87 300d Shift Problem - PeachParts Mercedes-Benz Forum

GEORGEPECKY · #1 01-29-2003, 06:08 PM

87 300D - WHEN TRANNY SHIFTS FROM FIRST TO SECOND IT ONLY STAYS IN SECOND FOR A VERY, VERY SHORT TIME BEFORE IT SHIFTS TO THIRD. SOMETIMES IT JUST REVS AND WON'T SHIFT UNTIL I LET OFF THE GAS. IF I HAVE THE GAS PEDAL ALL THE WAY TO THE FLOOR IT SHIFTS FINE. ANY HELP WOULD BE GREAT. THANKS

haasman · #2 01-29-2003, 08:58 PM

Welcome GEORGE

I don't know if you know that often readers of electronic mail and boards such as this regard typing in captial letters as yelling. It is in this way so that some emotion can be illustrated in the written word.

How many miles on your car? Sometimes low ATF levels will demonstrate those symptoms. Have you had the trasmission serviced recently? (ie new ATF and filer)? It seems there is a possibly vacuum leak, does it make any difference whether the car is cold or warm?

Haasman

GEORGEPECKY · #3 01-29-2003, 10:54 PM

Sorry about the yelling. I have about 185,000 miles. Yes, it has been serviced recently. It seems to have the problem of shifting through second gear all the time, hot or cold, except if I put the pedal all the way to the floor, then it shifts perfectly. When it's warmed up it seems more sensitive on wanting to shift back into third gear from fourth gear. After I changed fluid this problem improved greatly. What do you think? Thanks.

haasman · #4 01-30-2003, 12:05 AM

George,

If I understand correctly, it shifts quickly through the gears, unusually fast, compared to the way it used to shift.

Additionally you said "It seems to have the problem of shifting through second gear all the time, hot or cold, except if I put the pedal all the way to the floor, then it shifts perfectly."

I am not clear about "shifting through second gear all the time..."

It could be that the vacuum modulator could be out of adj or the Bowden cable needs to be adjusted.

How's the fluid level? 10mm below cold line at first start-up and at the max line after 15-20 miles on the highway?

There a several good postings on this site regarding this type of trans problem. He is one:
300E Trans Flaring

Also I presume that your car does have the vacuum modulator valve on the side of the transmission. It is operated by a vacuum line. Often the rubber tubing between the hard plastic line that comes down from the engine and the vacuum mod valve get hard and leak and/or break. Check it and see.

I used the search feature and typed this in and got some very interesting results: 300E trans vacuum Try it it.

Let us know,

Haasman

jcyuhn · #5 01-30-2003, 11:22 AM

These transmissions do have the habit of skipping through the intermediate gears darn quickly. My '87 300TD does this to a degree every now and again.

I'd start by adjusting the bowden cable. It sounds as if it has stretched a bit. The cable should be just tight with the throttle closed - no slack, but no real tension either. The bowden cable controls how long the transmission holds each gear. Opening the throttle more pulls on the bowden cable, telling the transmission to hold each gear longer. Too much slack in the cable and you get early upshifts - the transmission doesn't think you have your foot on the gas so much.

Driving style can also cause these stacked shifts. If toe way into the throttle to get started from rest then back off as the turbo spools up you are encouraging the transmission to shift up. Less throttle = less demand for acceleration = time to shift into a higher gear.

The vacuum modulator adjusts shift quality - quick and firm vs. soft and sloppy. It doesn't affect shift timing. Assuming fluid levels are correct, it sounds as if a bit more firmness might be in order.

This car also has a system which softens the shifts until engine coolant temperature reaches 50C. It's controlled by a three way valve mounted on the drivers' fender near the vacuum amplifier (round blue thing with ~5 vacuum hoses attached). If found this setup needed to be disabled upon switching to synthetic ATF - the trans would flare when the car was cold. Just unplugging the electrical connector from the three way valve disables it.

Good luck.

- Jim

GEORGEPECKY · #6 01-30-2003, 11:30 AM

Thank you all for your advice. Could someone please explain what is meant by flaring.

haasman · #7 01-30-2003, 01:10 PM

Simply, slippage- it is like a car with a standard transmission and the clutch is slipping. The engine revs but the car isn't going forward the corresponding amount of thrust.

I looked up the word flare in the dictionary and besides the obvious hot, bright burning warning device I got this:

1) : to become suddenly excited or angry (2) : to break out or intensify usually suddenly or violently --

The name flare is used here because the engine is not inducing forward motion AND as more throttle is applied the slipping increases rapdily. For me, I associate the flaring with the tach. It swings upwards in RPM rapidly with no correcpsojding forward motion.

