fan clutch part deux

Viking

Original poster
Member
Jan 2, 2013
2
Hi guys - wife was complaining about the engine roaring and no power - but when I drove it, it would not happen so I believed she was crazy. Anyway let my local guy look at it to placate her, he replaced the fan clutch - made sense to me.
So to the point - been fine since repair, except when I drove it the other day, started that "roaring" when I first started it, and kept doing it for about 1/4 mile, and I noticed little engine power when this was occurring. Eventually it stopped and seems to be fine, but I know its eventually going to do it again while she is driving and I'm going to get 'that call' .
Think I got a bum part and it needs another new fan clutch? Or is there something else I should be looking at before I go throw the keys back at my mechanic?
Viking
 

CaptainXL

Member
Dec 4, 2011
2,445
Welcome to GMT Forums.

If the clutch was replaced then the shop would need to update the calibration for the PCM. Just wondering if that was done? Also I believe nothing but an OE replacement clutch is recommended. Dormans in particular have a high failure rate a few months down the road.

Definitely bring it back and ask for a quality job.
 

CaptainXL

Member
Dec 4, 2011
2,445
The issue showed its face some time after it was relaced. So in my estimation there is a problem.

I have never had the clutch lock up on me after changing it.

While some initial noise is normal, it should only last a few seconds.
 

Viking

Original poster
Member
Jan 2, 2013
2
CaptainXL said:
The issue showed its face some time after it was relaced. So in my estimation there is a problem.

I have never had the clutch lock up on me after changing it.

While some initial noise is normal, it should only last a few seconds.

Thanks Captain, I'll ask the shop if the PCM was updated. If def not normal, had the truck for 9 years and you are correct it should only last a few seconds at startup. This AM wife reports it did it for almost a mile before it stopped.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Fan clutch lockup is an issue we discussed on the OS in 2007, with a GM insider who should have known everything.

Fan clutch 101 - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

I can't reprint his whole post without permission, but here's an excerpt:

The inherent problem with visco clutches is the fluid used to drive the fan for speed. It is a visco-elastic fluid. It is chemically indentical to Silly Putty - if you pull it slow it stretches, if you pull it fast it snaps. This is what happens to the fluid during startup conditions - there is not enough fluid in the working area of the clutch to sustain the shear of the startup condtions. The primary input from the clutch goes from 0 rpm to ~1100 rpm in no time flat and the fluid shears. The problem becomes one of an underspeed fan - it isn't turning fast enough to pull air through the a/c condenser and the a/c cuts out to save it's own life. While the head pressure is building the software is telling the clutch to speed up by opening the valve at 80 pwm - there's that damn ticking again. The difficulty here is that at such low speeds there isn't much centripital pressure to encourage the fluid to move out of the internal reservoir - so the problem has the potential to continue beyond the time of safe a/c head pressure and may not resolve itself until you hit the road and ram air helps spin the fan up to speed.

This called for another solution. In the latest design of the clutch there is a feature that allows fluid to drain back into the clutch when it's in the off state (parked overnight kind of thing). Now when you start up there is enough fluid to spin the fan up and keep the a/c working before you get out on the road. This feature came online with the launch of the '06 TB.

Ok, so you'd think everything has been resolved - there have been the occasional leaky valves - and here comes the morning fan noise complaints again. This isn't as bad as it sounds - you can do a neutral rev disengagement of the clutch - meaning leave the truck in park and run the engine up to about 2500 rpm (you probably have a rev limiter that stops you there anyhow). The clutch will disengage (you'll hear the noise fade away) and you'll be on your way - or you can go to get a new clutch put in to solve the noise complaint. All clutches are now leak tested during assembly now to make sure this doesn't happen again.

BTW, if you want to go through the trouble - the leaky valve is a postional problem, meaning sometimes it will give you a noisy clutch and sometimes it won't. It depends on what postion the clutch stops in after shut off. If you pop the hood and examine the clutch (rotate it by hand) you'll find a 4.1 mm ball bearing stuck in the side of it (you have to look through the fan blades to see it). Put that ball bearing down - it is opposite the valve opening. What this does is put the valve up out of the fluid in the reservoir and it can't leak back. If you want to do this then make you life easier by marking the fan opposite the ball and just put the mark on top
 

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