New 941 Dewitts and still overheats, 65 L-75

[John C.]

Well, I thought I had it. Went ahead and bought the 941 Dewitts along with a Mock pump, flushed the block and filled her up with Zerez G-o5 and distilled water. Drove around for a few minutes and the temp never budged off around 170. Turned on the A/C and there she goes, just like it did before the new radiator and pump, climbing and climbing until it hit about 210 or so and I turned the air off and put a fan in front of the grill to help it cool down and let it idle for about 10 minutes just to see how long it would take for the temp to drop down into the normal range. It runs about 10 degrees cooler just driving around, but gets hot just like it did with the old radiator and pump which was ok, just not correct to the car. Common sense tells me the only thing left is the fan clutch. The car never over heats, any speed, any outside temp, UNTIL, the A/C is turned on.
So, unless any of you fella's have some advice that I've missed, I'll start the search for a fan clutch. And on the subject of fan clutches, will a correct fan clutch do the job on an A/C L-75 or will I have to use a replacement clutch that performs better than the original design ?
Any help is greatly appreciated !!!
JC

[Edward J.]

Re: New 941 Dewitts and still overheats, 65 L-75

John, Do you have the correct Fan for a A/C car? They are making a reproduction of the original fan clutch that will turn on at about 160 Degrees , I believe the company is K&B they are listed in the new drive line. John also as a side note 210 is a normal operating range for todays cars, and will not damage any thing, using a infrared gun check the temp when hot at thermostat housing and see how close it measures to the cars readings. remember there maybe a small difference between the two.

[John C.]

Re: New 941 Dewitts and still overheats, 65 L-75

Pretty sure it has the right fan, 7 blade and appears to be the original. I've tried to perform the clutch test as laid out by various people and it appears it could possibly be partially "not up to snuff" for lack of better terms. I do know this, the noise coming from the fan never changes. It's the same when first cranked up as it is when the temp exceeds 200, which leads me to believe maybe the clutch is not working properly. I'd think the clutch would completely lock in at 200 and the fan noise would increase as it attempts to bring the temp down.
Apparently the clutch works well enough to cool the engine under normal conditions but can't handle the load applied by the A/C.
I'll look into the K&B fan clutch Edward, thank you sir for the knowledge !
jc

[Joe L.]

Re: New 941 Dewitts and still overheats, 65 L-75

Pretty sure it has the right fan, 7 blade and appears to be the original. I've tried to perform the clutch test as laid out by various people and it appears it could possibly be partially "not up to snuff" for lack of better terms. I do know this, the noise coming from the fan never changes. It's the same when first cranked up as it is when the temp exceeds 200, which leads me to believe maybe the clutch is not working properly. I'd think the clutch would completely lock in at 200 and the fan noise would increase as it attempts to bring the temp down.
Apparently the clutch works well enough to cool the engine under normal conditions but can't handle the load applied by the A/C.
I'll look into the K&B fan clutch Edward, thank you sir for the knowledge !
jc

John------

Testing a fan clutch is very problematic. If it's old, I'd replace it. The last available GM fan clutch was GM #3916141 but it's discontinued. The K&B reproduction is a good alternative.

By the way, if you want to go through a little extra effort and expense, there's a way of ruling out a fan clutch problem. Obtain a fan clutch eliminator. This replaces the fan clutch and makes the fan direct drive. That represents the most cooling you could possibly get out of the fan and what the fan clutch delivers at maximum "engagement". If the cooling problem goes away, then you know the problem is the fan clutch. If it does not go away, then you know the problem is not with the fan clutch.

[Duke W.]

Re: New 941 Dewitts and still overheats, 65 L-75

With a 15 psi cap and a 50/50 ethylene glycol/water mix the boilover temperature is 265F.

I consider the normal operating temperature range to be 180-230F, and operating the AC in low speed driving will get it into the upper range.

Most current replacement viscous fan clutches are "tuned" for later engines with 195F thermostats, which means they don't tighten until a higher radiator exit air temperature than in the sixties, but some can be adjusted to tighten at a lower temperature.

If you shut off the engine at 210 after extensive idling, the fan should feel a lot stiffer than when cold.

Duke

[Domenic T.]

