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Discussion Starter #1
Hi everyone...

I recently returned from a 5500 mile month long trip out west towing my Airstream. My truck developed a new issue, and it started suddenly when driving solo; NOT pulling the trailer. My truck is a 6spd manual and has Lonnie's LS7 clutch that he installed in 2012 along with 4.56 gears. I would guess the clutch has 25-30k miles on it. Truck itself has just over 60,000 miles on it. Clutch has been 100% trouble-free as has been the transmission. The fluid in the clutch is frequently flushed (before every trip and in the middle of every trip with Motul RBF600) and I last flushed the tranny fluid in 2015 with Dexron III. This problem I'm about to describe ONLY occurs when downshifting to slow the vehicle, never during upshift and never during downshifting to accelerate.

I frequently downshift through the gears to slow down the truck coming down big hills/climbs, especially pulling the trailer. I've been doing this for decades with all my vehicles without issue. I've never rev-matched until I had this problem (read on).

When I was in Ouray, Colorado on Hwy 550 (one of the steepest/curviest switchback roads in the country) driving the truck alone I noticed a grinding vibration in both the clutch pedal and gearshift when I would let out the clutch to downshift to slow down coming down the big climbs. There was no one instance I can point to when it started. The vibration/grinding sound does NOT occur when actually OPERATING the shifter....it only occurs when letting out the clutch to downshift after said gear is selected. The grinding vibration is accompanied by a slight grinding noise. It only does it when I start to let the clutch out and whatever gear I've chosen begins to engage. It DOES seem to do it more frequently if I'm downshifting to third gear but it will do it downshifting to any gear-though not as prominently. There is a "sweet spot" in the clutch pedal travel where the grinding vibration occurs. It only does it right when the clutch starts to "grab" and ONLY when I try to downshift to SLOW the vehicle. If I downshift to speed up, it NEVER does it. Also, if I rev-match when downshifting to slow the vehicle, it doesn't grind. I push the clutch in, give it a little throttle, and let the clutch out. It works silently like that. But, rev-matching when trying to slow down isn't ideal. Also, the grinding vibration occurs ALOT more when the transmission and clutch are cold (i.e., just starting out). The warmer everything is, the less likely to grind, even if you try to make it do it. Again...it never does it on upshift.

I'm not sure if the problem is the clutch or the transmission. The clutch works as smooth as the first day it was installed when upshifting. Smooth as butter. There is no throw-out bearing noise, and I think if it were the throw-out bearing it would do it any time the clutch was engaged or disengaged. But, I'm not sure. There is no burnt clutch smell, no problems with travel or change in release-point. It grabs at the same spot it always did.

I was thinking it also might be a synchronizer failure in the transmission since when I rev-match on downshift it doesn't exhibit the problem . But, I can put the transmission in any gear upshifting or downshifting without the clutch with no problem (the old way where you would nudge the shifter a little and wait for it to "fall" into gear without the clutch). I would think if there were a synchronizer issue it wouldn't slide into any gear without the clutch as easy as it does. But, again I'm not sure. The transmission shifts perfectly smooth and is silent. There is never a problem with it NOT going into any gear. Even though something may vibrate/grind, the tranny will always actually downshift. There is no problem getting it into any gear and it always does so smoothly.

My girlfriend and I flushed the clutch in Colorado with 2 full quarts of RBF-600, probably over 1000 pumps of the clutch through 5 flush cycles to get the old fluid out. Even after just a couple weeks and 2500 miles, the clutch fluid had some sediment (which has always been my experience). We flushed it until what came out looked exactly like what we were putting in. This didn't eliminate the problem.

My plan is to drain the transmission and look for any bits of metal in the fluid. Also a possibility is that the fluid has moisture in it. My truck sat at Eric's for a few years and no telling what happened while it sat. The vent is not too high and I have washed the underside as it was filthy. I could have got some water in it. My rear end had some moisture in it after sitting at Eric's (I flushed that back in April). In fact, before this last trip, I changed all the fluids in the truck except for the tranny (since I just flushed it in 2015). The fact that the problem isn't nearly as prominent when warm lends credence to possibly contaminated fluid (I'm cautiously optimistic but realistic that this might be what it is). I have a 2002 VW Beetle that got some water in the manual tranny from washing the engine (stupid vent design allows water intrusion), and it would barely downshift at all (but it didn't grind). Changing the fluid fixed the problem in that vehicle immediately.

I read an account of a similar vibration/grinding issue in a T-56 transmission on the F-body forums. But, his vibration/grinding was exhibited on upshift as well and turned out to be a badly worn clutch down to the rivets. But, there are lots of similar downshifting grinding/vibrating T-56 tranny issues documented on the LS1 forums that turned out to be worn synchros and friction cones. My problem started suddenly. Hope it's not the tranny.

