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Discussion Starter #1
I have an 03 duramax 4X4 CC dually with a ZF 6 at work that last week had a new dual mass flywheel put in it and the customer wanted to upgrade the clutch so he ordered a 1000 ft lb clutch from TST.. so I installed it also. At this time he had no box on it. He then put the TST PowerMax box on it..


Ok today he comes by the shop with his travel trailer hooked up to it and says his pedal is getting hard sometimes under a load with his trailer.. I say ok show me.. We go for a ride and it was doing exactly what he was saying.. Going up a hill with his 12k gooseneck trailer and the programmer on 2 you could mash the clutch pedal and it would go down to the floor but was hard about half way down ..shift out of 6th but would not go to 5th then you could release the pedal and it would only come halfway up then you could mash it again and it would go on into 5th gear. I have never seen anything like this.. I mean we can go through town and it runs fine or turn the box off and it doesnt do this, but it wont hold speed up a hill with a load.


I felt of the bellhousing and it was hot as hell.. He had to go out of town with the trailer so I told him just to turn the programmer off since it wouldnt do it like that and run it. He is going to bring it back in Monday for me to look at..


Anyone seen this before or heard of it?? Any ideas.. before I pull this tranny again.. The clutch has to be slipping to produce the heat that I felt on the bellhousing, but according to TST that clutch should hold.. I dont know whats going on really..Edited by: tanner
 

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My guess is the hard pedal is due to the clutch fluid overheating - I could be wrong but I'm pretty sure all GM clutches are hydraulically activated.....so once the fluid is hot (from the slipping clutch) gear changes become rather difficult, to say the least. If it is mechanically activated, then the heat is causing something to bind up.


So....it would seem the clutch is slipping under extreme conditions. If so you should have been able to see the revs increase w/o a speed increase....or the speed lowly drops off but the engine revs hold.....


I've exhausted my quite limited knowledge of clutch engineering, but if it's slipping, and TST rates it for this power/usage, then either it's defective or something went wrong during the install. Either way I'd say it's going to have to come out. Once you glaze a clutch it's not going to hold regardless...........


Keep us posted, will be interesting to hear the outcome........
 

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I had to have a trany cooler put on mine just driving on long trips


without pulling anything.
 

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It's strange that it happens with a replacement clutch. I think I know which unit you must have but if you pm or call me I will talk you through what is happening.


geerhead how long have you had the cooler installed? Has it been warm since then and you still had no hard pedal symptoms?
 

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only had on for one week so I don't know yet. the trany was boiling the clutch fluid. ford uses the same trany and they put a cooler on from the factory. by the way my truck is bone stock.
 

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It seems most coolers help the problem but don't eliminate it. It is a combination of hot fluid and the self adjust feature of the pressure plate so the cooler seems to keep it from happening quite so often. Anytime the factory clutch setup slips it will make the pedal hard from what I have found.
 

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CPMac, advice please! My clutch/hard pedal problem seems to have been worst during the winter. Now that warmer weather has arrived, the hard pedal doesn't seem to be happening. I have a regular trip of about 25 minutes that produced the hard pedal during the winter. I am making the same trip now that the weather is warmer and not having the problem. I am running the TST on level 4 and 4. I generally run around unloaded or with a 700 lb mower in the bed. A gentle trip with no excessive throttle would still yield the hard pedal and I seriously doubt the clutch would slip on a 25 minute flat drive with the cruise set at 70. I agree the problem is temperature related, but don't believe it is related to a slipping clutch. My 2001 truck has 26,000 miles relatively easy miles. 1 short tow of 9,200 lbs and my general trailering is between 5 & 6,000 lbs. Sorry for being long winded, but as much as I love my GM vehicles, I can't understand why GM missed this problem and doesn't seem to have a solid fix.


GM, are you listening??? You need to address this issue!


I'm hoping the aftermarket comes up with something soon for those of us that prefer to shift our own. I have found that pressing on the clutch pedal lightly while driving seems to prevent this problem. I'll push the pedal just hard enough to feel the throwout bearing contact the clutch disk, but not hard enough to make the clutch slip. Seems that moving some fluid around helps some.
 

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I stated it wrong because I know the slipping isn't the only cause of the hard pedal but it will become hard every time the clutch slips. I know of others that have the pedal get hard after trips unloaded with with gentle driving. I don't know what causes it either. Tanner is the first person I know to have seen the hard pedal symtoms after changing part of the stock clutch and flywheel. Every time I tested anything (disc, flywheel pressure plate, or combination of both) different than stock the hard pedal issues went away even if the clutch slipped.
 

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Discussion Starter #9
Craig.. I really do appreciate your help! The owner has decided to go back totally stock, I dont know if he still plans to use the power box or not, but he was not happy with the clutch after seeing it.. I am still not sure what caused this.. Like you said and I was thinking too the only thing could have been the pressure plate, but it only had 22k on it so I dont know.. Anyways thank again for the help and I hope you get something in the works soon for these guys..


Tanner
 

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Edited by: JRmac
 

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One more piece of info for the puzzle. Last week I unscrewed the clutch res. cap, it had a slight amount of vacuum, put it back on, drove it about 275 miles with very slight hard peddal. Drove another 250 and it gradually started to return. Resivoir was only warm. I did notice that when I topped it off last fall (it had been slowly going down, but not since), the hard peddle went away for awhile. Don't know what to make of this.
 

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You may have already covered this ground, but what does the juice clutch use for fluid? Brake fluid is hygroscopic (absorbs water). Minute amounts of water can dramatically lower the boiling point. Maybe changing the fluid would help? The vacuum could indicate that at some time it actually boiled. Just a thought.
 
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