ZR51 Performance- 6-speed Manual Transmission Specialists
Q) After installing one of the new ZF S6-40 transmissions, during operation the transmission now whines while in neutral with the clutch out.
A) These transmissions are very prone to alignment issues and will whine if not installed perfectly flush. 1991-'96 L98/LT1/LT4 Aluminum/Magnesium Bell Housings have insulator plates affixed to both mating surfaces of the bell housing. 1990-'95 ZR-1 and 1989-'90 L98 Corvette Bell Housings are all aluminum and DO NOT have insulator plates affixed to either mating surfaces. With regards to engine/bell housing/transmission stack, with "Aluminum" Bell Housing configuration there are four (4) mating surfaces requiring inspection/detailing. With "Aluminum/Magnesium" Bell Housing, there are eight (8) mating surfaces requiring inspection/detailing. Scotch-Brite all surfaces checked for high spots due to dings/dents or possible mishandling during removal. Any high spots are to be filed flush, insulator plates carefully reinstalled and the transmission is not to be forced back into position. It should mate perfectly together when put back into place if all surfaces are detailed and the clutch disc has been aligned correctly. With all mating surfaces detailed and all fasteners cleaned, optimal stack parallelism will be achieved with engine/transmission driveline assembly.
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NOTE (he he): Oxidization Conundrum (OC) A common problem with Aluminum/Magnesium bell housings that have the insulator plates are that the plates tend to form oxidation buildup during operation which can easily be missed if the insulator plates are not removed to inspect and detail undersurfaces.. Oxidation formations tend to puff up after de-stacking engine/bell housing/transmission case/mating surfaces. A 0.0005" thick formation developed while the stack is tightened to spec. once unloaded can displace (puff up) as much as 0.003"-0.004". This condition will result in 0.002"-0.003" interference of stack alignment regarding mating surface parallelism per insulation oxidization conundrum occurrence (2 plates per bell housing)..
Q) After installing one of the new ZF S6-40 transmissions, during operation with light-weight flywheel, there seems to be more gear-clatter or gear-rattle than one would expect to hear.
Q) What is the most common mistake made when replacing a C4 Clutch Kit?
For questions to Bill Boudreau, please post following information:
Q) My ZF6 was replaced back when the car was under an extended warranty, but now this one has gone bad! I was hearing a nasty sound that I thought was the throw out bearing, but after replacing it, the noise still exists. It is definitely coming from inside the transmission, and the best way to describe it is to say it sounds like it is running dry and grinding (but obviously the fluid has already been checked). The sound can be heard with the engine idling and foot off the clutch, as well as in any gear from about 1000 to 1800 rpm and then again from about 2500 to 3500 rpm or any time it is under a good load, say putting it in 6th and giving it some throttle to accelerate. The hydraulics are brand new and I do not have any problems shifting through the gears at all.
A) This type of transmission problem (Gear Buzz) can be corrected by removing the transmission from the vehicle and performing a more stringent main bearing shim selection process than the factory did when the unit was built. We recently had a brand new transmission (540Nm) develop the same symptoms. Instead of fumbling with trying to return it under warranty, we elected to go into the unit to investigate the cause. We found the front countershaft bearing was able to accept a 0.005” thicker spacer shim between the bearing and bearing retainer clip. The front and rear main bearings were found to have correct shim thickness installed. By installing the thickest spacer possible, the bearing and countershaft are not allowed to move along the longitudinal axis which disallows the occurrence of gear buzz. The transmission was reinstalled into the vehicle and road tested. All of the previously reported symptoms were gone and the transmission operated smoothly as if the problem had never existed. This repair procedure is provided in all 1990 through 1996 Corvette Shop Repair Manuals.
Q) On my car I can power shift 2nd and 4th, but I'm having problems with 3rd. During normal driving going to 3rd does not feel as smooth as going to 4th or 5th. Sometimes I don't even feel like it went into gear. The shifter grabs third gear, but it feels like it is half way there, like it has an additional 1/8" of travel left to go, not like all the others gears where you feel the shifter securely engaging the gear. Third grabs, but you feel like the shifter did not go in all the way.
A) There may be an issue with linkage that is slowing down the actuation of the 3rd gear shift event. How is the "return to center" spring action of the shifter feel? This is how well the shifter springs back to the 3/4 neutral position when released from the 1/2 neutral position or the 5/6 neutral position. The return to center action should be quick and crisp. This return to center action is initiated by the mountain detent of the shift shaft (internal) and the ball detent (roller) and two springs located just forward of the CAGS bung. If the ball detent gets crud in it, the return to center action becomes diminished. This condition is of the easiest to correct. It requires a new Ball Roller($15), 2 Compression Springs($5) and less than 5 minutes to install once access under the vehicle is gained.
