Another fan clutch option

I stumbled across something that may be of interest to those of us that use mechanical fans.I run a Hayden 2747 fan clutch, and a modified black fan on my '87 5.3 engine.Quite happy with it.Strangely enough, I use the same Hayden P/no in my SBC equipped old truck.
It seemed to be weakening, and I found in my garage another fan clutch with the same bolt pattern, and a little deeper , but it fitted my engine. I suspect it is a Hayden 2797.
The 2747 is described as a H.D. The 2797 is severe duty, and moves some serious air. Both fan clutches appear to be bolt-on items for our Jag engines, as long as the black modified fan blade is used. The photo shows the difference.

1 Like

Does the severe duty clutch disangage when cold? I read somewhere that if the fan is too small ( has too little resistance while spinning) it will not disangage.
I’m currently running with Hayden 2747 HD cluch and I’m quite happy with a cooling but when engine is hot and if I rev in above 3000rpm it sonds like cessna flying by :)))

Most XJ-S owners would rather err on the side of too much air.

You raise an interesting point, and I don’t know the answer.I used the severe duty clutch on my truck, with an 18 inch metal seven bladed fan. It was noisy, so I suppose on a quiet Jaguar engine with a lightweight fan it might sound like a turboprop taking off. What got my attention was that the bolt pattern and overall dimensions would allow the severe job to work on a SBC and a Jag! I too use the 2747, aluminum rad- runs cool.
Kirbert, I think so too!

I am running 2665, which is what Hayden catalog says should be fitted on all Jag v12s:

The first several pages of the catalog describe the difference between Standard, Heavy-duty and Severe-duty clutches. I get it that the Standard spins slower, but I am not sure I understand what the reference to “lighter pitch” and “deeper pitch” fans means.

I’ve seen other post on the use of Heavy-duty fan clutches. Can you (or someone else) explains the rationale for going H.D.?


Hi Steve,
I have always thought of “Pitch” being measured in degrees, but it seems Hayden is comparing the width of the blades. The Jag black fan blade is about three inches wide. (Lighter) The blade width on my old truck is four and a half inches wide. (Deeper) Perhaps there is an aeronautical engineer out there that can explain things more clearly. (or accurately!)
When I started learning about V12 cooling problems, I had a cracked yellow fan, and a partially plugged up radiator, and the climate control flat out did not do anything. I read all I could about good cooling, and replaced the fan blade. At the same time I replaced the almost thirty year old clutch, and got the H.D. version to increase the airflow. Anything that would help seemed to me to be worthwhile.I have been very happy with the results, and don’t notice any fan noise. I suppose that if my cooling system were in perfect condition the 2665 O.E. fan clutch would be adequate. The P.O. had replaced the alternator just before my ownership, so I did not want to make the change to electric fans. Hedging my bet? Perhaps.

Thanks for the quick response Dave. The 1994 and 1995 v12 with the 6.0L V12 have the black fan from the factory, no concern about the width of the blade for us.
I am curious to hear more opinions. The link to the PDF I provided earlier says that Severe and Heavy duty can be used interchangeably, but no such mention for Standard.

For full disclosure - I am running 100% stock set-up and have no cooling issues whatsoever - the reason for me asking is purely academic.

It’s simple enough, I think. These fan clutches are standardized to some extent, which is why we’ve found several that will fit the Jaguar V12. Thing is, though, the applications vary widely, notably in the amount of torque it takes to spin the fan at some specified RPM, say 2000. Larger diameter, more blades, broader blades, and steeper pitch all increase the amount of torque needed to spin that fan. Presumably the clutches are designed to provide the specified torque, allowing the fan to “slip” a bit at higher engine RPM to reduce energy consumption and noise.

So, if you take a fan clutch that was designed for the relatively lame fan in the XJ-S and bolt it into your big honkin’ truck with the 18" blade, it might fit but it may not drive the fan as fast as intended and the cooling efficiency of the radiator may be compromised. Conversely, if you install a fan clutch intended for the big honkin’ truck into the XJ-S, it’s liable to drive the fan much faster than intended. That wastes energy (fuel), makes noise, and may overstress the fan blade.

Like I said above, XJ-S owners will always choose to err on the side of too much air. This is not necessarily a good idea, though. The problems with this cooling system occur when the fan clutch malfunctions, not because it’s not heavy enough. The problems typically occur at idle or in stop-and-go traffic where the engine is turning slowly and where any fan clutch should be driving the fan at nearly engine speed; none of them, heavy duty or whatever, should be slipping significantly at such low speeds. All things considered, I’d recommend a regular-duty fan clutch for the XJ-S.

The problem with a fan clutch, IMHO, is the difficulty in determining whether or not they are malfunctioning. Even if the thermoviscous fluid inside has all drained out and the clutch is basically freewheeling, it still looks like it’s turning the fan when you peek under the hood. It takes either a significant level of experience or some really elaborate testing procedure to confirm that it is in fact working as intended. Failing that, I’d recommend owners simply replace the #$%^ thing every five years or so just to make sure. It’s simply too important to wait until symptoms convince you it’s time to take action.

