4-Matic Transmission Question ('87 W124 300D Turbo Diesel)

By the way, I’m brand new to this forum and to Mercedes. Thanks in advance for helping out a novice.

Short version of question: For maximum acceleration, what are the correct shift-points (in rpm) for the auto transmission on a '87 3 liter turbo diesel?

Longer version of question:

Having recently purchased this diesel with over 300k miles, I brought it in to my mechanic for some overdue maintenance and repair. While he did fix the primary problem with the auto-trannny (hard shifting) he apparently miscalibrated the shift points. Now, when I floor it for maximum acceleration, the car shifts as follows:

first to second – 3900 rpm
second to third – 4800 rpm (actually, the tranny doesn’t shift at all until I let up on the throttle a bit. I think there’s a governor that limits engine rpm to 4800.)
third to fourth – 4300 rpm.

I know it’s a relatively trivial matter to readjust the tranny, but I’m wondering now, “what are the optimal shift points for maximum acceleration?” It appears to me that this car benefits little from high rpm acceleration-wise. By this I mean the car seems to accelerate about as well from, let’s say, 45 mph to 50 mph, in third gear as it does in second. The difference being in second gear the engine is turning around 4500 rpm, while in third it’s closer to 3000 rpm. I’m not sure about the jargon, but this engine seems to have a very flat power curve, and doesn’t appear to benefit as much from high rpm as, say, your typical Toyota.

Your thoughts?

Covi,
Welcome to Benzforum. I don't normally floor my diesels(not completely,except the 240d), I don't think I've ever seen the RPM at 4800. When I experience unusual shifting in my diesels, I always look around for a loose vacumn line. You didn't mention what type of maintenance and repair your mechanic did on your car, is there a chance he adjusted the modulator valve on the transmission? Also, is it possible he adjusted the Kickdown cable? Hopefully, this is just a simple adjustment or a loose vacumn hose.
Keep us posted.

Bigbenzman,

Thanks for your reply. Your absolutely right. My service receipt say’s “REPLACE TRANSMISSION SHIFT MODULATOR” and “RESET SHIFT POINTS.” Apparently, the shift points were reset incorrectly.

The repair was motivated by my complaint that the transmission was shifting extremely hard. When up-shifting, it was literally slamming into gear and rocking the car. The repair did involve replacing a bunch of worn vacuum hoses. As a side benefit, both the rear headrest switch and the center AC vent now work (they didn’t before). But more importantly, the transmission now shifts as smooth as silk.

I wanted to check with you folks first, and your comment that you’ve never seen 4800 RPM confirms my impromptu assessment that the diesel has no business operating in this range of RPM. I’ll bring the car back to the shop and have my mechanic reset the shift points.

Cheers,
Brian

Hey what was the fix for the rear headrest and center ac vent? I have a 87 300d that has the same problem. Also my speedometer is about 6 0r 7 mph off. Any fix for this?
thanks

G200,

The center AC vent and the rear headrests run off vacuum pressure, as do many other components in the car. So the fix is to replace the broken vacuum hoses as they become worn and brittle with age.

My mechanic was primary trying to tweak my transmission, but having gone to the trouble to remove the air filter housing, I think he just went ahead and replaced any worn hoses he saw. Ergo, the incidental repair of my AC vent and rear headrests which I wasn't expecting.

Brian