1969 clutch fan question

[Mark A.]

Hi Guys,
Where can I find an explanation of the exact way a fan clutch works? Also what is the best way to test a used one to see if it is good.? Mine has good resistance the first few times you rotate it, but as you keep rotating it the resistance goes away. I am assuming that means mine needs to be rebuilt. Also I am looking for one for my 69 390hp, with build date of Sep. 69. Thanks for the help. Mark

[Joe L.]

Re: 1969 clutch fan question

Hi Guys,
Where can I find an explanation of the exact way a fan clutch works? Also what is the best way to test a used one to see if it is good.? Mine has good resistance the first few times you rotate it, but as you keep rotating it the resistance goes away. I am assuming that means mine needs to be rebuilt. Also I am looking for one for my 69 390hp, with build date of Sep. 69. Thanks for the help. Mark

Mark-----

Does the clutch look like the one pictured below? If not, it's probably not original to the engine.

[Jack H.]

Re: 1969 clutch fan question

There have been a few articles on how fan clutches work. Basically, they're filled with oil and rotation causes the oil to move between a storage reservoir and the race pathway inside the clutch. The more oil that's in the reservoir (vs the race), the more relative slip you get between drive shaft and the outer ring of the clutch where the fan mounts.

Also, the clutch has a bi-metal actuated control valve that's temperature controlled. This valve forms a controlled 'leak' between the internal reservoir and the race passage. So, the degree of slip is a function of BOTH rotational RPM and ambient temperature.

Fan clutch designers define the part's operation with a 3-D graph. One axis is %-slip which ranges from 0% (fully locked) to 100% (fully decoupled). Then, there's an axis for RPM and another for ambient temperature. It's a reasonably complex little animal!

As far as how to test, the bottom line is you DON'T... To get useful performance data, you need a stroboscope to measure shaft rotation and another to measure fan blade rotation (with a SPECIFIC fan). AND, you need to be able to precisely control ambient temperature as well.

When you have these three ingredients, you can re-create the fan clutch's performance silhouette and compare it to the manufacturer's specification (presuming you have access to that data). It's a time consuming and test equipment costly proposition.

You'll find 'good ole boys' who will take a fan clutch and pronounce it good or bad by feeling it with their hands. You'll also hear about how many RPM's the fan ought to turn on engine shut down. But, these approaches only sort real 'basket case' failures...

Fan clutch restorer, Fred Oliva, once put on a technical seminar where he had several physical samples of various fan clutches for examination. He'd tested each and kept the test data private.

He invited various members of the audience to come up, hold/feel the clutches and sort them good/bad. Then, he continued his seminar.

In the end NOT A SINGLE 'expert' from the audience had correctly diagnosed the clutches!!!! The correct answer was ALL of them were 'bad' because they failed to comply with their governing specification in one way or another...

So, I know I don't know how to measure/assess and don't even try to determine whether a given fan clutch is 'good' or 'bad'.

[Michael W.]

Re: 1969 clutch fan question

Mine has good resistance the first few times you rotate it, but as you keep rotating it the resistance goes away. I am assuming that means mine needs to be rebuilt.

My fan clutch performs exactly the same way- I believe this behaviour is normal as the clutch engages/disengages properly during engine operation.

[Mike E.]

Re: 1969 clutch fan question

Jack--
I appreciate you and Joe and Duke and Clem and the few others who are able to explain to us laymen, in relatively laymen's terms, what different components are, (often what their part numbers are), what they are designed to do and how they are designed to do them. With the clutch fans, for most of us, after figuring out the correct configuration, we then have to eyeball their performance and do an inexact assessment of where they are on the scale from "as-designed" to "non-fuctional".