Bernie

Hey All,

So I have a clunk in the rear of my 85 AT when shifting between reverse and drive.

The tranny was always a little on the slow side to shift between these two but now there is a noticeable clunk when I do it.

Already changed out the tranny fluid and I also want to work on the shift firmness as the car shifts pretty hard on acceleration. My be cool for some but for me it has become an annoying. I am thinking I have no vacuum to the tranny but it will take a while to confirm this.

So I am thinking it is in the rear end, maybe the CV's are worn. What method can or should I use to confirm that there is or isn't an issue with the CV joints?

TIA!

Bernie

mj1pate

I'll leave it to others to offer diagnosis for the V joints, but if you have no record of them ever being regreased, I would do it or have it done. A competent mechanic should be able to do the job and diagnose any wear issues, just like for any other independent rear axle car. provide him with the torque settings for the half-shafts.

Your automatic should not slam from one gear to the next. This is usually caused by an open vacuum line on the long run to the transaxle. Do not let this go unattended, as sheared torque tube rods ultimately result. The normal path for vacuum is a long metal tube attached high on the torque tube. You can choose to reconnect flexible hose to both ends of it, or rerun a temporary flexible metal line along the outside of the frame. Yes, you really may be able to reach and replace the flex hoses on either end of the existing metal vacuum tube, with the exhaust heat shields off. The aft section is reachable from the passenger side, just above and in front of the transmission on top the torque tube. You feel it, more than see it. The fore section is reachable from the passenger side, on top of the torque tube...again, easier to feel than see. You can yank the old flex hoses off and with the aid of some sensor safe lubricant, press on new silicon hoses. Be ready to exercise your 4 letter vocabulary in the whole process.

Good luck

troy928

The cv joints are probably a little worn and that's causing the clunk. Roger at 928's rus told me a trick last summer when I was replacing my worn out boots. He told me to switch one side shaft with another, they are both identical. Regrease the joints and install the boots, you're good to go...no more clunk!
Call Roger to order the boot kits and have him confirm this procedure.

Cheers,
Troy

Cosmo Kramer

How high is your idle? If it is too high you will get that problem.

GregBBRD

While you might have worn/dry C/V joints and you could indeed have vacuum issue these transmissions shift fairly hard from 1st to reverse and back again into 1st. Once the internal friction discs start wearing (and most all we see are very, very worn, at this point), the "run-up" distance the pistons have to make gets longer and they engage the discs fairly hard.

You'd think that this would get better, when the transmissions get rebuilt, but the new style Mercedes discs tend to be fairly aggressive and they can clunk also. We got so tired of this, that we invented and make up a little restrictor that softens the initial piston contact, without changing the internal pressure, so the ending "clamping force" is unchanged. We install one in every transmission that we rebuild....but I wouldn't suggest installing one in a worn transmission.

bobradder

Check out the following from POrsche 928 Email Keepers site (-http://members.rennlist.com/pirtle/z0_keep_main.html)

----Original Message-----
> From: Gary [mailto:faast@iinet.net.au]
> Sent: Monday, July 24, 2000 11:24 PM
> To: 928
> Subject: [928] Clunk when changing Park-Rev-Drive
>
>
> Any ideas or common problems?
> My car (an '87 S4) has always had a clunk sound when changing from park to
> rev, and rev to neut, and neutral to Drive.
> Seems worse Park to Rev if I have been going forward previously ,
> and rev to
> drive if I have been going backwards.
> If going forward, stopping, & going Park to drive quickly doesn't seem too
> bad.
> I have just had a transmission pipe (near the top of the radiator) changed
> because of a small drip after fixing the cooling fan (I think the movement
> of the old hose made the small hole near the join). This has
> been my first
> leak (apart from power steering a little while back).
> I showed my mechanic the clunk (I don't think it was as quite bad before I
> took it to him, but it was still noticable & I could be mistaken) - he
> checked the transmission fluid & topped it up a little. He said with age
> they all make a clunk, but he was also worried about the 3 or so
> very small
> clunks after the big one.
> The car still drives really well, and changing gears automatically whilst
> dribving is still very smooth.
> He thinks it could be friction plates or pinnion slackness (as it is much
> worse when changin direction.
> Any common faults I could tell him when I take it back?
>
> We have a German Car day this Sunday. He said he should ovehaul the
> transmission - do you think it will be OK to leave this until next week?
> The drive on Sunday is about 3 hours (and 3 hours return on Monday), but
> mostly straight driving & I should be changin Park - Rev - Drive
> very much.
>
> Any help greatly appreciated.
>
> Thanks,
>
> Gary.
>
>
>
>
From: Jim [jim@928intl.com]
Sent: Thursday, July 27, 2000 12:44 PM
To: 928
Subject: [928] RE: Clunk when changing Park-Rev-Drive

The 86 model and some 87 automatics used rivets to secure the torque
converter drive plate to the hub/input shaft inside the front of the
transmission it is a triangular flat -- these rivets can become worn and
allow some movement -The Porsche approved fix is to replace the rivets with
bolts which is how all the other transmissions are made .

Jim Bailey
928 International
jim@928intl.com
79 928 5spd
80 928 5spd
68 911 5spd

Hope this might help you...it did me.

Thanks

Bob Radder
87S4

BIMMERMIKE

I get the same clunking but only after the vehicle has reached operating temps. I'm conveniced it's a high idle issue/vacumm leak!!

Bernie

Mine is an 85 so I am not sure if the rivet problem exists?

First thing I am going to do is get to the vacuum line at the transmission and see if I am getting any vacuum there at all.

What amount of vacuum should I be seeing pulled through that line? I dont have a complex vacuum gage but I do have a mity-vac which will maybe read low enough to tell me if there is vacuum present?

What amount of vacuum would I expect to see if the gage is at the end of the transmission vacuum line?

Cheers
Bernie

WallyP

Ears and fingers will tell you if you have a healthy vacuum on the hose.

You MityVac will read the vacuum and give you an exact number. It should basically be the same as the vacuum at the intake manifold.

Mark SF

A clunk when going from forward to reverse is normal and a normal part of the aging of the transmission. If it's shifting normally between the forward gears, forget about it unless it's getting ridiculously loud.

More positive shifts between the forward gears is a good thing as it reduces wear of the clutch plates.