The LS7 clutch has been great but I have concerns it might not be strong enough for a 5000lb truck pulling a nearly 5000lb trailer. Perhaps the original dual flywheel would be better for the weight, but that setup while competent always felt a bit clunky (and was more easy to smoke/slip upon takeoff pulling the trailer). But again, the LS7 setup has been fine since 2012. But 25-30k miles isn't durable enough (if it turns out to be the clutch). I haven't used the truck since 2015 and when I do use it it is only for a month long 5-6k mile trailer-towing trip once a year. Other than that it sits.

To sum up:
1) Issue started suddenly
2) Only does it on downshift to slow the truck, not when downshifting to accelerate
3) Upshifts are fine
4) Does it when downshifting to any gear but more likely to do it when downshifting to 3rd
5) Does it much, much more when cold
6) Always goes into gear regardless of grinding
7) Does NOT occur when actually OPERATING the shifter....it only occurs when letting out the clutch upon downshifting
8) Problem doesn't occur if I rev-match on downshift

Any ideas?

Thanks!
Jeff
 

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Ex SoCal Nut
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Sounds as if you have a trans problem going. Reverse load on the rotating assemblies opens up some tolerances. Not sure if the trans has full on bearings or bushings involved. Kinda the same load on the main bearings when downshifting.
 

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Discussion Starter #3 (Edited)
Hi everyone. It's been a few months, but I finally flushed the tranny fluid. I carefully examined the old fluid. No evidence of metal shavings, sparkles, or debris in the fluid whatsoever. It was a little dark and looked like it had a tiny bit of moisture in it. I last flushed it in 2014 or 2015. Anyway, flushing the fluid didn't make a difference. Something still grinds when downshifting from 4th to 3rd just as you let the clutch out. If it starts grinding and you push the clutch back down to the floor, the grinding stops. Every now and then, it grinds when downshifting to other gears besides 4th to 3rd. I even noticed just a tiny bit of grinding just a couple times UPSHIFTING from 4th to 5th. But, 95% of the time it happens on the 4th to 3rd downshift. And, it doesn't always grind...maybe 50% of the time. Works perfectly normal the other 50%.

I honestly don't know if it is the clutch or tranny.

Here are two videos. In the first video you can hear the grind better (when the clutch is partially engaged when downshifting). In the second video you can see how it grinds on part let-up of the clutch pedal.

I should note that once you release the clutch all the way up it stops grinding. It never grinds when actually in any gear with the clutch fully disengaged. Nor does it seem to grind when the clutch is fully engaged to the floor...only when you start to release the clutch.



Any ideas?

Thanks!
Jeff
 

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One of the SoCal Nuts
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I have not driven a Stick since I had my Vega, Before that it was a Jeep M38A1 so I know nothing about clutches or 6 speeds. Sorry I can't help , but I read your post.
 

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Haven’t worked on manual (or auto) transmissions in years but manuals haven’t changed much and my guess is a bad throwout bearing. At first I thought it might be a pressure plate and/or possible hot spots on the flywheel. But since the sound is intermittent and particularly since it varies with the clutch pedal position/pressure it seems more like the throwout bearing to me.

At idle in neutral, say in your driveway, try slowly depressing the clutch pedal from just barely engaged all the way to the floor. Do you hear any “ringing” or sort of metallic sounds? it would probably be heard with just light pressure and then disappear when the pedal is at the floor.

I see in your earlier post that you discounted that, but it might be worth another look.

BTW, (y) (y) for posting the videos!

@Autoprof and others will probably chime in with some more knowledgeable feedback, but that’s my WAG.
 

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BAD BOW TIE
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Too much exhaust sound in the videos to hear what you hear. Drove and worked on a ton of sticks over the years and my experience with noise when you put the clutch in or out was always related to throw out bearings failure. Might check with the guy that put the clutch in and the Tremec folks if they know what you are talking about. Might be something that either one has had happen before. Have you tried to research Tremec problems on the Search community here on the fanatic site?

Here is a place to start:
GOOGLE: Tremec six speed grinding noise on down shifting
 

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Discussion Starter #8
The first video is the one where you can really hear it.

I would think if it was the throw out bearing it wouldn't matter which gear I was up shifting or downshifting to.

There is no classic throw out bearing noise when idling in neutral. It is silent.
 

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The first video is the one where you can really hear it.

I would think if it was the throw out bearing it wouldn't matter which gear I was up shifting or downshifting to.

There is no classic throw out bearing noise when idling in neutral. It is silent.
IIRC from my past life, throwout bearing noise wasn’t either/or when it’s beginning to fail, it was intermittent.