Note: I looked under the hood of an XJ6 once and noticed that someone had drilled a couple of holes clean through the fan clutch and installed bolts through it, hence totally locking it up. That’s one way to make sure it turns the fan, I suppose, but I don’t recommend it – especially with an OEM white/yellow fan with a few cracks in it.

Finally, remember that my long-standing recommendation is to rip that belt-driven fan outta there and toss it over the hedge. Go to your friendly neighborhood junkyard and buy a 16" electric fan assembly (with shroud) out of a Chevy; it should cost about $25, and any junkyard worth its salt will actually guarantee it for a few days until you can confirm it’s in working order. Assemble it all onto your existing fan shroud, or fab a new one. Add a temperature switch of some sort and possibly a relay and some wiring, and you’re done. Electric fans flow less than belt-driven fans at high speed but they flow more at idle where it’s most needed. Most importantly, if you look at it and it’s running, there’s no question it’s working.

Steve, did you open up the rear wing for an intake to cool the rear inbord brakes??
Your opinion on Flex Fan ala B. Emden?

Best, JW

Hi .
I have xj12c 5.3v12. I replaced the aluminum radiator, the fan clutch, the fan and the thermostat. Nevertheless, after 30 minutes of idling (rpm650) it goes up to 100 degrees. However, if the fan turns quickly by stepping on the accelerator, the temperature goes down.
I read your article about Hayden 2747 fan clutch.
Will it be more powerful than normal clutch in idling time (rpm650) if I replace it with 2747?

The problem of overheating in my car has not been solved for a long time. Please give me an advice.
Thank you in advance.

Your electric auxiliary fan should come on before the coolant hits 100C.

I’ve put the Hayden 2665 Standard and it’s adequate indeed.
The mechanical fan does not do much on idle and a heavier duty clutch will have almost no difference while idling, there is where you need the Aux. electric fan.
As Greg said, it should come on way before the temp goes up to 100°C, rather around 90°C.

Well at 650rpm your mechanical fan is turning at about 80% of that so your at 550rpm on the fan, according to the Hayden site the heaver clutches turn at an higher % with the HD being at 90%. That is why giving it some gas helps with fan speed and cooling, Your electric fan should be really starting at 85-90c to cover idling, the rpm’s on the engine being to low. Like Kirbert I think the electric fan is what needs looking at. I went with a Spal aux fan to replace the stock electric fan, I got the high amp one and drive it with a big amp relay connected straight to the battery with a 14g cable. In town it comes on at about 195f and will cycle on and off, and moving even 20mph makes a difference. Also your shroud needs to have good pliable flaps under the electric fan and the foam that separate’s the big fan side from the electric side must be there or it will not pull air through the radiator. Here is my stock and big fan shrouds when I switched them out.

Not sure what your XJ12C shroud looks like but it should be similar, fans at

My lump 's E fans keep temp’s a 90 F. usually off unless after a bit of idle or in slow traffic,.

In my book, just right .


Hosung, generally speaking, a H.D. fan clutch will move a little more air than a standard duty clutch. Whether that alone would solve your overheating is unlikely, IMHO.
You say you replaced “the thermostat.” Do you mean “thermostats?”
I agree with the other guy’s responses, too.

Remember, the fan switch isn’t looking at the same temp that the temp gauge is. The temp gauge is looking at the hot side of the circuit, and the fan switch is looking at the cold side. So even the same fan switch will come on at different engine temps depending on the temp rise and fall around the circuit. For example: If your radiator is 90% plugged up so coolant is only oozing through it at a trickle, the fan switch will see very cool coolant indeed and will never come on – despite the fact the engine is dropping valve seats.

I know I’ve posted this elsewhere, but these details are I think helpful.

Like Kirby said, the aux fan thermostat measures coolant coming out of radiator-

OEM original aux fan thermostat: 197F ON, 180F OFF (92C/82C)
For me, this translated to approx temps at the rails where my thermostats are to 205F ON, 195F OFF (96C/90C)
Cooler aux fan thermostat: 190F ON, 174F OFF (88C/79C)
For me, this translated to approx temps at the rails where my thermostats are to 200F ON, 190F OFF. (93C/88C)

Thank you for your answer.
I didn’t see the auxiliary fan running even when the engine was overheating.
So I put the thermostats aside so that the auxiliary fan could operate. now it works.

The OEM auxiliary fan thermostat turns on at 92C and turns off at 82C. :thinking:
I thought it was related to the air conditioner because the auxiliary fan was not working.

Thank you for your detailed answer.
I’m going to replace my current electronic fan. I think the current fan is not doing its job.
But I don’t know which product is the most suitable. Can you recommend me the brand or size of the fan? I wonder if I should consider the speed of the fans or the CFM rating(?).
I attach a photo of mine.

My car is replaced by an aluminum radiator.
I’ve never seen the auxiliary fan running, despite the engine’s temperature rising.
So I’m going to replace the auxiliary fan.
Do you recommend any other way to lower the temperature while idling?