C4 ZF 6-speed gearbox question
Discussion
My Lotus shares this gearbox with the C4 ZR1. In recent weeks it has taken to making a bit of a knocking sound until the clutch pedal is depressed, whereupon the noise stops. There are as yet no problems with the actual shift or day-to-day use. Suspicion is a worn gearbox bearing, despite the gearbox being only 8000 miles old. Are there any UK specialists who could provide advice, parts or repair this?
Same gear box, slightly different input shafts. It's normal for a ZF to clang and bang at idle. The Vettes were fitted with dual mass fly wheels to reduce this but many go with a light weight fly wheel which results in exactly the sound you describe. Perhaps your Lotus Omega also had a dual mass fly wheel and that is no longer doing its job as well as it once did. The ZF is a fantastic box and there's ZR1's with 600+ hp still running ZF's without any issues. I'd be surprised if its failed. Out of interest, what lube does Lotus recommend as factory fill? GM has it down as non serviceable but it actually takes engine oil. BMW M5 10/60 is the recommended fill. ZF were based up in Notts but I think they are all but gone. Some whine in the low gears is also normal. How much whine and banging depends on the box. Some vette boxes have a blue serial number tag, some black. The black label ones are German and reportedly stronger but noisier. The blue labels are from ZF USA and are a bit quieter but supposedly not as strong. It's put luck what one a vette is fitted with. Mines a lack label with a light weight fly wheel and quite modded. It sound f
ked at idle
This guys good but in the US. Take a look at his site zfdoc.com
Hope that helps.
ked at idleThis guys good but in the US. Take a look at his site zfdoc.com
Hope that helps.
Edited by Gixer on Friday 31st May 16:36
My 94 LT1 had the same 6 speed I believe. As it got a bit older i do recall a few extra drive-line vibrations that would go away with depression of the clutch. At 120k miles I had a pretty worn clutch which was definitely starting to slip. The noise I would have guessed was a worn release bearing though. Sorry this is probably not much help as I sold the car before replacing a clutch.
I'd be surprised if your problem was the box itself. I thought they were pretty bullet proof, unless yours has been badly rebuilt?
There are several C4 ZR1 & LT1 owners on here so I'm sure you'll get a better answer soon.
Edited to add: Gixer beat me to it!
I'd be surprised if your problem was the box itself. I thought they were pretty bullet proof, unless yours has been badly rebuilt?
There are several C4 ZR1 & LT1 owners on here so I'm sure you'll get a better answer soon.
Edited to add: Gixer beat me to it!
Edited by z06tim on Friday 31st May 16:40
Edited by z06tim on Friday 31st May 20:25
How many miles on the clutch? Without hearing the noise, could be the clutch release bearing / thrust bearing which would be a better option pricewise.
You have probably heard a release bearing squeal that goes away when you depress the clutch, could be a similar thing if it is a light knock from the bearing.
That's where I would start anyway. Sometimes there is an inspection hole to enable you to get some WD on the release bearing to see if the noise changes - just make sure you are careful not to get it on the clutch plates.
If it is deeper in the box, I am not sure about g-box specialists in your area, but down here, I have used Hardy Engineering (Leatherhead) some time ago and more recently, B K Transmissions in Overton (Basingstoke) who were very good.
You have probably heard a release bearing squeal that goes away when you depress the clutch, could be a similar thing if it is a light knock from the bearing.
That's where I would start anyway. Sometimes there is an inspection hole to enable you to get some WD on the release bearing to see if the noise changes - just make sure you are careful not to get it on the clutch plates.
If it is deeper in the box, I am not sure about g-box specialists in your area, but down here, I have used Hardy Engineering (Leatherhead) some time ago and more recently, B K Transmissions in Overton (Basingstoke) who were very good.
From a Q&A section on ZFDocs site, hopefully may go some way to putting your mind at rest -
(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.
A.) Dean, it would be a nearly impossible for a ball bearing to fall out of the main bearing case. If the noise goes away when you push the clutch in at idle,... that could be 1 of 3 things, either a bad dual-mass flywheel, or the flywheel is not an original dual-mass type but instead is a single mass type. The ladder situation will produce a sound known as gear clatter while in neutral which is harmless to the transmission. The springs in the clutch disc, which are not present in the disc of a dual-mass set up, will oftentimes produce a rattle like resonance in the 1K - 2K RPM range when under moderate to heavy power load settings. These are the drawbacks of converting over to a single-mass flywheel.
A bad input bearing in the transmission will normally produce a whirling or whining sound proportional in pitch to engine revs. In 1st gear the whine is most prominent decreasing in intensity in each subsequent gear to the quietest level being 4th gear (direct drive energy flow).
A bad dual-mass flywheel with a failing damper will sound like a diesel truck engine at idle or a clacking noise. Under certain load settings, this sound may be heard occasionally in 1st gear too.
The main bearings in a ZF S6-40 are very different from those used in Muncie & Borg-Warner 4 speeds. The ZF main bearings are self-contained non-preload non-tapered double-row type bearings. One row is comprised of cylindrical type bearings and the other row is comprised of ball type bearings. This type of bearing would be more similar to the type of bearings used in turbine engines.
If it sounds like a diesel engine at idle, the flywheel dual mass damper may be failing. What I've seen happen before is that when a flywheel is determined to be bad, some people choose to replace it with a less expensive single mass type flywheel where they can buy the flywheel, pressure plate with throw-out bearing and clutch disc for $800 instead of $750 for OEM flywheel and $300 for the clutch kit.
If it is more of a whining or whirling sound, the input bearing has been compromised. This results from the alignment of the transmission during reinstallation is off slightly where the trans, bell housing butt up to the block. I've never heard of a ZF S6-40 main bearing going bad on it's own. FYI, I've rebuilt 125 of these units and have at least 7K hours of experience with the ZF S6-40 design.
