w126 1984 300SD Automatic Transmission Rough first shift, then everything Smooth?!?! - PeachParts Mercedes-Benz Forum

Theseus · #1 03-14-2023, 07:21 PM

Hello fellow Mercedes enthusiasts,

I'm seeking some advice regarding a peculiar issue with my 1984 300sd. Whenever I start the car and shift to 1st gear, it runs smoothly. However, there's a strange delay of about 3 seconds when I try to shift to 2nd gear, and the car revs up to 4000 rpm before it shifts. This only happens the first time I start the car in a day, and afterward, it drives like a normal car.

I'm wondering if this is a normal behavior for a 300SD, or if it's a sign of a potential problem. I've checked the vacuum lines for leaks, and they seem fine. Although, I did find a slow ATF leak in the input spline area. Also, it takes around 2 seconds for transmission to engage park to reverse.

If anyone has experienced this issue before or has any insight on how to diagnose it, I would greatly appreciate your input.

Thank you in advance.

jdulle · #2 03-15-2023, 11:36 AM

I have heard that these cars hold the gears to a higher rpm when they are cold. I think it is done to help speed warm up. Mine definitely does this, not quite to 4k but close. I usually let up on throttle for a few seconds to make it shift earlier. You could also try adjusting the boden cable looser. This would make it upshift sooner, but it might then upshift too soon when it's warm.

MB140300SD · #3 03-15-2023, 10:52 PM

Quote:

Originally Posted by jdulle

I have heard that these cars hold the gears to a higher rpm when they are cold. I think it is done to help speed warm up. Mine definitely does this, not quite to 4k but close. I usually let up on throttle for a few seconds to make it shift earlier. You could also try adjusting the boden cable looser. This would make it upshift sooner, but it might then upshift too soon when it's warm.

That is a feature that some years early to mid 90s of gas models have to warm up the converters faster. Diesels do not have this feature and certainly not something as old as a 84.

JHZR2 · #4 03-19-2023, 11:24 AM

I’d say it isn’t normal, unless your car had a replacement at with such a function from a gas car. Even then, 4000 rpm is too high. I have no reason to take any of my cars to 4000 except my 190D when driving over 70. No 5/6 cyl car should have to go that high.

My first guess would be poor flow of ATF and/or drain back. Not enough that the car won’t move, but enough that you can’t access that gear until fluid flows.

When was the pan dropped and filter changed last? What did the pan look like in the bottom? Sludge? Chips? Nothing?

Starting with a clean filter, and correct ATF level of good fluid (and some lubeguard red for good measure).

What history do you have regarding the transmission, its replacement, its servicing, etc.?

vstech · #5 03-23-2023, 02:43 PM

Check the carpet under the gas pedal...
If button is pressed or sticking, it will do this.

BillGrissom · #6 03-23-2023, 09:52 PM

I wouldn't say normal, but it is somewhat common, though I wouldn't describe it a 3 second delay, rather I've seen some go to high rpm, maybe 3000 rpm before thunking into second, even with light accelerator. I especially recall test driving a ~1982 300SD which did that. The "shifting too high rpm" is said to be much improved if you go thru a checklist you can find here (or links, forget) to insure your VCV is optimally set. You also need good vacuum from the pump, and a good vac modulator at the transmission. The VCV makes a vac signal which simulates a gasoline engine's intake manifold pressure, using just the position of your accel pedal.

The Bowden cable adjustment also matters. Tighten the cable to make the tranny think your pedal is more down to help shift at lower rpm (I think). If your plastic adjuster (atop valve cover) is botched (common), a trick I did is to cut 1/4" tubing, with a sprialled axial slot (to slip it on) to hold the cable housing out to tighten the cable.

1985 had the "Blue Saucer" vacuum amplifier. That sends a stronger (and quicker?) signal to the transmission's modulator. I put one on my 1984 since seems it could only help, but don't recall noting a difference. They continued that in later models, and I've seen many M-B vac amplifiers when searching ebay for stuff. Mid 1970's Chryslers used similar vac amps in the engine controls (emissions stuff).

If your engine flares up in rpm between shifts, that is a sign of worn clutch plates slipping, though could be other things. I had that in our 1996 Plymouth minivan, tore the tranny apart for rebuild and found the clutch plates in the L/R pack worn to bare steel (was flaring the 1-2 shift so makes sense). Fine since I fixed rebuilt it 15 years ago. Your M-B transmission is more intimidating inside (tore a 1983 apart to save the hard parts). Only a few here have rebuilt one, but one member did and posted great photos.

Theseus · #7 03-24-2023, 11:53 AM

Thank you for the responses. I have noticed that this issue only occurs ONCE. Then the RPMs get high, and after that, the transmission works "normally". It seems as though the problem is solved because some fluid got filled or air got cleared from somewhere after that point, but it's hard to say for sure. I have also noticed a slight flare between the 2nd and 3rd gears. First and second an a hard crisp shift.

Just to give some background, I had previously installed the blue UFO vacuum averaging device from the 1985 model w126, which helped improve shift quality, but that was before I started working on this particular shifting issue.