I'm guessing that the reason my 92 300D 2.5 Turbo W124 bangs into gear is because the vacuum going to the tranny is too low.

At about 1/4 throttle the vacuum drops from 10" to about 5". At half throttle it is near 0". I have checked for vacuum leaks - haven't found any. I don't think there are any because the vacuum level is correct (10 inches mercury), and is the same when the tranny is connected or not.

The vacuum valve does make a buzzing sound when it is dumping vacuum though, and I don't get any vacuum at the "ventilation" port on the side when it is dumping. Maybe it is buggered? If so, it seems strange that it provides a smooth (albeit exaggerated) vacuum control.

I have changed tranny fluid and filter (mobil 1 synth) and the results are the same. The car shifts perfectly at low throttle (which makes me think the vacuum modulator is set correctly).

I took the vacuum valve off to look at it, and it seems fine - by that I mean the amount of vacuum seems to correspond to the position of the thing that turns it on the injection pump. That thing (wassit called?) seems to turn a lot for a little throttle movement at low throttle and then reduce how much it turns.

Anyway, I did the accelerator linkage adjustments from the FSM and everything was in spec.

The only thing I noticed out of place was the vacuum diagram for this car doesn't match exactly - instead of splitter coming out of the vacuum source to the vacuum valve, my car just has a single line. I'll attach the vacuum diagram with a note where the problem is.

Any help with this would be GREEAAATLY appreciated. I have been trying to fix this for months (on and off). I've got pretty much everything on this car going great. It's just the rough gear shifting that is the problem.

Thanks!

Attached Files

07.1-0500 12s.pdf (45.5 KB, 226 views)

How did u check ur tranny fluid ? I suspect u r low on fluid. Car has to at operating temperature, not having driven hard, on level ground and in NEUTRAL.

Have u replaced all the vacuum line connectors in the car ? WHat is the state of ur air filter, clear fuel filter ?

Another suspect is the Kilma relay. It is behind the battery. There is a newer one out for the past couple of years and u should change it - costs about. $100.

Checked tranny fluid as you described - it is fine.

Fuel filter replaced about 5000 miles ago. (Don't see how it could affect the tranny though)

There is no indication of a vacuum leak, so I am reluctant to replace all the lines (there's lots of them).

What does the Kilma relay do?

I did not suggest replacing the lines. I said the connectors. These r made of rubber and connect the vacuum lines to either each other or the vacuum lines to components. U may be have slight leaks and this has been known to cause shifting issues.

There were issues with this relay and MB introduced a number of mods over the years to iron out the problems in the car. The latest relay out is one that made a difference to the way my car ran. If u r not convinced, don't waste ur money.

After you have checked to see that you have 16 to 18 inches coming out of the vacuum pump and that all of your vacuum lines and connectors are OK you may want to focus on the Vacuum control valve. You should have over 15 inches of vacuum at idle diminishing on down to about 8 as you move the rack. I say that bcause I had hard shift problems with my 300SDL and it turned out to be a faulty vacuum control valve.

__________________
Tom Hughes
St. Louis
84 300SD
92 300D
86 300SDL

Hughet - I'll check the vacuum pump output. Thanks.

Is there really supposed to be 15 in going down to 8 going to the tranny? I thought it was 12 to zero. At least that's what I think the FSM says.

I think I will take the vac. valve off and test it by itself.

DieselBeagle, I appreciate your replies - I didn't intend to offend you.

There is an input line to the VCV that should have the same vacuum reading as the pump. As far as the output line goes, I am not certain what the readings ahould be on a 124. The readings I gave you are what I remembered from when I had to troubleshoot my 86 300SDL for rough shifting. That was with a new vacuum pump and a new VCV. The 300SDL uses the same VCV and pump but it has a vacuum amplifier and a switchover valve between the VCV and the transmission.
You can see what I went through and get some ideas by reading this whole thread. Be sure to check to see that the line to your vacuum modulator holds a vacuum.

Hard shift on 300SDL. How do you troubleshoot?

You will see in one of the posts that my 92 300D measured 22inches output from the vacuum pump and 16 inches from the output of the VCV on my mityvac. I don't remember what the readings were as I opened the rack

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Tom Hughes
St. Louis
84 300SD
92 300D
86 300SDL

I read through that post - very helpful.

I treated myself to a good mityvac pump and brake bleed kit ($35 at harbor freight,. $45 at prep boys), and now have a favorite tool. It took me about 2 minutes to find that the modulator valve was leaking - the vacuum would leak down from 15 to 0 in about 4 seconds. I jacked up the car and tested the vacuum on the modulator valve directly - same result.

As far as I know, the vacuum is supposed to hold on the at modulator valve -right?

So I ordered a new one. Coming soon.

but now the turbos aren't spinning up....but that's another story I'll have to get to soon.......

I'll let you know how the at modulator turns out... thanks for the help chaps.

Hi pdrayton,

Any luck with tracking down your vacuum issue? I've a similar car that I'm starting to work on its vacuum as well. 1992 300D 2.5 Turbo.

Also, can you tell me where the vac line from the tranny comes up to the engine compartment and where the VCV is? I've got some diagrams, but I can't seem to locate it in the engine area.

Thanks!

-m

__________________
Now:
2018 Tesla Model X
1999 S500 Grand Edition 164k
1992 300D 2.5 Turbo 287k
2005 E320 4MATIC wagon
1991 Alfa Romeo 164L 99k (sleeping for a while)

Then: 96 Lincoln TC, 93 Lincoln TC, 87 560 SEL, 87 300 SDL, 80 300D, 89 560 SEC

I found the modulator cap (on the tranny, next to where the vacuum line goes into the tranny) had a crack in it and was leaking vacuum.

The vcv is on the IP just behind the Engine stop lever. Is that what you needed to know?

Hi,

Yes that helps. I'll have to look around there.

Where does the vacuum line connect to the tranny? Can you see it without putting the car on a lift?

Thanks.

-m

__________________
Now:
2018 Tesla Model X
1999 S500 Grand Edition 164k
1992 300D 2.5 Turbo 287k
2005 E320 4MATIC wagon
1991 Alfa Romeo 164L 99k (sleeping for a while)

Then: 96 Lincoln TC, 93 Lincoln TC, 87 560 SEL, 87 300 SDL, 80 300D, 89 560 SEC

lift up the drivers side behind the front tire with the tire jack, and you'll see the modulator sitcking out the side of the tranny on the drivers side, about parallel with the tire jack when you're lying on your back under the car. Modulator is either blue or red plastic and has a black cap and a tube to attach the tube from the vcv.

Somebody has probably posted pics and everything about this on the forum. Have a ssearch.