Fan Clutch

Newbe here... 1990 cherokee, set up for offroading, but needs some work. Owned for a month.

This is my first car with a fan clutch, most my prior cars didnt even have a radiator. (classic vws, porsche) Have some watercooled cars, a triumph, vanagon wasserboxer, and a 914 with a Buick V6, they all have electric fans on their radiators, otherwise most my cars are aircooled.

how does one determine if the fan clutch is operating correctly? I plan to off road, slow speeds, and want to make sure the fan clutch is up to the task of cooling.

At what rpm does the fan clutch kick in and fully engage, disengage?
How does one determine correct operation? Know if it is slipping too much or not?

I read about a ZJ clutch as an upgrade for offroading, how is that clutch different in regards to being fully engaged, specifications?

Sorry for all the questions, this is all new to me.

thanks

keep jeeping!

The fan clutch operates off of temperature, rather than engine speed. When the engine temperature increases, a bimetal strip (similar to a thermostat in your house) activates a viscous fluid coupling inside the fan clutch. It's a bit of JFM, but in a nutshell, at lower engine temps, the fan clutch slips a bit so as not to unnecessarily suck up engine power. At higher temps, it slips less so the mechanical fan can get some air flowing past the engine. A quick, easy, stinky way to test them is to just let the engine get up to operating temp and then let it idle for 5 minutes or so. Shut it down and try turning the mechanical fan by hand. It should have some resistance to it because of the fan clutch.

Really though, a bone stock XJ should have no problems with over-heating if everything's up to par, so if you're worried about overheating, I'd start with a basic coolant flush. It's also a great time to replace the cap, T-stat (stay away from the low temp ones, a bone-stock 195 unit should be fine), hoses, clamps, fan shroud and anything else that might need attention. If you want to get really crazy, most XJ's have a secondary electric fan that normally only kicks on when the A/C is on or, IIRC, the engine temp gets too high. If you want to do it the easy/lazy way, you can simply wire a switch to ground on the low-amp side ground circuit to turn the fan on (this will set a CEL and code on an OBDII XJ, not sure about a Renix though.) but there's also plenty of write-ups and info on here about how to wire in a second relay that will prevent that from happening.

Watch this video.

https://www.youtube.com/watch?v=pbhNbQ_YHBc

I was not having any problems in my '00 as far as cooling goes but when I saw this video replaced the fan clutch.

Thanks both of you for the info. Ill test my jeeps fan as soon as I get her running again, power steering pump needs to installed right now.

I did some interweb searching and found there are three main types of fan clutches

one is strictly a viscous clutch where the fluid gets thicker with increasing temps, another is the one like you have described above, a thermoswitch that opens a valve to allow the fluid to circulate and run the fan when hot, the third is on newer car, electromechancial valve controlled by the ecu that operates a clutch, much loke an AC clutch.


Thanks again!

If you want to lose the fan clutch all together you can and go with a three electric fan setup and you’ll lose the drag that the clutch fan puts on the motor. Plus the fan only puts out low cfm’s at best.

Mechanical fan will flow as much if not more than a 3fan electric setup. I know electric setups don't work in the desert when it's 120 outside. Since you have a 90 i would upgrade the original electric fan with one from a 2001. Flows more and operates quieter.

Can also get a adjustable fan controller. I got mine from the parts store. Think it was a Hayden. Use a thermostat housing from a 91 or newer and screw the sensor into the port on the tstat housing.

Also switch to the newer style radiator if you are still running that janky coolant bottle.

cwyman6 said:

Mechanical fan will flow as much if not more than a 3fan electric setup. I know electric setups don't work in the desert when it's 120 outside. Since you have a 90 i would upgrade the original electric fan with one from a 2001. Flows more and operates quieter.

Can also get a adjustable fan controller. I got mine from the parts store. Think it was a Hayden. Use a thermostat housing from a 91 or newer and screw the sensor into the port on the tstat housing.

Also switch to the newer style radiator if you are still running that janky coolant bottle.

Click to expand...

I agree. Well. If you want to keep it, you can keep the 90-down closed cooling system. I did and it works fine though if the coolant bottle fails it's getting swapped to a 91+ open cooling system and I'll never look back.

The mechanical fan setup flows way, way more air than any aftermarket electric fan setup, and it doesn't load your electrical system down necessitating a larger alternator. Electric fans typically *start* at around 40 amps, even for the stock one, and go up from there. That's 34% of the available power from a stock alternator gone, right off the bat. The only possible justifications for changing to an electric are so you can turn the fan off during deep water crossings (I've never really had an issue with just letting it spin... water cooling the fan clutch off reduces the amount of force applied anyways) or if you need every single last horsepower at the drag strip.

As for replacement - here are some typical symptoms and their causes:
overcools when moving quickly - thermostat missing or stuck open
overheats when parked or moving slowly, but does not when moving >20mph: fan clutch worn out, fan shroud missing or damaged, radiator fins corroded off to the point that it doesn't have enough surface area to get rid of the heat
overheats at low RPMs, but not at high RPMs (under low load): water pump impeller corroded badly

These can be combined in various ways... it's not always clearcut, for example my '88 overcools right now even with a brand new fan clutch, thermostat, and radiator and no shroud at all. But if it's got more than 100k or so on it and is overheating when moving slowly or parked, I'd start by checking the fan clutch for sure and maybe toss a spare on to see if it gets better, then replace it if it does. There are a few upgrades people like to run (such as the 4.0L ZJ fan clutch) but I'm not overly convinced they're necessary. Run em if you want, all it can really do is cause a mild fuel economy hit.

Fan clutches seem to have short lives in hot climates.
The ZJ clutches work but are engaged and noisy all the time and will cost a 1-2 MPG drop, in freeway driving.
For a trail only machine, they work well. Not so good for a DD.