'81 300D modulator? - PeachParts Mercedes-Benz Forum

#1 08-30-2002, 09:42 AM

I HATE automatic transmissions, so as a result, I ignore them when they give trouble. This thread will be evidence of that.

I rebuilt the engine in my daughters car about a year ago and ever since it shifts way too early. I checked the line to modulator and all was well. It worked good before that and I can't think of anything other thing that could cause this except the modulator and associated components.

The car is down for some maintenance now, so I want to check the vacuum contraption on top of the valve cover and the modulator.

Any comments or suggestions are appreciated.

Have a great day,

#2 08-30-2002, 10:31 AM

Larry,

Something I found by accident once while doing a valve adjustment was where the bowden cable attaches at the top of the valve cover it can go past the stop and gets to much slack. This made for very early (but smooth) shifts. I thought I had broken a line or pulled something loose. Took some looking before I noticed what was wrong. I have seen another post here where someone removed the black box thing with no bad side effects so I don't know if it has anything to do with the tranny shifts.

surfblau · #3 08-30-2002, 11:58 AM

Aren't you describing textbook Bowden cable too loose?

For sure you know way more about these cars than me (just buying my first one six months ago, and not even close to being able to rebuild an engine), but is there some reason that you didn't look at the cable?

#4 08-30-2002, 01:42 PM

As I say, I HATE automatic transmissions. I did indeed R&R the black box contraption during R&R of engine. I now can't wait to get home to give this a quick look. I assumed the problem was vacuum related.

Thanks VERY MUCH for the tips. Rebuilding the engine, no problem.

Replacing a clutch, no problem.

Messing with an automatic transmission, ignore it and it will go away.

Thanks again and have a great holiday,

engatwork · #5 08-30-2002, 02:58 PM

Larry,
I have the same issue on my 300D. It is in high by the time I get to 40 no matter what. I have replaced the bowden cable, a couple under hood components and tested the modulator on the side of the tranny. Nothing works. I am going to eventually test the TD (shifts great) and see what the differences are in the vacuum signal to the modulator.

#6 09-10-2002, 04:38 PM

Well, I finally got to this and the '81 doesn't have the Bowden cable you speak of. There is only the vacuum switch contraption on top of the valve cover. Does anyone have any information regarding the vacuum profile from this thing?

If I knew what the vacuum at various throttle positions is supposed to be, I could put on a vacuum gauge and do some testing.

I feel that on this model, the problem is this contraption, a leak along the way or the modulator.

Thanks for any help you can offer.

Have a great day,

diesel don · #7 09-10-2002, 05:03 PM

Larry:

The following describes my journey and the data I've collected since trying to solve my "flaring" problem between 2-3. Much of it is the same for tranmission adjustment.

The most comprehensive discussion on this is from Steve Brotherton (some kind of tech) who seems to know these trannys inside and out. Perhaps as much as 6 factors need to be in balance for the trannys to shift properly. I have made great improvement even though its not perfect yet.

His information (5 page printout) is available at:

www.continentalimports.com

click on "articles" and then "mercedes-benz transmissions".

He has pics and the whole shootin' match.

Don

diesel don · #8 09-10-2002, 05:05 PM

engatwork:

did i see you with a gold (looked orange) non-turbo for sale on e-bay? looks like a nice car for the price.

don

diesel don · #9 09-10-2002, 05:08 PM

Larry,

Sorry for not including this in the former post but another very helpful article for me was from:

www.mercedesdismantlers.com

scroll down to the bottom and click on:

Automatic Transmission Technical Information.

Under that you'll find a couple of great options depending on your tranny.

Don

engatwork · #10 09-10-2002, 05:44 PM

yea Don - that's me. I am selling it for a friend. The only thing it needs is rebuilt 1/2 shafts - they are noisy. Reserve is only $1200.

diesel don · #11 09-10-2002, 06:23 PM

Jim:

I was trying to get a friend to buy it today so we could take a road trip. I figured he is at two and I'm at four, so its his turn. He wants to but his daughter is only 2 yrs old and he thinks its a year or two premature.....sigh

Don

sean 300D · #12 09-12-2002, 12:36 AM

Larry...My 81 300D (180K) as of tonight, actually!...does the exact opposite...it tends to rpm up pretty high and shift late...I'm not a transmission person either!..so I don't know?!? Almost feels as though it's "slipping"...however..here's the scary part...the fluid level looks good...and it shifts much smoother when warmed up. Which here in the middle of 100 deg. every day weather Kansas..that doesn't take long! What's going on with this???..any ideas?..I really like this car, and don't mind spending $ on it..(it's a 2nd car)...and...to anyone else out there...I'm looking at a '87 300sdl in Kansas city this wknd..any suggestions?

thanks
sean

#13 09-12-2002, 06:45 AM

Diesel Don,

Thanks very much, this is helpful information. I printed the Steves article yesterday and digested it. Then I just read the wrecking yard article and it really started my thinker to thinking.

Thanks a bunch for sharing your research.

Sean,

In your case, it sounds very much like you have a lack of vacuum, the opposite of my case in which I think I have too much. The system basically "bleeds" the vacuum to lower it. This is how the contraption simulates the vacuum of a gas engine. Do you have any other vacuum related problems such as door locks or slow to shut off? If so, start looking for a vacuum leak.

Good luck,

#14 09-12-2002, 07:39 PM

Sean; re: 300SDL

The 300SDL's engine and tranny are especially durable, compared to the trannies of earlier years. Make sure the shifts are firm and crisp. Pop the hood and look at the belt tensioner. It should not be loose, making a racket or the pulley angled. Make sure the sunroof works (big $$$). If you near some lifter noise, don't worry about it, its a characteristic of these engines. Make sure it starts easliy with minimal smoke. Make sure there is not alof of smoke at full throttle. Good luck and let us know how you make out.

John

Ridge · #15 09-12-2002, 11:40 PM

You know you guys could save yourselves years of adjusting automatic transmissions by just junking the damn things and converting all your cars to manual transmissions out of 240D or if you're really lucky a 5 speed out of a European 280CE.

Besides not having to constantly have to monkey with the damn things you'd also not have to make reservations and get a printed invitation to cross an intersection.

We're putting together all the components to do our fourth auto to manual on a 300Turbo diesel that we've also converted to probably the first intercooled 300DT in the US and maybe the world. The local MB club President gave my son a manual tranny that he'd run in his 280CE. These manual trannys are cheaper than dirt and so much fun.

Join the care free world, switch!!!

Ben