Hopefully this helps!

Haasman

haasman · #8 01-30-2003, 01:54 PM

Jim

I just noticed that the bowden cable on the 190E is very slack. Does your advice apply to it as well? Not lose but not too tight? I read where someone said: "The cable is adjustable (small knurled plastic sleeve) the pointer on cable should align with notch on sleeve" Could you expand on this. I am looking for the correct adjustment.

Separately, does the '91 300E have the same temperature for shift quality when cold. Ours is hesitant to shift until warmed up.

Thank you in advance,

Haasman

jcyuhn · #9 01-30-2003, 02:28 PM

Hassman -

Don't know for certain, but I would expect the answer is yes. The transmissions on all these cars are cousins, if not more closely related. Within a range, it's personal preference as well - do you prefer the transmission to shift early or hold each gear a bit longer? If you get the bowden cable too tight the transmission will want to hang in third gear and not upshift to fourth. I've occasionally had to loosen the adjustment - perhaps vibration causes it to tighten over time?

I don't think the '91 300E has the trans shift quality control. I took a quick look at the ETM - I could find it for the '87 diesel, but not the 91 gasser.

- Jim

haasman · #10 01-30-2003, 02:31 PM

Jim

Thank you for looking it up.

Haasman

GEORGEPECKY · #11 01-30-2003, 04:05 PM

Once again, thank you for all of your help. This weekend I will check the Boden cable and make an adjustment at the modulator and see what happens. I'll let you know.

howardjnl · #12 06-13-2004, 11:33 PM

To build upon the issue of soft up shifts:

Months ago, my 1987 300SDL abruptly began to run through the gears too quickly when accelerating to speed. I found the transmission modulator valve to have a vacuum leak, so replaced the valve, adjusting the pressure according to Mercedes specifications for my car and valve. The car continued to run through the gears too rapidly, and began to flare between 3rd and 4th gears, particularly when slowing down (foot off of the accelerator). The only positive change experienced was that the flaps internal to the air system inside the car worked much better.
I found that the vacuum control valve (mounted on the side of the IP) was not bleeding enough air, so replaced it. No help. The vacuum amplifier had an internal leak, so changed it. No help. Replaced all vacuum hoses that even appeared to be getting old. No change. Changed the two swithover over valve on general principles, cleaned all openings to the various manifolds, etc. During all of this, used a Mity-Vac to check vacuum pressure, and curently the vacuum pressure appears to be working properly, with the vacuum to the vacuum modulator valve on the transmission dropping slowly to about 1 or 2 pounds of vacuum at full throttle at normal temperature. Adjusting the Bowden cable during all of this has had absolutely no effect on shift points, and the electric button under the throttle pedal doesn't cause a downshift.
As an experiment, I've adjusting the modulator valve one turn to the right (more pressure) to see if that helps with the flaring or shift points. It may have, it's hard to really tell.
I'm wondering: how far should can I safely "tighten" the modulator valve? And why to both the Bowden cable and electric down shift button fail to alter the shift points?
Any ideas or suggestions are welcome. I'm wondering at what point to take it to a transmission specialist, but I'm also wondering who would be qualified to work on it. For example, our local MB dealership doesn't do transmissions!

John

howardjnl · #13 06-14-2004, 05:22 PM

I noticed a message on this forum mentioned that the vacuum drop to the vacuum modulator should go to zero very quickly when checking it driving down the road. It takes mine several seconds, and only goes to about 2 pounds. This sound "wrong"?
John

howardjnl · #14 06-14-2004, 05:38 PM

Sorry for the continued string of messages, but noticed also from Steve Brotherton's write up (http://www.babcox.com/editorial/ic/ic20242.htm) the following paragraph:

"The most important modification came with the 1985 model and is continued on all subsequent ones. It involved the addition of a vacuum amplifier to the system (see Fig. 7). The amplifier takes a large vacuum supply, the proportioning vacuum signal and a boost signal and creates the final signal to the transmission. This device did two things. First, it incorporated the input of boost to further tailor the load control of the modulator vacuum signal. The vacuum at the modulator now starts high - 12-17 in. - and is reduced to about five inches through straight throttle movement. As boost builds, the vacuum is further reduced to zero only at full boost."

My 1987 300SDL seems to be behaving like my vacuum is to too high. Disconnecting vacuum from the modulator valve eliminates the flare. I've changed the VCV, switchover valves, and blue vacuum amp. I'm wondering if there's an electrical component I should check related to operation of these electric switchover valves? I'm really lost here guys. Any advice would be helpful.
John