Hi Jimmy

Mines an 89, so I very much doubt if our Cars are wired up the same.

Thanks for responding anyway.

There have been several responses to your post, that were about different years than yours. I apologize if my response was inappropriate.
Also...this 89, is it not the 90 that is in your signature?
...Jimmy

Hi Jimmy

Not inappropriate at all, if anything its my fault, I didn't look at your Signature properly and as soon as I saw 94, I was thinking AJ16 :icon_doh:

What would you gain? The fan automatically goes on/off as per design. Did Jag get something wrong?

No but sometimes it would be handy to have the Electric Fan running as well, hence the desire for a manual override switch.

Why, other than a good way of draining the battery?

Running a fan after shutdown accomplishes very little beyond reducing the temperature of the coolant in the radiator as the coolant pump is not operating to circulate the liquid.

Well maybe it's just because, I want my Car the way I want it,
whether or not its right or wrong in anyone else's eyes.

Your 1990 should run the Aux fan when the engine shuts down until coolant temperature drops below about 90°C. You could easily add a manual over ride switch to turn on the relay, but the danger here is forgetting to turn it off and having a flat battery. Like Grant said you will not gain anything by running the aux fan longer.

If you want to cool the bay vents in the bonnet would be a better option.

I have to agree with this statement.


I also installed a lower 82/77 degree thermostat switch and it works very well. On days that I am running stop and go, the electric fan runs and when I get home it seems to run for an additional 2-3 minutes after shutdown. Not all the time but only on warmer/hot days.

Rather than speculation, let me just say that water is 24.1 times more thermally conductive than air. I would be interested to gather the actual data on the car, but I can assure you that cooling the remaining hot coolant in the radiator DOES affect the car cooling down. The remaining coolant will dissipate heat into that coolant much faster than the air will absorb heat coming off the engine.

Also, my electric fans draws about 11 amps. Average stereo is probably 5-10 amps. Anyone else sat in their car before with the engine off playing the radio? Does anyone get freaked out their battery will die? No, you sit outside waiting to pick someone up and listen to the radio while you play on your phone. The battery recharges when you drive it.

So why on earth would I care about a fan running for 3 minutes that pulls the same power?

Running a radiator fan for three minutes will do little to cool anything worth the effort.

What effort? The system is entirely automated. By that logic you should do away with the fan altogether because you only sit at red light for 30 seconds, so it is hardly worth the "effort."

The fan is not on a timer, it is on a temp switch, so I don't even know how that thought could occur. It is verifiable that is is cooling because it shuts off solely because it cooled off. Btw, I know you mean to make it sound preposterous because 3 minutes is a small number, making it sound silly, but on my fan that is 7000-8000 cubic feet of air, which is more than the size of an entire railway car.

This is just like that other thread where everyone is discussing improving airflow in the engine rather than improving the cooling system. Somehow you guys are okay with the engineers' designs and decisions in not making the engine air cooled. You of course agree that it is a no brainer that this engine can't be air cooled, yet you continue to randomly select instances where you believe, simply because you feel like it, that liquid cooling is inferior.

The switch and fan are already setup. It is apparent in the design that this was included because it could be, not because it was necessarily essential. I'm not arguing that the system vastly improves cooling, but I am arguing against the claim that is has some negative impact overall. The system is already in place, uses a negligible amount of power, and does in fact aid post-shutdown cooling. It's only beneficial, so why does anyone argue against it?

The idea that it does nothing because coolant is no longer circulating is retarded, the laws of thermodynamics still apply. Everyone seems to agree on the conduction and radiation going on through the engine and into the air, but magically the cooler coolant doesn't do anything I guess?

The volume of heat conducted through the radiator hoses from the engine to the rad is minimal. It's similar to a domestic hot water tank. Just a few feet away from the tank itself, the temperature of the water inside the pipe is pretty close to ambient.

If the pump was circulating the coolant, that would be a different story.

