ZF Trans noise

[69Crazy]

I am bothered by a problem with the six speed trans. in a '90 Coupe. When idling in neutral with the clutch out, there is more of a rattling noise than I would like. I was told that this might be inherent to the clutch design of the transmission and therefore normal. Is there any fix for this??

Shake, rattle, and then ROLL when you put it in gear!

Thanks for your help,

Louie

 

[Rob]

Louie,

Regarding the 1990 ZF transmission chatter, the following was posted over the ZR-1 Net on July 12, 2000:

From ZF:
"All S6-40 transmissions installed in the C4 Corvette contain 100% helical gears. In fact, all of the gearboxes ZF produces for passenger car applications, contain helical gears, the primary purpose of which is to increase gear contact ratio and thereby minimize gear noise.
There is one major difference between the model year (1989-1992) and (1993-1996) C4 Corvettes - the contact ratio for the later years was further increased in order to reduce the gear noise. There, however, was a very slight reduction in torque capacity of the gearbox (from 610 Nm to 540 Nm) when ZF increased the gear contact ratio in the later year transmissions.
The noise you hear in 1st gear is commonly termed "1st gear growl". This noise is inherent to the design of the 610 Nm gearbox and in no way is detrimental to the reliable function of the transmission and your [Corvette]. With the increased gear contact ratio in the 540 Nm gearbox, ZF also was able to reduce the "1st gear grow"."

Basically, it's normal. They all do that, including my '90 when it's parked, in neutral and idling.

 

[Ken]

...including my '90 when it's parked, in neutral and idling.

:L I read an article once where the guy swapped-out the dual-mass flywheel and he said the noise at idle in neutral was... I forget the word he used to describe it, but it was horrendous! :L

He said it was no problem, just noisy as hell. He could eliminate the noise by depressing the clutch.

Myself, I like the sound of an industrial gearbox, the "Rock Crusher" comes to mind.

_ken

 

[69Crazy]

Trans noise

Thanks for the information. My '69 has the Muncie and it is a sound that I am accustomed to......I guess I will just have to get over it!

Louie

 

[c4c5specialist]

Thanks for the education guys, didnt know any of that information. I knew it was normal, but thats about it.
Thanks again, c4c5

 

[Rob]

Re: Trans noise

Thanks for the information. My '69 has the Muncie and it is a sound that I am accustomed to......I guess I will just have to get over it!

Louie

It's one of those inherent C4 traits that I always mention from time to time. You just get used to it after a while and I think of it as....."my tranny is stronger than that '95 sitting over there.

 

[Topless]

Learn something new every day

That noise in nuetral was something I wondered about too. Good to know it is normal. Although, I think my clutch is on its last legs....oh well. Not slipping, but occasionally hard to get in to gear.

 

[Hib Halverson]

Generally, with the early ZFs, a higher level of gear rattle in neutral and clutch out than with any passenger car manual is normal.

That said, the gear rattle problem can get worse if the car's dual-mass flywheel is failing. Typically, if the dual mass is going bad, you hear a very short rattle after you turn the key off, however, if you suspect a bad dual-mass the only way to know for sure is to disassemble the powertrain and inspect the flywheel.

 

[69Crazy]

Thanks for the information! I just can't accept the noise that I hear as normal. I have to learn more about the dual mass flywheel. I hope that the Helm manual (that I have on order) has this information. Is there any particular referencce that you would suggest?

Thanks again,

Louie

 

[69MyWay]

Generally, with the early ZFs, a higher level of gear rattle in neutral and clutch out than with any passenger car manual is normal.

That said, the gear rattle problem can get worse if the car's dual-mass flywheel is failing. Typically, if the dual mass is going bad, you hear a very short rattle after you turn the key off, however, if you suspect a bad dual-mass the only way to know for sure is to disassemble the powertrain and inspect the flywheel.

Hib,

I first learned about this wonderful little trait on our old 89. With that said, I did a lot of research when I was replacing the clutch and went to get the flywheel surfaced. The G.M. manual says that is a :nono

Anyway, I said, what the heck, I will just buy a new flywheel. Back about five years ago, G.M. wanted $810 for a new one

Don't tell G.M., but I took her flywheel down to a local trans shop and they resurfaced it anyway for about $20. Here is the good news, it worked like a charm and the new clutch and all was strong as an ox. No worries at all.

We are way overdue for a new clutch right now in the 90. Everything keeps getting in the way. I actually tried to beat them down on the price when we bought the car because I kept telling the guy it clearly had something wrong in the trans.....:argue but, being that he was a Corvette dealer and recycler, I could not convince him that the rattle rattle clunk at idle was anything bad. Hey, it was worth a try.