Re: New 941 Dewitts and still overheats, 65 L-75

John,
I think the fan clutch advice may be it but there is another thing related to the A/C. I found that a overcharged system created much more heat at the condenser in front of the radiator. Either to much gas or oil will do that. I fought that problem with my one of my cars and found it overcharged with both oil and gas.

Dom

[John C.]

Re: New 941 Dewitts and still overheats, 65 L-75

Thanks guys for the input. The fan clutch has the same resistance all the time, cold, hot, etc. And since the age is unknown and the cost of the eliminator is about half the cost of a new K&B, I just went ahead and ordered a new clutch. So, hopefully, with the new radiator, a new Mock pump, spotless fluid and some luck, maybe the new fan clutch will fix the problem. We'll see. I'll let you know how it comes out once I get the clutch and install it.
Thanks Fella's
JC

[Duke W.]

Re: New 941 Dewitts and still overheats, 65 L-75

Chances are a new fan clutch will be "tuned" to a 195 thermostat, so consider modifying it to tighten at a lower temperature. See the attached pdf on how to do it.

Duke

[Leif A.]

Re: New 941 Dewitts and still overheats, 65 L-75

Chances are a new fan clutch will be "tuned" to a 195 thermostat, so consider modifying it to tighten at a lower temperature. See the attached pdf on how to do it.

Duke

Duke,
The K&B reproduction fan clutch is set to come on at 160 degrees...it is NOT the "tuneable" (spring) type.

[John C.]

Re: New 941 Dewitts and still overheats, 65 L-75

Well dang, if it locks in at 160 degrees, it will basically be fully engaged constantly. I'm not an engineer, but why would it be built to stay fully engaged all the time ? I can see maybe having it engaged at around 190 or so, maybe even as low as 185. Wondering now if I should install it once I receive it, which will be this Tuesday. I'm not completely convinced I want a fan howling all the time, even with the A/C turned off. I guess I do have the option of returning the K&B and going another option. Any suggestions on how to obtain a rebuilt clutch or even a new one that can be set at around 190. I only put about 100 miles a year on the car. It's for the most part an unmolested, unrestored car which was repainted several years ago, all original interior except for new carpet. I'll get it out once in a while just to keep the fluids moving so I do need a fan clutch that functions properly and the way I see it, even if it's not the exact correct clutch, if it does the job, it can always be replaced later if absolute correctness is desired. What do you guys do for occasionally driven cars when it comes to fan clutches ?
JC

[Joe L.]

Re: New 941 Dewitts and still overheats, 65 L-75

Well dang, if it locks in at 160 degrees, it will basically be fully engaged constantly. I'm not an engineer, but why would it be built to stay fully engaged all the time ? I can see maybe having it engaged at around 190 or so, maybe even as low as 185. Wondering now if I should install it once I receive it, which will be this Tuesday. I'm not completely convinced I want a fan howling all the time, even with the A/C turned off. I guess I do have the option of returning the K&B and going another option. Any suggestions on how to obtain a rebuilt clutch or even a new one that can be set at around 190. I only put about 100 miles a year on the car. It's for the most part an unmolested, unrestored car which was repainted several years ago, all original interior except for new carpet. I'll get it out once in a while just to keep the fluids moving so I do need a fan clutch that functions properly and the way I see it, even if it's not the exact correct clutch, if it does the job, it can always be replaced later if absolute correctness is desired. What do you guys do for occasionally driven cars when it comes to fan clutches ?
JC

John------


The clutch starts to "engage" at 160 degrees (or, whatever these things are actually set for). It is not fully "engaged" until a higher temperature. Noise is not going to be a problem with this clutch. It might get a little noisy when you get to a temperature that you really need the extra air flow. There's no way around that if you want it to cool properly.

This clutch is as close as you're going to get to an original clutch, both in function and configuration. I would not go with a rebuilt clutch for driving as I don't think it will perform as-new or as well as the reproduction. If your existing clutch is original to the car, I'd save it and, maybe, have it rebuilt for judging use only.

[John C.]

Re: New 941 Dewitts and still overheats, 65 L-75

Oh good, thanks Joe. I'll bolt it on this week and report back to you. The fan clutch is the last piece of the cooling puzzle that hasn't been replaced. Thanks everyone for your help !
JC