Agreed, there won’t be any noise at idle until the clutch pedal starts being depressed, engaging the clutch. It would not be heard when the pedal is fully up or at the floor, but might be audible, if only faintly at first, anywhere in between.

Also agree that it might be something else completely though, but that’s where I’d start if it were mine.

I guess if I’d done everything you’ve done to date about the only option now would be to drop the trans for a full inspection to be sure everything’s aligned properly and go ahead and replace the throwout bearing, pressure plate, clutch and resurface the flywheel. It’s probably not going to get better and I’d hate to do any damage to the gears, etc.
 

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Discussion Starter #10
There is no throw out bearing noise or vibration when idling at any position of the clutch pedal.

It only does it when you to let up the clutch up while downshifting from 4th to 3rd, as the videos show.

I'm thinking a synchronizer is failing in the tranny...
 

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BAD BOW TIE
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Like I said above before I spent any money or did any labor I would have a phone conversation with the guy that put the entire clutch set up in and Tremec. They possibly have knowledge of what is going on and then you can spend money and do labor. A while back there was about three guys on here having tranny trouble and Tremec was very helpful in pin pointing the problems. Everything I have read is the syncros are very fragile especially if you don't RPM match before down shifting. Just trying to save you time and money.
 

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BAD BOW TIE
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I just did this in the search community at the top of the page and there are many posts about Tremec Transmissions and what they did to fix them. Some folks have never used this feature but it is very easy. At the top of the page it says Search Community. Type in Tremec Transmission 6 speed problems and then click the little spy glass and it will take you right to the sites and posts. Good luck.
 

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HobbyBob
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Look in your owners manual under trailer towing. Our SSr's are great vehicles but they are not big trucks. The max. towing weigh for a trailer is listed at 2500 pounds. That is about half of your Airstream. The GVW is shown to be 8000 pounds, which still does not match up. Even with the best dual disc clutch and a good rebuild on the transmission, you are still exceeding the design limits of the vehicle. Having towed trailers for years, I would highly recommend a larger vehicle to tow the Airstream, like a Suburban or a Ford Expedition. For safety, use a stronger vehicle designed for pulling. The longer wheelbase will give a more secure ride. Did you ever hear of the tail wagging the dog? That is when a short wheelbase is used to pull a trailer that is too big. You can lose control as the trailer sways side to side. PLEASE CONSIDER YOUR SAFETY AND YOUR PASSENGERS. .02
 

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Discussion Starter #15
Been towing with it for 14 years 60000 miles. The only thing I use it for is to tow my trailer, its bern up and down the highest peaks in the counyry. It's old, it's allowed to break.

BTW, best tow vehicle I've ever owned. The truck drives better towing with a properly set-up WD hitch than solo. It's all about set-up.
 

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If the clutch has 30K on it and you have been pulling a trailer that weighs twice the recommended capacity of the truck then I would guess your clutch may be worn and slipping and that's the gears grinding that you hearing. There is no clutch that is going to take that kind of service load without premature wear.
Your moving +/- 10,000lbs up and down steep hills, that's torture on any clutch. Might be time for a new clutch.
 

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Discussion Starter #17
I would think if the clutch is slipping it wouldn't only exhibit the problem during the 4th to 3rd downshift...wouldn't it slip at other times?

Once the clutch is fully pushed down, it goes right into gear. It never makes the grinding noise when the clutch is fully up or fully down. It only grinds when you partially let up the clutch as the second video shows...fully down, nothing...let it up just a bit and it will grind. Only on the 4th to 3rd downshift.

It never grinds when you push the clutch pedal down. It only does it on release.
 

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I would think if the clutch is slipping it wouldn't only exhibit the problem during the 4th to 3rd downshift...wouldn't it slip at other times?

Once the clutch is fully pushed down, it goes right into gear. It never makes the grinding noise when the clutch is fully up or fully down. It only grinds when you partially let up the clutch as the second video shows...fully down, nothing...let it up just a bit and it will grind. Only on the 4th to 3rd downshift.

It never grinds when you push the clutch pedal down. It only does it on release.
I'm really afraid that you're beginning to see the effects of worn blocker rings/syncros, but first I'd double check clutch release. If I'm testing that I'll find a very slight incline where the vehicle will barely roll backward when in neutral. Then try it in gear with the clutch fully depressed - it should roll backward at the same rate. Good Luck!
 

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BLU SSR
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Btw, if I rev-match it on downshift it never grinds...giving credence to the synchro being the culprit.
Important piece of info! Bettin' you just identified you problem. For future reference, rev-matching is a good habit to get into, as someone who grew up shifting things without syncro's, it's a habit I acquired early at the strong encouragement of my Dad when he would hear that grinding noise!! Just another way to treat your baby's power train nicely.
 
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