Very strange downshifting problem
 

Yesterday my sons car started a very strange problem. I hope I can describe it good enough on here maybe someone knows what is causing it.

The car drives fine, upshifts are smooth and clean no noise no problems at all all the way through all 5 gears.

Downshift from 5 to 4 no problem, 4 to 3 no problem -

Downshift from 3 to 2 causes a strong vibration/Shudder/thumping noise from under the car.

If you accel in 2nd and then decell by totally letting off the gas no problems at all.

A simple visual inspection under the car shows nothing strange.

Any thoughts?

front or back of car?

it's prob too early to start guessing but i'm thinking either motor mount or tranny mount

Sound more middle to front.
Car doesn't vibrate at idle like bad motor mounts.
Is there a way to inspect the trans mount?

Also if it was mounts wouldn't it do this in all gears? Why only downsifting to 2nd?

check the nuts on the axles/cv joints?

3-2 have a lot of torque..perhaps some are loose?

try rev-matching to see if that cures it.

probably clutch chatter - the disk is skipping over the flywheel - this is due to one of a couple of different things

it can happen with a flywheel that is too light

it can happen with a DMF that is failing

it can happen if you have deposits on the clutch disk or it is glazed - this is similar to brakes, where you would get vibration in the steering wheel, and is often misdiagnosed as warped rotors

it can happen if the clutch disk is missing pieces

it can happen if you have a rubber center clutch disk that is failing

the reason this only happens going from third to second is that it is the largest gear spacing where you downshift while you are moving at a good speed, and the point where the most torque is being transitioned - the engine and transmission are having the biggest speed difference to overcome

quick test - on a 3-2 downshift, get the car to a speed where you know it happens - then, step on the clutch and while between gears, blip the throttle and rev the engine up to the point you would be at after the downshift - then quickly downshift and release the clutch smoothly - make sure the rpms don't fall before you release the clutch - this will take some good timing - this is called gear matching - we do this on the track - if this eliminates or dramatically decreases the chatter, you have found your problem

I will try that later, and report back

I am assuming if that works the car needs a new clutch even though it's strong and not that old?

mikeran,

I agree with flash968 on the clutch disc. Take out the starter and look for small 1/4 or bigger chunks of rubber disc. There is an inspection hole but the chunks seem to end up by the starter. My rubber disc went out last year and it happened shifting from 2nd to 3rd.

I saw you have your '86 turbo for sale. Your price on both cars is fair. I hope you reconsider restoring the turbo. PS is cheap to replace. Rebuilt rack, tie rod ends and misc parts about $400. Forget about parting out the cars. It is against your neighborhood CCRs and you will spend months on EBay and your back. My 2 cents.

GL
John_AZ
1988 924S 58K + 1987 924S 112K DD

I had the same symptoms, bad rubber center to my clutch. New clutch, no shudder downshifting. It got to the point where I had to rpm match downshifting into 4th and 3rd, originally it was just dropping into 2nd.

Mikeran, ever get this figured out? My car is doing the exact same thing.

Mine did the same thing. Soon there after, the clutch went completely. For some reason it was only as you explain with second gear. As it got worse my second gear would "jump" a lot. A lot of jerking in second when on and off the gas. All other gears were fine.

Clutch "went" how? Rubber center failed? My disc is Sachs spring centered.

Not sure exactly what the failure was. I shifted into first at a light one day, went to second and nothing. Went back to second and nothing, went to first, nothing, went to third, nothing. Rolled into a parking lot, tried every gear and nothing. The only warning was the problems with second gear.

I had a shop do the work for me. This was before I found this website. They charged me $1800 for a new clutch and all the good junk that comes with it.

It could also be the TT bearings. Mine did that on 3-2 downshifts, but not every time. Terrible vibrations and a brrrrzzz sound. Found that one of the bearings had fallen apart in the tt.

Not exactly vibrations, more like the entire drivetrain flopping about like a walrus caught in an electric fence.

I know this sounds strange, but check out the exhaust system and all the hangers. I went through this same issue on a car (nephew's) trying to track down the problem, but never solved it before the car spun a rod bearing and got parted. I read online later somebody with the same issue found an aftermarket exhaust / exhaust hanger problem that solved it. After I read this I realised the car I had been working on had a modified exhaust and was probably missing the center hanger. I figure that second gear downshifting must hit a resonant frequency in the exhaust that starts the vibration going. Maybe try and remove the catback part of the exhaust and see if goes away.

I also bet the vibrations aren't there if you rev match the downshift, correct?

They aren't there while rev matching, but the drivetrain is violently hopping around. I don't think the exhaust has enough mass to cause that.

sounds like a pretty typical clutch failure issue - severe glazing, cracked or broken friction material, or a cracked or broken rubber center will do this

I know, It doesn't make sense. I know the exact feeling of the shake you are describing. My nephew's car did it with 2 different good clutches, so i know it wasn't that. His CV's all looked good and were cleaned and re-packed and his trans mount looked good as well. We even tried his transmission in my car to eliminate that. I was at a loss untill the car spun a rod bearing.

Good luck!

yep, +1 on robm-951. check your exhaust hanger and supports especially the ones close to the torsion tube.

Rasta Monsta recently installed a 951 plastic gas tank and went to a single transaxle mount.
http://www.924board.org/viewtopic.php?t=29582&highlight=plastic+gas+tank

It seems that the shudder developed or worsened since the change.
http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/493094-tranny-mount-evaluation.html?highlight=tranny+mount
OR
http://forums.pelicanparts.com/porsche-924-944-968-technical-forum/492382-severe-clutch-shudder.html?highlight=tranny+mount

If the direction now is on exhaust hangers and supports, maybe this should be addressed first. I have no input on the single transaxle mount.

GL
John_AZ
1988 924S + 1987 924S