Fan keeps running

[Jagdcat]

Hi everyone,

Newbie here. I have purchased a 1996 XJS Celebration; low miles. I am having it transported up to me from the auction house but the transporter chappie recently rang to advise me having driven it up the ramp and turned the engine off, the fans were still going 30 mins later.

It is due to go into a garage for re-comm towards the back end of next week but it would be good to be able to talk techie with the mech on the issue.

Feedback gratefully received.

JC

[AlastairCM]

I'll qualify this response with fact I'm not familiar with facelift cars.

But the pre-facelift cars have a bootstrap circuit built into the auxillary fan that keeps it powered post shutdown while the coolant remains above a certain temperature. The relay (during running and post shutdown) is operated by an on/off temperature switch that on pre-facelift cars is on the water pump elbow. If the ignition is turned off while the fan is already running and the coolant temperature switch is stating that the coolant is above a certain temperature then the bootstrap circuit continues to power the fan, if not the fan gets no power from the bootstrap circuit post shutdown.

I assume that the engine wasn't run extensively before it was put on the trailer such that the coolant was up to the temperature level where the auxilliary fan would switch on? If it was, this could just be the fan behaving normally if it took 30 mins post shutdown for the coolant to fall back below the level at which the temperature switch would shut the power off via the bootstrap circuit (on the basis the car would be stationary with no additional airflow).

If not perhaps there is a fault on the coolant temperature switch that is turning the fan on prematurely and engaging the bootstrap circuit post shutdown, or turning it on permenantly if the auxilliary fan runs form the moment the ignition is switched on.

Assuming the post facelift cars use the same system of course and someone hasn't tried any rewiring like bypassing the fan relay so the auxilliary fan is always getting power or installing a manual overide switch.

[Jagdcat]

Hi

Brilliant response. I shall print off and arm myself with this when I see the mech next week.

Thank you

JC

[oldtimer]

Alastair,
The vehicle has come from a auction house, possibly not attended too

Just a thought, what do you think the effect would be if the coolant in the cooling system was at a very low level?

[AlastairCM]

Yes it could heat up the remaining coolant and produce a quicker than expected response from the fan, but if it wasn't run beforehand, just enough to put it up on the trailer, then I doubt it would have time to get that hot to trip the fan switch even with low coolant.

Easy way to check would be to see if the coolant level warning light came on, but certainly another one to mention to the mechanic and a lot easier to remedy than trying to remove the temperature switch.

Could also be a faulty relay if it has managed to fail with the switch closed, but that would mean it was on permanently when the ignition is on.