380 SLC vibration - PeachParts Mercedes-Benz Forum

ERM · #1 02-22-2009, 08:53 AM

I am looking for help trying to diagnose a vibration.I am working on a 1981 380 SLC (European model) with a 4 sp.auto model 722 304.I get a vibration at about 45-50 mph,the vibration gets worse a the speed increases.The vibration ONLY happens in 4th gear, if I downshift 1 gear it goes away no matter what speed, as soon as it shifts into 4th gear the vibration comes back.This vibration does not seem to be tied to the engine speed.There is no vibration in neutral,or in park.Here is a list of what I have replaced.
motor/trans mounts 2 years ago
both flex disks/rebuilt & balanced driveshaft 1 year ago
rebuilt axles 6 months ago
The transmission has no other issues ,shifts smooth,no delays going into reverse or drive,no slipping or flaring on up-shifts.Could this be a torque converter issue ?Any ideas?thanks

mbdoc · #2 02-23-2009, 08:11 AM

Either the trans mount OR the rear subframe mounting.

ERM · #3 02-23-2009, 11:11 AM

M.B.DOC thanks for your reply .The trans. mount was replaced 2 years ago and has less than 1800 miles.I checked the rear subframe mounts when I installed my re-built axles in Oct. 2008 they were in good shape.Would bad mounts cause a vibration only in 4th gear?This is the only time I get the vibration is in 4th gear.I can drive at 55 mph with the vibration, and if I floor it to hit passing gear the vibration goes away,as soon as it shifts back to 4th gear the vibration comes back.

freeride · #4 02-23-2009, 11:23 AM

Then maybe You should check Your center shaft support... Most like the bearing will be fine but the housing is rubber. If there is play there must be vibration

dpkreuze · #5 02-23-2009, 12:55 PM

Can you describe the vibes with more detail please? Location? Frequency? Related sounds? Does drive-shaft angle effect the vibe?

ERM · #6 02-23-2009, 06:25 PM

Freeride: center bearing and support was replaced when driveshaft was re-built and balanced 6 months ago.Driveshaft balanced with new bearing and carrier
dpkreuze:
location:hard to say, I feel it through the seat and the floor.
frequency:It seems to be the same frequency as the driveshaft-rearend,as I said before the vibration "only" appears in 4th gear.No matter what the engine rpm.
sounds:no sounds associated with the vibration.
driveshaft:no, I tried shimming the center bearing on the driveshaft no difference.

thanks.

dpkreuze · #7 02-24-2009, 01:11 PM

I have never heard of the clutches in a trans causing the symptom you describe. HOWEVER, I have an open mind (too much lisurgic I guess). The rear clutch is engaged in 4th gear to provede direct drive thru the trans with the front clutch engaged also. The rear clutch is not used in any other forward gears just reverse. Wanna drive flat out in reverse?