C2 fan clutch - how?

[Tom Merkel]

To follow up on the C2 overheating thread, just how is the fan clutch supposed to operate?

For example, when the motor is cool (as in fresh start), it should free-wheel freely, right? By hand, you should be able to turn it.

Then, when motor up-to-temp (as in driven for an hour or so), it should turn by hand much more difficultly (or not at all), right?

I thought this might be two easy experiments to run, since I am observing what seems to be pretty close to what the gentleman recently described - 200+ at idle; 180 steady on the road.

FYI, my fan shroud hasn't been tampered with in any way, and I'm running a brand new DeWitt's AL in my '67 L-79. Coolant level and timing all OK.

Thanks for your comments.

Tom

[Jim T.]

Re: C2 fan clutch - how?

Tom I have a original owner 68 L79 that I replaced the aluminum radiator in 73-74 with a copper brass replacement because of the cost of a new aluminum one and it has continued to serve my driving needs. The fan clutch was replaced with one from Auto Zone about 20 years ago. No air conditioning. No problems at idle and on a hot, hot day running from Bowling Green down I65 and then west on I40 at 70-75 the temp got up to 210, dropped the speed and the temp would come down some. Have 3:70:1 rear ratio, so turning some rpms at 75. I am using 10 to 12 degrees BTDC on initial timing and the vacuum advance is original and works.
Posts from Duke about vacuum advance being absent at idle warn about how a engine will run at a higher temp.
Another helper in my engine temp is that coolant continiously flows through my heater core as it originally designed.
Appears to me that your L79 is generating more heat at idle or your cooling system is deficient in air flow thorough the radiator to cool the heated coolant from the engine.

[Scott Butville]

Re: C2 fan clutch - how?

A quick test of the fan clutch is to put a piece of masking tape on the end of one of the fan blades, get the motor hot as it gets, then shut it off and try to count the number of rotations the fan makes. Generally speaking, if it makes more than five revolutions after the motor stops, the clutch is suspect. Make sure it is fully up to temperature for a while to get the most accurate results.

[Craig S.]

Re: C2 fan clutch - how?

Tom - I have never seen the fan clutches exhibit a hard lock like you describe when the engines are off, but a well functioning unit will cycle when running, and you can sometimes hear the fan couple up and roar more. Conceptually, what you describe is happening though, it just doesn't happen in a constant state like you describe. It is pretty hard to guage a fan clutch's performance manually by feeling the internal friction. If you have an older unit, save it for judging, and buy the current GM supplied Eaton unit through an online supplier like Scoggin Dickey or gmpartsdirect, they list for around $160-170, and you can get them for around $115 with shipping. As Jim mentions, there are a multitude of other items that influence the temperature, timing being a big one to investigate. Also, you didn't mention your engine, but has it been modified or is it stock, is it overbored much, etc. Sometimes with larger overbores it's hard to keep them cool at idle, too much heat transfer to the coolant through the thinner cylinder walls. By using a DeWitts, you have that issue knocked off already, so I would look first at the low rpm timing, and follow some of Dukes posts on setup in the archives...Craig

[Tom Merkel]

Re: C2 fan clutch - how?

Scott - That's a unique approach you offered. Never thought of that one.

Get the engine up to "above normal", i.e. 210-ish, and shut it down from an idle condition, while counting the revs the clutch and fan make until it stops.

You suggest 5 revs. That would seem to me to be a case where the clutch is fully engaged, and would stop just about when the fan shaft stops, which itself will stop when the engine stops.

I'll give it a try, and advise.

Someone also suggested getting a replacement, and put the original (it is) on the shelf.

Thanks.

Tom