Okay guys, I have searched the forum to death & posted this question a couple of times over the last couple of years with no real answer that satisfied me. So here goes again. (I'm sure someone here can help.)

I have a '96 CE LT4 with the ZF6. I’m sure (I think I’m sure; I hope I'm not losing my mind) that when I bought the car with 52k on the clock the transmission shifted as smooth as silk between all gears regardless of driving circumstances. But for last 7K miles or so it has felt notchy to me (64K on clock now). But only when jerking quickly from gear to gear, as if it resists being pulled into any gear quickly.

Any of you with a ZF6, please try this for me: Start your Vette and sit still with the clutch pushed in fully. Shift as quickly as possible from1st to 2nd, 2nd to 3rd, etc. Does it go as smoothly as silk? Mine does!

Now, start the car rolling; 5mph will do. Shift the same way again. Is it still just as smooth? Mine is not! It is notchy. But, if I shift slowly (normally) from gear to gear it seems to shift okay.

Is this normal functionality for a ZF6? My local dealer says so, but the tech did not seem sure & I'm not sure I trust his judgment. Plus, I could have sworn mine did not behave this way when I first got it, but I do not have a ZF6 Vette nearby to test drive myself to see. I really need to know if a new tranny will fix this, or if I'm imagining a problem that is not there.

I have replaced the following since first seeing this problem: Clutch, fork, throw-out bearing, pressure plate, clutch master cylinder (twice), clutch slave cylinder (twice), and ZF fluid (several times with GM part#).

I'm putting this in General & in Tech/Perf so that I might find a few members to try my little experiment on their cars & give feedback.

Thanks in advance guys!
David

 

I have the B&M so I really can't comment (although mine feels the same either way) BUT you should contact Bill Boudreau @ www.zfdoc.com

He is the ZF guru and can answer any & all questions you may have.
You can email him or even call him...

BTW, have you checked the clutch fluid recently?
Also, the ZF fluid should be changed regularly as well...

Since I live in Phx, Bill has done regular maintenance on my trans + he installed my B&M. He rebuilds ZF's and knows everything there is to know about them.

 

Yeah, I talked with Bill about it two years ago. He quized me pretty good about my clutch fluid & cylinders, then said it could be the ZF (he mentioned what internal parts, but I can't remember now), but then as he pointed out, he couldn't feel it for himself. He did say that when he has rebuilt one you should be able to pull it from one gear rop the next with the tips of two fingers. He said it should be "like butter". Of course mine is as long as you are sitting still. Still not sure.

A nice guy, Bill. Very willing to help & respond to questions as I remember.

David

 

My car seems to shift more slowly and notchy when going easy at low revs in traffic, going into the next gear at less than 2K. When pulling revs and driving hard it is smooth as silk with my Hurst.

The synchros don't seem to work as well at very low revs, or when the car is cool from just starting out. Especially so on the 2-3 shift. 108K miles on mine with a fresh clutch and fluid fill with GM fluid.

 

So when shifting at high revs yours is just as smooth as when stopped???? Are you sure? Is yours smooth as silk even when shifting quickly (i.e. jerking from 1st to 2nd)? This is just the info I need!

I also have a hurst, and mine is the same amount notchy at low or high RPMs. Only axle movement seems to affect it. (Notchy when rear wheels are rolling, not at all when stopped.)

David

 

Sounds like worn synchros to me, but I'm no expert on them.

 

Would worn syncros act the same at 5mph as at 60mph? My notchiness seems the same rolling at any speed, but is gone when stopped.

 

This may be somewhat related:

Before I had my Vette I was driving a Thunderbird SuperCoupe 5 speed (awesome car!). It would be much harder to shift at high engine speeds but road speed didn't make much difference. Before I got rid of the car one of the gears was starting to grind during normal shifting from time to time. That was a Mazda transmission similat to what Ford uses in the pickup trucks.

My ZF6 (92 LT1) shifts smoothly at all road speeds and engine speeds.

You may not want to hear it but you might have to open it up to find the problem if there is one.

I am sorry I am not much help.

 

Thanks for the info quickcat. Does your ZF shift just as smoothly as when stopped, even when jerked quickly from first to second, for instance?

Thanks again,
David

 

I have the same tranny and did the test as you asked. No problems. It shifts as smooth as the day I got it. You mentioned you had a Hurst, could it be you may need an adjustment on the linkage? Other than that I'm stumped. Good luck.

 

Thanks LT4-FUN! No, the notchiness was there before I installed the Hurst. Wish it were that easy. Also, I would think I would see the effect of some type of linkage/shifter problem even sitting still.

Thanks again for the feedback.

David

 

Yes....it shifts the same regardless of speed. I hope you get her worked out. Even if everything is ok it would be nice to know why it is behaving like it is.

I know this may sound silly but is the transmission installed properly? I have heard that it is possible to install the transmission in such a way that it will not be properly aligned, and that this condition could affect performance.

Thought I would just throw that one out there.

 

After discussing this issue with Bill (via email) myself and learning how the ZF tranny works, I'd be inclinded to say your synchros may have gotten some wear before your first CHS change. And, regardless of the speed you'd feel more resistance when the car is moving vs. stopped -- if they have enough wear.

Before changing/rebuilding your ZR, I would make sure your CHS parts are really good. Despite changing the mc/sc twice, you might still have inferior parts. I've heard about several people doing this service more than once in a short period of time. (When you replaced these parts, did you replace the hose that connects them? If not, the lining of that hose breaks down and would quickly contaminate the new mc/sc). I think the reason there were so many problems had something to do with inferior quality. And, I think recent months have started to show better production -- especially for slave cylinders.

Was there any improvement immediately after mc/sc/ZF fluid change? Have you done all at the same time?

Beyond these suggestions, I think Bill is still your best authority. I'd quiz him thoroughly before spending that much money!

 

For ZF clutch/performance issues, it would be worth looking at this thread....

http://www.digitalcorvettes.com/foru...ad.php?t=56557

 

Worth repeating.

 

I'll take the LT4 out tomorrow morning and try this out after warming her up a bit. I'll post on the matter then.

 

My 94 coupe with about 28000 miles on it can be shifted fast but it has never had a smooth feeling. If I want it to fell smooth I have to slow down the motion and it will almost take the next gear by itself.

But if I shift it too quickly it might feel a little tight to engage the gear.
I like the ZF6 but have found it a little dissapointing. I thought that the 6 speeds would just let me play around with it more, and found out that #6 can rarely be used ; mostly on straight boring roads, which of course I do not usually take.

 

That's how mine is. It even hangs up going into 3rd if I really baby it. Normal driving and banging gears at the track it works excellent. It's out of a 89 or 91.

 

Are the 89 and 90 ZFs the same as the later ones?

Mine is RPO ML9 and I believe the later ones were MN6. Curious to know if there's a difference.

 

Sorry, couldn't do it yesterday due to other committments and it rained this morning. But - I just took her out and tried the experiment. No difference sitting in the garage while idling and shifting very fast through the gears and out on the road shifting as fast as I could. Smooth with no hangups. Car only has about 44,000 on it.