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Unread 05-07-2015, 09:52 PM   #1
jason280
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4.0 mechanical fan differences?

'94 YJ started climbing a little on the temp gauge the other day while idling, got as high as 225-235 while sitting in 85* weather. I was able to get it to cool back below 200 by running the heater on high, and ran at correct temp on the road. Checked it once I got home, and it began creeping up again while idling. Popped the hood, and noticed the mechanical fan was barely turning. I checked the fan once the engine was completely cool, and it was somewhat easy to rotate. I couldn't free turn it a full rotation, but it felt "loose".

Figured it was a bad fan clutch, so I went ahead and pulled the fan assembly. I have a couple mechanical fans off '93 & '94 Grand Cherokees, and they are definitely more difficult to free turn. Decided I would go ahead and simply drop in one of the GC assemblies, and noticed they are set up completely different than the YJ fan. The GC fan has 7 evenly spaced blades, whereas the YJ has 5 blades spaced in an odd configuration (certainly not evenly spaced).

Any reason for the difference? I assume it would be for the GC to move more CFM for a trans cooler, but I honestly have no idea. Any reason the GC fan wouldn't work in the YJ?

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Unread 05-07-2015, 09:59 PM   #2
Chrisnvegas
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I got a NOS Mopar (Delco) fan clutch on e-bay for 15 bucks. Still new in the box. No need to re-invent the wheel
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Unread 05-07-2015, 11:07 PM   #3
jason280
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Not sure what you mean, but the cheapest I have found a new fan clutch is around $40-45, and would require me driving into town to pick it up....I have two GC fans that were basically free, are sitting at my disposal, and are ready to be installed right now.
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Unread 05-07-2015, 11:39 PM   #4
Drednot
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I recall reading about people retrofitting grand cherokee fans. They locked up a little sooner and had more blades like you mentioned. It was like a poor mans heavy duty cooling.

If it fits, and turns the same direction, go ahead and mount it.

Let us know if it fixes your problem.

I got an NOS , OEM fan clutch off ebay for $35 shipped. If the GC fan doesnt work out.

Fan clutches are one of those parts where OEM is way better than aftermarket
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Unread 05-08-2015, 06:43 AM   #5
jason280
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The fan fits fine, bolts up just like the stock YJ fan. I cannot imagine why it wouldn't work, as it came off an identical year 4.0L (just out of a GC instead of a YJ). Blade rotation and bend is the same, big difference is simply the number of blades. Definitely seems to move a decent amount of air, and maintained temp around 195*. Of course, I really won't know if there is any real change until I drive it a bit in warmer weather. As for now, seems to be doing OK.

Here are a few comparison pics of the fans.




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Unread 05-08-2015, 06:53 AM   #6
Chrisnvegas
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Try it.
You can see the oil leaked out of your old clutch.
The new one looks like it could be close too
Check it by the bi-metal spring. Looks oily.
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Unread 05-08-2015, 06:53 AM   #7
Que89YJ
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Looking at the blade pitch they are the same. It's all about moving air. If the pitch is the same direction it should work. The fans don't move air the same way. The GC has a supplemental electric fan too. The radiator core is a different size and shape too. Run it and watch the gage. As long as you air moving air it should be fine.
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Unread 05-08-2015, 07:12 AM   #8
jason280
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Quote:
Run it and watch the gage. As long as you air moving air it should be fine.
That's what I was thinking. Comparing the two, its a little difficult to determine which would move more air. The GC fan has smaller blades, but more of them. The YJ design is a little interesting, and I'm curious why the blades are positioned the way they are. I assume its to maximize efficiency, but it really is a unique way of spreading them out.

If it doesn't work, which I am still not sure why it wouldn't, I'm really not out anything (other than the time necessary to swap over). I can always go back with an OEM style, but I really won't know anything until I get to drive it on a few warm days.
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Unread 05-11-2015, 12:38 PM   #9
jason280
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Well, so far so good....its 91* today, and temp gauge has stayed around 185* (driving and at idle). Highest I have seen the gauge reach is just under 210*, but that was only very briefly. I'm keeping my fingers crossed, but looks like the GC fan is working out so far.
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Unread 05-11-2015, 09:09 PM   #10
Drednot
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Quote:
Originally Posted by jason280 View Post
The YJ design is a little interesting, and I'm curious why the blades are positioned the way they are. I assume its to maximize efficiency, but it really is a unique way of spreading them out.
They are oddly spaced to quiet the noise the fan makes. The spacing makes out of sync pulses that dont harmonize and get loud
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Unread 05-11-2015, 09:26 PM   #11
Chrisnvegas
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Cool. I always wondered too.
How does the fan not vibrate though? It looks like it would be imbalanced with odd blade placement.
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Unread 05-11-2015, 09:31 PM   #12
mike134
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Quote:
Originally Posted by Drednot View Post
They are oddly spaced to quiet the noise the fan makes. The spacing makes out of sync pulses that dont harmonize and get loud
that's interesting!

I even gave you a thumbs up.
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Unread 05-11-2015, 10:14 PM   #13
imstillatwork
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Quote:
Originally Posted by Drednot View Post
They are oddly spaced to quiet the noise the fan makes. The spacing makes out of sync pulses that dont harmonize and get loud
It also makes it much more convenient to unbolt parts like the thermostat housing or power steering pump with bigger spaces between blades.
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Unread 05-12-2015, 12:45 AM   #14
Drednot
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Quote:
Originally Posted by Chrisnvegas View Post
Cool. I always wondered too.
How does the fan not vibrate though? It looks like it would be imbalanced with odd blade placement.
They have 5 cylinder engines, so I'm sure they can balance them out.
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Unread 05-12-2015, 06:08 AM   #15
fishadventure
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Quote:
Originally Posted by Drednot
They are oddly spaced to quiet the noise the fan makes. The spacing makes out of sync pulses that dont harmonize and get loud
I think it partly has to do with a brilliant use of the Doppler effect.
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