Under the hood temperature and battery cooling fan

Hello all,
Among the many intriguing quirks cooked up by Jaguar engineers, the battery cooling fan stands tall on my list of favorites. It deserves to be resuscitated.

Also, I just got the battery box powder coated and it looks awesome, so it now has to be used :slight_smile:

Both the fan motor and the thermoswitch have gone belly up. Modern replacements should be easy to come by in the computer age when a lot of items in boxes need to be kept cool.

Would anyone know what the normal temperature range would be under the hood of a XJ6 so I can select a suitable thermoswitch?

Thank you.

Doesn’t that thing draw air from within the engine compartment? If so, its operation shouldn’t be based on engine compartment temp but rather battery temp – as in keeping the battery from getting too much hotter than the engine compartment.

Of course, if the fan draws cool air from outside, that’s another matter. I’d probably want it to run whenever the temp in there is over 100F.

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Yes the ambient atmosphere is the air in the engine compartment.

My thinking is that if the temperature in the battery box is say 5-10 Centigrades above the “normal” ambient atmosphere, then the fan should kick in.

100F is about 37C. This sounds about right. The manual says that the airbox flap door actuator system should be on (if it works) until 40C.

For the battery box I am thinking that the fan should kick in at 50C.

What causes the battery to run hotter than the engine compartment?

Exactly, that would be a question for Jaguar Engineers :slight_smile:

Charge and discharge cycles I believe.

Iirc in my 76 xj6. (Now long parked) the fan draws hot air out of the battery box and there is a rubber tube that attaches to the firewall for cool air to be pulled into the box. Not sure if the source of the cool air is from the air conditioning system or just outside air from the bulkyead vents.

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Aha! So is it drawing air from the wiper compartment? Because that should be at outdoor temps. That’d truly keep the battery cooler than the engine compartment.

If the fan is toast, seems like it’d be a simple matter to utilize a 12V computer fan. Not original, though.

You are right, I remember now, there is a now-impossible-to-find rubber pipe with a bifurcation that acts as drain for the scuttle vent and connects to the battery box!

So, assuming that I can emulate that pipe with a home made contraption (3D printing with the new soft materials?) I need to figure out the optimum temperature for battery operation and find the appropriate thermoswitch! :slight_smile:

And in my spare time, I will make mud bricks…

Not really. Mine actually was not working so I removed the whole thing and just attached the battery with a generic tie down. Used an optima and never had a problem.

Yes, that would be the simple logical solution if course :slight_smile: but where is the fun in that?

The unfindable bifurcated pipe, with a thermoswitch and a fan is so much more… Jaguar no? :wink:

The reason I would argue for 100F is that it’s high enough to keep the fan from coming on in all but the most miserably hot weather, but within range so V12 underhood temps will readily bring it on. Still, it’ll likely come on immediately when you start the car in a sunny parking lot.

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BTW, what’s up with your original fan? Motors can usually be refurbished.

Motor is not functioning. I need to investigate.

And that is the start of a nut and bolt rotisserie restoration😀

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Yep :slight_smile:
Although I was first investigating a noisy tappet on the exhaust side… then just like that…

Google Photos

The battery is heated both by the engine and charging/discharging, Eric…

Battery capacities are usually quoted at 20C (68F) - which may or may not reflect an ‘ideal’ battery temp. Certainly, at freezing temps discharge capacity drops quickly, reducing cranking current. The charging, with relatively low currents is little affected by temps…

However, the main enemies of the battery is large temp variations and certainly vibration. Whether a battery fan can reduce temp variations and prolong battery life is arguable - with the battery rattling about in the engine compartment.

A life expectancy is likely some 4 - 5 years - though it may perform OK for longer if the demands is not too great…

xj6 85 Sov Europe (UK/NZ)


On my Series II the air is drawn from the cowl vent. That thermal switch looks a whole lot more sophisticated than mine, (on the series II). On the Series II it’s nothing more than a by metal spring that when it warms up it pushes on a micro switch which intern fires up the fan. Like the termostat at my Grand parents house.
When I was a kid, I was fascinated by it. pulled off the wall to play with it. Back in the day when it was cool to play with mercury.

Got my ass kicked for breaking the thermostat.

So the Series III’s had battery boxes? Again, all of my XJ’s are rescue kitty from the crusher. No battery boxes here.

Can someone with the dead stock system in operating order please tell us how often the fan runs? Is it pretty much always running when the car is warmed up? Or does it only come on in extreme conditions, driving through the desert or something?

Not that I’ve ever seen, no.

Just the battery sitting on a tray with a hold-down system.

The V12s, at least some of them, used a hold-down that incorporated a small heat shield, as seen in this pic


One of those hangs on my shop wall, replaced indoors by a complex programable thermostat.

In the shop, it merely reads temperature!

A local AC contractor enjpys smashing replaced thermostats to get the mercury. why???

If you are in for gold mining and smelting, Mercury is useful…