83 300sd trans shift problems - PeachParts Mercedes-Benz Forum

Steve H · #1 08-25-2018, 09:24 PM

Last week I purchased an 83 300sd, nice body, nice interior, but the running gear needs help, odometer stopped at 308K+ miles. Engine runs strong, but the first issue is the trans shifting. I loosened the boden cable to where there is considerable slack, 1/2 shift is slightly severe, 2/3 is about the same, but the 3/4 shift is quite severe and won't shift until you take pressure off the accelerator. Tomorrow I will check how much vacuum I have and if the modulator holds a vacuum. Past that I am lost, any and all help will be appreciated.

Thanks, Steve

tyl604 · #2 08-25-2018, 10:29 PM

I would go back and put the Bowden cable back where it was. Then check the vac pressure with a Mityvac on the VCV. You should have something like 12-15 iHg between the VCV and the tranny. When you push the accelerator the vac should quickly go down to zero because you do not use vac to upshift. (Technically not exactly an accurate statement but close enough.)

If not good, you can prise off the plastic dome from the VCV and adjust the vac. I did this on mine to increase it from 10 to 12 or so;

If that looks good, I would unhook the vac line behind the VCV and hook it in to the Mityvac. Drive around with zero vac and see if you get a smooth upshift (the downshift will clunk). Then drive some more but before coming to a stop, pump up vac with the Mityvac and see if you get a smooth downshift.

If all this works, your vac modulator and tranny are probably good. So experiment with adjusting the vac modulator. If I remember right, CW is for firmer shifts.

If you do all this and still have problems, I would guess it is actually a tranny problem and not a vac problem.

If you are interested you can follow my steps to success in my thread called something like Tranny Goes Clunk. I had another tranny installed and the indy had no idea how to adjust it. With a lot of help from the forum guys here, I got it done and have had no problem for several years now. 1981 300SD.

Diseasel300 · #3 08-25-2018, 10:44 PM

Definitely check that you have vacuum at the modulator. If you have an extra rubber tee, attach a vacuum gauge and go for a ride. As mentioned above, you should have 12-15" of vacuum at idle or when coasting. WOT should be 0", and anywhere in between should have a value between 0 and 15".

Since the vehicle is new to you, it may be a good idea to start with a transmission service. Change the fluid and filter. Drain the torque converter as well. Install a new pan gasket and dump in fresh fluid. Dexron III. Use something cheap if the fluid you drain was pretty nasty, you'll want to change it again in 5000 miles or so.

Also do a search for the K1 accumulator spring. It's well known for falling apart and causing sloppy or delayed shifts from 2-3. It's possible a previous owner overcompensated for the flaring by turning the modulator up way too high.

My SDL went from "shifting by Helen Keller" and feeling like being rear-ended with every gear change to being extremely civilized "slightly firm" just with a fluid and filter change.

tyl604 · #4 08-25-2018, 10:57 PM

Agree with Diseasel300. Change of fluids, etc. might be the ticket.

Steve H · #5 08-27-2018, 01:21 PM

Thanks for the help guys. All I have checked is the vacuum pump- 21/22" Hg, and the modulator which will hold 25". I'll work on the rest of the suggestions as they were laid out.
Again thanks
Steve

Steve H · #6 08-27-2018, 07:29 PM

I checker the vacuum to the modulator and at idle it is 15"Hg. However, WOT in 1st foot on the floor it never got below 6". I ordered a new trans filter and gasket plus new fuel filters so will do that when they arrive. Back to the trans do i need a new little green vacuum control valve?