
>
> - Engine waits until it's one mark beyond the 'normal' temp mark  
> (the more
> distinctive 'centre' thicker splodge of white mark on the dash),  
> then both
> fans kick in (full speed?) for about 20 seconds. They then turn off  
> again
> without having cooled the engine.
>
> I'm guessing that a) first speed fans aren't working, as I remember  
> them
> kicking in before that mark, and staying on longer, then 'speed 2'  
> kicking
> in when sitting in traffic for ages, bringing it down to 'speed 1'  
> temp.
>
> If anyone has info, or fault finding for this system, could you let  
> me know
> please?
>
> TIA
>
> James