'84 928 auto transmission question

Tim Boyer · Tim Boyer's Garage

Hi,
I am new to the 928. The one I bought is automatic . When it shifts between gears there is a "clunk" but still shifts smoothly and quickly. Is this normal or should I have my mechanic take at look at it?
Any thoughts or insights would be helpful.
Thanks,
Tim

Danglerb · 12-04-2016, 02:49 PM

If your mechanic has a LOT of experience with Mercedes auto transmissions, or a lot of experience with the 928, then maybe. If not than generally no don't let them touch the 928, its VERY expensive to teach a new mechanic how to work on them.

Be more specific on the clunk.

Tim Boyer · Tim Boyer's Garage

not to worry. Shop only works on high end cars, mostly Porsches

The clunk occurs occurs between shifts. It doesn't sound like there any kind of metal to metal going on just the clunk. I think that it maybe just the gears engaging but I never had a 928 so I don't even know if this is unusual!

Danglerb · 12-05-2016, 03:49 AM

That is the time to worry, Porsche experience has almost nothing to do with the 928, except your typical experienced Porsche mechanic will make a lot of incorrect assumptions with the 928.

Between shifts or just as the new gear engages? Clunk to me implies something is moving more than it should and abruptly being stopped. Do you hear anything with non shifting throttle changes, ie sudden acceleration from deceleration?

stepson · 12-05-2016, 08:30 AM

Quote:

Originally Posted by Danglerb

That is the time to worry, Porsche experience has almost nothing to do with the 928, except your typical experienced Porsche mechanic will make a lot of incorrect assumptions with the 928.

+1 A Porsche mechanic is not necessarily a 928 mechanic!!

Do not spend your money training a Porsche mechanic to be a 928 mechanic!!

MainePorsche · 12-07-2016, 02:23 PM

Sounds like you have mechanic(s) do the work on your car(s).
Send a Message to MrMerlin - he is a true and renowned 928 mechanical expert. He is somewhere in PA.

Mrmerlin · 12-07-2016, 06:10 PM

a clunk could be the TT damper sliding back and forth,
or loose CV joint bolts,
or loose drive shaft pinch bolts,
or axle nuts
or worn motor mounts

stepson · 12-08-2016, 10:30 AM

Tim,

I would recommend that you get your car to MrMerlin (Stan) (especially since you are in the same area as he). Stan can fix whatever is presently wrong with your 928 and also tell you what else will cause you trouble before it becomes a problem.

Tim Boyer · Tim Boyer's Garage

Thank you for the advice!
I will have to connect with Mr. Merlin. My daughter lives in Philadelphia near Independence hall.

MPDano · MPDano's Garage

A quick free check is to get your car up on jack stands, put it in neutral and grab each side of your CV joints. Verify there is no play when you twist them.

If you plan to hire someone to fix your trans, if that is the problem, get out those credit cards. It will be very expensive.

Motor and Trans mounts could also cause a clunk. You shouldn't be able to be able to move your engine much if you grab the intake and shake it side to side.

Post pics of your engine. MM's could be gone and we could tell by how low the engine sits.

Simon928 · 01-11-2017, 07:30 AM

The "clunk" that you hear could very likely be the vacuum modulator on the side of the transmission being set to make harsher shifts. Some prefer it that way believe it or not. The modulator is on the driver side of the transmission and it is a key/pin which can be found underneath a round rubber cap which can be pried off with a flat screwdriver. Pulling out the key and rotating it clockwise makes shifts harsher and rotating it counterclockwise makes the shifts softer. The car that I just bought had the exact same symptoms so I just rotated the modulator key CC until the shifts were buttery smooth. Just make sure that you don't turn it too far or else the shifts will slip.

Tim Boyer · Tim Boyer's Garage

Thank you! This sounds like a possible cause/solution!
Tim

Simon928 · 11:03 AM

It's a free fix, so hopefully that turns out to be your problem. If you don't have a lift it's a bit of a PITA to lift the car multiple times to get the setting just right, but I wouldn't rotate it more than 180º on my first try, and then maybe 90º or less in each subsequent turn. If you turn it too far and the shifts get too soft apparently it can do damage to your transmission if you keep it that way for any length of time.

If the vacuum line that goes to your transmission modulator is cracked or broken all of your shifts will also be harsh no matter how you adjust the modulator. Before you mess around with the modulator try to identify the vacuum line that goes to the transmission and inspect it for breaks. There is also a procedure to check for vacuum pressure on that line but I've never had to do it myself so the details escape me. If you confirm that the line is in good condition and can hold vacuum then you can start adjusting the modulator. You just don't want to get it really far out of whack only to find that it wasn't the cause of your problems to begin with. It would probably be a good idea to make a record of how much you rotate it in case that wasn't the source of your problems and you want to restore it to it's previous position.

Good luck!