When installing a Hurst Billet/Plus gear shift handle, there is the possibility that the shifter setup may be binding somewhat at the axis points. I have found this to be the case when installing the Hurst shifters unless removing all of the powder coat from the contact areas at the pivot points prior to installation into the shifter. There should be no hang-up in movement in the x and y axis and no play in the axis joints. The condition of the shifter frame (Remote Control Arm) is also important. If you can rock the frame (Remote Control Arm) and it rolls more than just the slightest little bit, it has a delaminated rubber eyelet at the front mount point. This condition will also work against you when transitioning from gate to gate (1-2 gate, 3-4 gate, 5-6 gate).
Once you have cleared these items as possible issues and you are certain that the clutch hydraulic system is good, you can be certain that the problem lies internally at the specific problematic synchronizer.
I had my transmission rebuilt (1995 corvette ZF6) and a clutch installed
about a year ago. If I am at a stoplight I sometimes have trouble shifting
from neutral into first gear. I also have trouble getting into third gear
sometimes. Sometimes the shifts are very smooth and other times it seems
like it goes halfway into gear but I have to pull back and try again or shift
into another gear first before it will go. Does this sound like an
internal transmission problem or a clutch problem? Recently I noticed my
clutch fluid was low so I refilled it but the problem still remained.
I plan on changing the clutch master and slave because of the low
fluid. The car also shutters a little bit when I let out the clutch going into
first gear. I have put about 15k miles on the clutch and transmission
since the work was done and I don't drive the car to the extreme. Kevin
With out a doubt, this sounds like
you are experiencing clutch hydraulic problems. Continued use of the
transmission while in this condition will cause excessive wear to the
eventually the need to have your transmission rebuilt again. The
clutch hydraulic master and slave cylinders should be replaced as a set. Regarding
the clutch shudder during engagement, if
it has shuddered from day one after the clutch replacement, I'd say that the
pressure plate was not tightened down
evenly with a uniform star pattern
you have any detailed instructions relative to the "PROPER" way to R&R
the Clutch hydraulic fluid for a 1984 Vet?
A local shop replaced the Clutch Fluid
(DOT 3) - before they did the R&R, the clutch worked great (30K original
when I drove away from the shop, the clutch seemed different, and produced
gear-clash when I tried to get into reverse. I let the car sit for the weekend,
and a few days later the clutch had lost all of it's pressure and shifting the
manual transmission was very difficult. I took it back to them and suggested
that there was air in slave, and gave them instructions to tilt the slave Cylinder at 90* when bleeding because
bleeder valve location -they did it, and the clutch pressure came back, could shift into
the 4 gears and occasionally
ground when going into reverse. I
drove the car for 32 miles, and I lost 85% of the clutch pressure, and was
unable to shift into any gear. Mike
– C4 Corvette owner
(R&R) of the clutch hydraulic fluid for the '84-'88 Corvette is generally
(Q.) Bill, I own a 1991 Corvette with the 6 speed ZF. I have a transmission noise which many other Corvette & Camaro owners report as "normal". This noise is most prominent in neutral & 1st gear and is commonly described as the "rocks in a can" sound. Two local companies that rebuild (only) Muncie & Borg-Warner 4 speeds have heard this description and believe it to be the front bearing and possibly one of the ball bearings rolling around inside the trans case. From your experience is this "really" the problem which many people report hearing only in THIS model trans? Dean of north Chicago.
(Q.) I just bought, at the local BMW
dealership, 3 liters of the Castrol TWS 10W60 oil. I was surprised to see
that it is motor oil. Not being a lubrication engineer myself, who did the
investigation and determination that this product is compatible with and
good for our ZF transmissions?
Jim – Grand Sport Registry
Q.) Bill, I don't know if this is in your ballpark but I have a "growl" from the release bearing ( a least I think its the bearing) every time I push the clutch and/ or let it out. The trans still shifts smooth but the "growl" is getting louder. In your opinion would this noise be the release bearing? If so, would you recommend changing the clutch plate at the same time? The clutch feels good and does not slip. Also, could you / would you recommend a capable mechanic to do this work in our area (I live in Tucson). Any suggestions on the best place to buy the kit? Thanks for any help you can give me.
A.) If you can make the growl sound fluctuate by wavering the clutch
Q.) I have a noise that I can only describe as gear lash or slop. It is most noticeable when the car is coasting and when I gently touch and release the throttle I can hear a clunk. Also, from time when I shift from first to second I can hear the same type of sound. The transmission shifts fine, no gear clash. When I had the car on a lift in gear and moved the drive shaft back and forth the noise seemed to emit from the transmission. I don't feel the drive shaft movement was excessive. Also, there is a different pitch whine when the transmission is in gear with the clutch in verses when the transmission is in neutral and the clutch out.
Q.) I was hoping you could answer this question I've asked many times and have never gotten a real answer: What is the cause of the noise from a ZF with the clutch out when it is changed to a single mass flywheel? So far everyone just tells me it does it without telling me why. If you could tell me or direct me somewhere I can find out, I'd very much appreciate it.
Q.) I would also like to know what your typical turn-around time is for rebuilding a zf6.