 

[69Crazy]

Better references

Is there a better reference for information about the ZF trans. than the service manual?

Louie

 

[Andy C]

I have had 2 '90s and they both had the flywheel rattle. The first one had some bad fuel injectors that caused a very slight rough idle and that made the flywheel rattle more. Bad injectors were common on the 90 and if you haven't had yours checked you may want to. My current one rattles only on shutdown and, as far as I know, that is completely normal. I remember some article about updated stronger springs in the flywheel to reduce the rattle but it seemed like alot of trouble to disassemble the trans to elimate a minor character quirk of this really nice car.

 

[69Crazy]

What is actually rattling?

I am sure this reflects my ignorance about the dual mass flywheel; but, what actually is rattling????????:confused :confused

 

[Andy C]

As the dual mass implies, the flywheel is actually two flywheel rings connected by springs. It's the two parts rattling against one another that create the noise. You can replace it with a standard single mass flywheel if the rattle at idle really bothers you, but then you trade that for gear rattle as you drive along because the flywheel no longer dampens out the engine pulses that get trasmitted through the transmission. That's why GM came up with the dual mass. As far as I know it's specific to the ZF because of the transmission design.

 

[Hib Halverson]

Dual-mass flywheels are unusual, but not unheard-of, in passenger car applications but fairly common in medium and heavy truck applications.

With 89-96 C4 manuals, the early dual-masses were more prone to problems with age than are the 91 or 92-up units.

The reason you shouldn't (I say "shouldn't" not "can't") resurface a dual-mass is that when you resurface the secondary mass you remove material further reducing its mass. Reducing its mass reduces its effectiveness in damping the grear rattle as well as reducing its heat-sinking ability--which, I might add, is already marginal. No doubt there are a number of resurfaced dual-mass flywheels out there but they don't work the same way they did when new and are less tolerant of heat build-up.

This doesn't mean resurfaced dual mass wheels won't work at all but it does mean they don't work as well and probably will not be as durable.

 

[Topless]

I can't help myself....

Can you ditch the dual mass and replace it with an aluminium flywheel? What would be the reprocussions? More problem than benifit?

-MIke

 

[Ken]

TPIS has this:

Flywheel Kits

"Corvette, 1989-1997, six-speed Corvettes with six speed transmissions have a dual-mass flywheel that is very expensive ($862 dollars), very heavy (over 40 lbs.), and they wear out.

We have a solution - we use a 1987 Corvette 4+3 flywheel, PN 14088646, which we can furnish, or you can get it from your dealer. Our Kit includes a special clutch pivot ball and a special clutch disc which features a unique friction material for long life.

Unless yours is badly worn, you can reuse your existing pressure plate. The end result is a very lightweight clutch assembly that will accelerate much faster and be easier on synchros. We have found the stock pressure plate to work fine - even on 500+ HP applications.

Note: There is a downside to this setup -- with the car idling with the clutch out there will be a clattering noise coming from the transmission. This will not hurt anything but it can be annoying. With the clutch depressed the noise goes away. The noise has always been there but has been dampened by the dual mass flywheel.

_ken

 

[Topless]

"Note: There is a downside to this setup -- with the car idling with the clutch out there will be a clattering noise coming from the transmission. This will not hurt anything but it can be annoying. With the clutch depressed the noise goes away. The noise has always been there but has been dampened by the dual mass flywheel."

Well, since mine does that now, there really would be no down side. :L

Thank you.

 

[69MyWay]

Vibration

Ken,

In your travels and research have you seen where anybody has mentioned that a failed dual mass will cause a vibration?

We have had a minor vibration in the 90 since we bought it. It hits about 4,000 rpm and will shake the gear shift handle. This is whether you are in neutral, clutch in, out, or going down the road. I have ruled out everything, but the front balancer maybe slipping or the flywheel.

Any thoughts?

 

[Ken]

Dual Mass Flywheel (DMF)

I'll keep looking, but check this out. It may help you.

Center Bearing: A sealed double row center ball bearing carries the load between the inner and outer halves of the DMF. The leading cause of center bearing failure is often related to out of balance vibration caused by not aligning the pressure plate with the DMF dowel pins. The center bearing may also fall if the clutch pilot bearing is destroyed by a worn transmission input shaft (see Pilot Bearing) or if the rated load/towing capacity of the vehicle is exceeded.

Tech Bulletin

_ken