Thermo fan switch on/off temps

I have twin thermo fans on a 1982 4.2L xj6 s3. It currently has a thermo fan switch that comes on at 100 and goes off at 95. I think this is a little too high, so I’m thinking of changing to 95 and 90. What temps would forum members recommend for the switch?

Maybe 90 is too close to the thermostat temp??

go with what you’re comfortable with, i opted for a in hose capillary thermal sensor switch with a temperature setting dial so i can set to lower temp summer & higher for winter, personally the closer to thermostat setting the better, achieving what the thermostat is designed to do, open up so water can be cooled.

Is there a chance of cavitation if off temp is near the thermostat temp?

i was suggesting the on temp…keep in mind that the sensor & gauge are an indication not guaranteed to be 100% accurate, if you’re curious i use a hand held laser temp sensor to check all areas of the motor including the radiator…very handy
90C thermostat is the recommended temp for yours

FWIW I think the switch is intended to trigger “on” at 94ºC. Can’t remember of “off” spec. So yours seems a bit on the high side.

Does your coolant temp often hit 100?


The fans are not for cooling, but to prevent overheating, so as long as they come on before the car boils over, there is no need fiddle with it. I would be comfortable with 100->95°.
100 is not near boiling, and it shouldn’t reach 100 often.

OK …so what do the fans actually do if they don’t perform a cooling function

As David says, Tom - to prevent overheating…:slight_smile:

In theory, and ideally, the engine temp is controlled by the thermostat - and should not then exceed the thermostat temp. There is no danger to the engine at ‘reasonably’ higher temp - which may occur occasionally…

The advantage of the electric fans

[quote=“Tom_Thomas, post:7, topic:438393, full:true”]

…is that they only come on when needed; under normal driving the air flow is sufficient for cooling - and the fans are not needed. This is the advantage of electric fans; drastic noise reduction - as opposed to the mechanical fans that makes noise all the time…

If Zenit’s fans comes on, engine temps high enough to start fans, regularly/unreasonably often; he likely has a cooling problem. Either a stuck thermostat or somethin else - which should be pursued. Otherwise he is OK as is - his call…

xj6 85 Sov Europe (UK/NZ)

I think that the 100/95°C is what it should be.
If you go to 95/90°C with a 90°C thermostat, you risk that the fans will never turn off, it’s too close to the thermostat rating.

As mentioned, the aux fan is there to help in conditions when there is not enough air flow, i.e. very slow driving or the car is not moving.
It also gives a cooling boost when the AC is on, but that is controlled normally via the AC relay and not the fan switch.

Hopefully quite a bit before boiling over. No sense waiting until the last minute :slight_smile:

Right. My concern is thinking that changing e-fan triggers points will solve a more deeply rooted problem.

That would be on a V12 car. The six cylinder Series IIIs did not have a tie-in between the e-fans and the climate control.


I never had AC but it might make sense to hardwire the fan to the AC clutch like in the XJ12 and E Types.

Speaking of E Types, the fans rarely come on and if so it is in traffic or after a short stop, I forgot to plug the fans in recently, shame on me, and it took quite a bit of city driving on a very hot day to get it to boil over which of course only happens after shutdown, so yes, you want them to come on before the boilover temperature is reached, which will be above any normal operating temperature and far above the thermostat rating. Nice sentence, David. So, 100° is what you want - it wouldn’t yet boil over when you parked it at 100°.

Thanks for all the replies.
David, that makes sense to me. Here is a bit of a history and observations of the thermo fans.

When the engine was rebuilt many years ago, the mechanic changed from the factory fan to twin thermo fans (from some Ford model). They wired them so that there was no thermo fans running until the thermo switch cut in. And when they switched on it was in high speed mode.
I noticed a problem with this setup when I use to drive home and the last 2km was all uphill. The fans would start when I was half way up the hill, the temp would be around the 95/100 range. But the thermo fans where unable to cool the motor any further as I went up the last half of the hill. It would result in some coolant overflowing out the expansion chamber.
So I put a manual switch inside the car to turn on the thermo fans when I wanted, in times where the thermo switch had not cut in. I manual switched the fans on at the bottom of the hill and there was no problem.

I only recently found out these Ford thermo fans have low and high speed modes. So it is rewired so that low speed is always on and it switches to high speed with the 100/95 switch. And that has been running fine for sometime. But recently, the engine has been getting hotter in certain conditions, such hot days. So, I now wonder if there is an underlying problem (possibly intermittent) as Frank suggested. I did have a welsh plug with a small leak, so I put some Risolone to seal it.

I was advised by a reliable mechanic that it is safe to use but wonder if that could of made the thermostat a bit sticky. On mild days the temp sits roughly between 85 and 90 and seems fine.

I’m going to continue monitor it before thinking changing anything.

The coolant level reflects the highest engine temp reached, Zenit - adding coolant; the process would be repeated…

That the fans did not drop the temp at the last leg of your uphill may just be too short time?

Personally I don’t like such additives - over time they have a tendency to clog up elsewhere unwanted…

‘Our’ Jaguars are not exactly overcooled with age - usually not alarming, but monitoring such changing behaviour, as you mention, does not come amiss. It’s more a matter of ‘how’ - and what to look for…

Running the fans at all times is of course not harmful, but it defeats the prime purpose of electric fans - noise reduction.

xj6 85 Sov Europe (UK/NZ)

Replace all of the welch plugs - don’t compromise using stop-leak snake oil.

Nothing wrong with that but if one plug is leaking, they will all be getting thin so eventually you have to do them properly. The rislone is not good enough in the long run. But it does help! Some plugs are easy to reach, others are not, I know.

The little coolant seeping out after the uphill stretch, is that just from further expansion or actually boiling over?
The expansion tank finds its own level, I‘m sure you know that. When it gets warmer it will find a new slightly lower level.
Of course if you have to shut the engine off without a cooldown lap it is more likely to boil over even if it wouldn’t be a problem in normal operation.

Yes, I know the Risolone is just a temporary fix. Not able to replace the welsh plugs at the moment, this just gives me a bit of time before I do it. Both manifolds have to come off and I assume there will be one at the back of the motor, so the box has to be pulled back.

The coolant seeping out was from further expansion, as there wasn’t a cooldown lap after the hill climb. That got fix when the fans where wired to always run on low speed. When this was done the engine temp was good all the way uphill and high speed mode was not needed.

No, that’s a plate, but the starter is in the way (the small plug can stay, that’s just an oil return).
You could put a second switch in place that goes to low speed just above operating temperature. I’d think they sell adjustable switches? Then keep decreasing the set temperature until they only run when needed, that would make sense to me. You don’t need them on all the time, then again if it works it works!


As Frank pointed out, a couple of kms is not enough time for the fans to cool down the engine, sothe fans does not seems to be an issue.

That on the other hand is an issue.

The crucial question is: at what speed ?
If above 60 km/h, fans or no fans makes no difference, there is enough air flow and the engine should stay cool regardless, the problem is elsewhere, most probably or faulty thermostats.
If below 50 km/h, again a clogged radiator could be the problem, but otherwise maybe too much coolant? What is the coolant level after the engine cools down?
Does your pressure cap seal properly?

FYI, on my V12 I had the exact same symptoms and it was the radiator.