(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.
A.) Dean, it would be a nearly impossible for a ball bearing to fall out of the main bearing case. If the noise goes away when you push the clutch in at idle,... that could be 1 of 3 things, either a bad dual-mass flywheel, or the flywheel is not an original dual-mass type but instead is a single mass type. The ladder situation will produce a sound known as gear clatter while in neutral which is harmless to the transmission. The springs in the clutch disc, which are not present in the disc of a dual-mass set up, will oftentimes produce a rattle like resonance in the 1K - 2K RPM range when under moderate to heavy power load settings. These are the drawbacks of converting over to a single-mass flywheel.
A bad input bearing in the transmission will normally produce a whirling or whining sound proportional in pitch to engine revs. In 1st gear the whine is most prominent decreasing in intensity in each subsequent gear to the quietest level being 4th gear (direct drive energy flow).
A bad dual-mass flywheel with a failing damper will sound like a diesel truck engine at idle or a clacking noise. Under certain load settings, this sound may be heard occasionally in 1st gear too.
The main bearings in a ZF S6-40 are very different from those used in Muncie & Borg-Warner 4 speeds. The ZF main bearings are self-contained non-preload non-tapered double-row type bearings. One row is comprised of cylindrical type bearings and the other row is comprised of ball type bearings. This type of bearing would be more similar to the type of bearings used in turbine engines.
If it sounds like a diesel engine at idle, the flywheel dual mass damper may be failing. What I've seen happen before is that when a flywheel is determined to be bad, some people choose to replace it with a less expensive single mass type flywheel where they can buy the flywheel, pressure plate with throw-out bearing and clutch disc for $800 instead of $750 for OEM flywheel and $300 for the clutch kit.
If it is more of a whining or whirling sound, the input bearing has been compromised. This results from the alignment of the transmission during reinstallation is off slightly where the trans, bell housing butt up to the block. I've never heard of a ZF S6-40 main bearing going bad on it's own. FYI, I've rebuilt 125 of these units and have at least 7K hours of experience with the ZF S6-40 design.
Edited by Gixer on Friday 31st May 18:23
So the gearbox seems to have been overfilled by a pint or so and has been spitting out the excess through the breather. This has dripped down underneath and left a greyish residue as per the second photo. The residue could well be metallic particles and the gearbox oil does not look at all clear for an 8000 mile car.
Any suggestions?
Any suggestions?
Well you're not going to know for sure unless you open it up. However when my diff had a problem the oil that came out resembled metallic silver paint not anything like that. Personally I still think you are hearing normal ZF gear rattle. I'd refill with M5 oil and carry on using it. Where abouts are you? If your are near me you are quite welcome to come and hear how bad normal gear rattle is.
franv8 said:
Is this not the world's most pointless gearbox removal thread?
Brilliant post. Thanks, very helpful. 
FWIW the bellhousing is being removed to be modified as a preventative measure: a well-documented Lotus Carlton weak spot. Whilst they are at it I asked the workshop to see if they could discover the cause of the extra transmission noise, which has only developed during the past few months.
Gixer, thank you for your offer. I see you are in Kent, but I don't have any plans to travel that way from Northamptonshire in the next couple of weeks. If anything changes at short notice I will PM you.
My pleasure.
Even the Carlton sites say the box will be noisy. Yours isn't the most strenuous application for the box, and it's done naff all mileage. Change the oil as it's 20 years old and leave it alone.
Or just spend money having it stripped down and risk having it put back together wrong. Your choice. Good luck with it.
Even the Carlton sites say the box will be noisy. Yours isn't the most strenuous application for the box, and it's done naff all mileage. Change the oil as it's 20 years old and leave it alone.
Or just spend money having it stripped down and risk having it put back together wrong. Your choice. Good luck with it.
My pleasure.
Even the Carlton sites say the box will be noisy. Yours isn't the most strenuous application for the box, and it's done naff all mileage. Change the oil as it's 20 years old and leave it alone.
Or just spend money having it stripped down and risk having it put back together wrong. Your choice. Good luck with it.
Even the Carlton sites say the box will be noisy. Yours isn't the most strenuous application for the box, and it's done naff all mileage. Change the oil as it's 20 years old and leave it alone.
Or just spend money having it stripped down and risk having it put back together wrong. Your choice. Good luck with it.
Just to put this one to bed. It transpired that when I had the timing chains uprated (another LC weakness) 3 years ago, the workshop did not tighten-up the bell-housing quite square when they refitted the oil pan. Hard to believe but tell-tale washer marks gave the game away. The mating surface of the bell housing for the engine block and oil pan is flat so the cylinder block face and oil pan must be correctly aligned when the engine is built up off the car. There is a special tool for this but a straight edge will do the job. If these two parts are not correctly aligned problems will occur with the drive train, this is because the rotating parts are being forced to run at an angle putting far greater load on them.
The clutch plate was well worn so was removed and relined. Once the bellhousing had been modified everything was refitted in the correct manner. The good news is that the excessive transmission noise at idle which prompted this work has now all but gone, so is probably now similar to all other LCs. I'm also glad I got the clutch plate relined and refitted at little extra cost whilst the transmission was off.
So all's well that ends well. Next job is the cam sensor which needs repositioning: presumably another hangover from the clumsy timing chain change...
The clutch plate was well worn so was removed and relined. Once the bellhousing had been modified everything was refitted in the correct manner. The good news is that the excessive transmission noise at idle which prompted this work has now all but gone, so is probably now similar to all other LCs. I'm also glad I got the clutch plate relined and refitted at little extra cost whilst the transmission was off.
So all's well that ends well. Next job is the cam sensor which needs repositioning: presumably another hangover from the clumsy timing chain change...
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