I also question the logic of running the fan at all after shutdown, but that's a different story. The engine is well equipped to deal with heat soakback, so hastening the removal of that heat is of little value especially if it's done inefficiently.

Through the hoses to the engine? Are you serious? Do you understand that heat transfer is going on between the hot coolant and the cold coolant and from the engine to the coolant as it usually does? Maybe you meant "through the hoses" as in the coolant in the hoses, but you weren't clear.

The air a few feet away from the engine also isn't hot, do you take it that that means the air isn't cooling the engine either? Again you are using your intuition as "logic" rather than thermodynamics. There is absolutely heat transfer going on through the coolant and extra cooling was accomplished by doing next to nothing. Even if it is a small amount, it required little to no extra effort, so it is only beneficial.

Little value if done inefficiently? The circuit ALREADY exists, all it required was to design it with constantly 12v rather than ignition on power. How is that inefficient? They literally took the already designed system and simply let it automatically accomplish some cooling after shutdown. What it is so inefficient about that?

Again, you believe the engineers are totally competent and the engine can completely deal with post-shutdown heatsoak but you think they are morons for allowing the fan circuit to have constant power. I don't get it.

Yes, I meant the coolant in the hoses as well as the water in the domestic heater analogy.

They are the ones who determined at what water pump coolant temp the fan is turned off after engine shutdown. The suggestion here is that Jag got it wrong and the system needs to be improved. I am asking what those improvements would be and questioning at what cost. Again, the 3 extra minutes you suggested will achieve very little.

That's OK.

The average car stereo will draw less than 1amp unless you have it turned up very loud. The 1.2 is to allow for power loss inefficiencies.

10watts / 13.8v = 0.72amps * 1.2 = 0.87amps

10amps / 1.2 * 13.8 = 166 watts pretty loud in a small space

With average car speakers of 89db your SPL would be around 115db at 166watts, pretty loud and would be mighty uncomfortable. 110db is a chainsaw at 1m

While I don't think it is at all necessary, there is some merit to the idea. If the cooling system is cared for it shouldn't matter, but anyone that has had the system teetering on the edge of not working to it's fullest knows that the heat soak happens fast in a hot area.

Pop into the store for a soda and when you come back out the needle is way up into the gauge, so it would give the option of leaving the fan on while you are parking only temporarily.

Again, if the system is working well it will just cool off after driving for a bit, because the system should be able to "over-cool" the engine, thus the thermostat.

Right, my bad, I as thinking the power they generally are rated at/can use rather than what they would be using at a minimum.

Either way, turn the car off and leave the fans from ac/heat on, shut down and then put the windows up, etc etc my 90AH battery doesn't care about a fan running for 2 minutes.

All liquid cooled engines do that. It's called heat soakback. The coolant is no longer circulating to remove excess heat so it rises in temp to that of the engine components.

Blowing air through the rad with a stopped engine does little to cool the body of extremely hot liquid trapped inside the engine

You never implied a lower temp switch should be installed, you implied that the entire system accomplished nothing and was a waste of time.

Again the circuit was already there, no one is saying the fan being does at amazing job and cooling the car down post-shutdown, but it requires basically zero effort to set it up to accomplish some cooling and it obviously accomplishes some degree of cooling, so why not?

Do you know the stock opening and closing temps? Obviously if the fan turned off it cooled the system. I don't really care either way, but with so many XJS owners on a quest to reduce post shut-down heatsoak, it blows my mind that there is someone that would prefer the car to be hotter than it already is and thinks dismantling an automatic system that does some cooling is more beneficial than just leaving it alone.

So its "OK" to try to convince people on the forum of your opinion or advise them on topics even if you have no reason to believe whatever it is your are stating? What is the point in going about telling people things that may or may not be true? The whole point in the forum is to help people out. Why tell an owner the fan switch is a complete waste of time and does "virtually nothing" if you aren't sure of that?