All right diesel, vacuum, and tranny gurus, once again I turn to this forum for help. I have a 1990 W124 300D 2.5 Turbo and a hard shifting transmission. And just as a preemptive response, please don’t respond by saying:

“Don’t you know how to use the search function on this forum??? This question has been asked a million bazillion times and you’re an idiot if you can’t find the answer among all the other posts.”

I’ve used the search function, I’ve found and read 4 hours worth of stuff and most of what I slogged through, where there is good info is related to the 617 engine cars. Or the other w124 threads lack real details about how to go about collecting data when trying to trouble shoot a transmission/vacuum issue with the 2.5Turbo 300D.

I have the 602.962 5 cylinder. I’ve been working on getting the car to run smooth and make power for the last 6 months or more. I have all the fuel issues worked out and the car idles happy, and the engine feels and sounds good to my novice ears. Injectors are new. I’ve turned the Alda fitting’s adjustment screw counter clockwise until I started to make smoke under hard acceleration and then turned it clockwise ¼ turn and now have no smoke and a diesel that feels faster and punchier then ever before (about 2 CCW turns from where it was under its protective tamper cap). I have no leaking fuel at or around injectors or Injection pump and no noticeable misfiring cylinders. Under medium to hard acceleration I get a significant turbo lurch at ~2,000 rpm. My floored 0-60 mph time is 14 seconds consistently.

I’ve flushed the tranny and torque converter and changed the tranny filter about 3 months ago with no real improvement. Last night I read all about Vacuum issues and VCV adjustment, Transmission modulators and bowden cable adjustments. I found the attached Vacuum diagram for my engine/tranny combo and started working through the maze of hard lines. Of course my car has the most complicated Vacuum system of all the 30 diesel tranny-engine combos MB made from 1978 on. Blue flying saucers everywhere, seriously I have 4 or them not to mention all the EGR crap!

After doing some vacuum line fishing I found an unplugged VCV damper (#72), and two tubes going to the wrong places just to the side of the IP where all the vacuum lines come together. I plugged in damper, secured it with a zip tie to the VCV, fixed the mis-routed lines and checked the placement of the VCV. I couldn’t wait to drive it victoriously! As I checked the VCV, I noted that it was rotated in to the furthest CCW position possible. I took the car for a test drive and the transmission was noticeably less harsh when shifting since finding the leaks, but was still shifting stiff. I loosened the VCV and pulled the throttle to max setting while the car wasn’t running, and I could feel resistance from the very beginning of turning the VCV clockwise. I turned it to the half way point in its adjustment range, tightened the 2 nuts down and went for another test drive. The transmission shifted harder then before, grrrrr.

I turned the VCV back to its original position and then hooked up my Mighty vac with a Tee between the Damper (72) and the Vacuum amplifier (123) as seen in the attached diagram. I ran the engine at idle with the AC running and I was getting 12” of Hg which is all good according to the FSM, so that should mean that the VCV isn’t shot. I took the car for a spin with the Mighty vac clamped to the windshield by the wiper. In medium acceleration, the vacuum reduces to 4-5” Hg and even through the shifts only drop a couple of inches of Hg and then rebound back to the original value pretty quickly but never goes to 0 under light and moderate acceleration. Under a floored petal, the gauge read 0, and under 25% throttle the gauge was reading between 8-10” of Hg.

I then made the following test runs:

25% Pedal
Gear shift RPM MPH Shift Stiffness
2 2200 ~10 Nice and soft
3 2400 ~25 Nice and soft
4 2600 ~45 Nice and soft

50% Pedal
Gear shift RPM MPH Shift Stiffness
2 3400 20-24 Med Hard
3 2900 ~30 Pretty soft and nice
4 3600 65 Med Hard

100% Fully Floored to the button
Gear shift RPM MPH Shift Stiffness
2 4700 27 Chirping tire with whiplash Harsh
3 4700 50 Chirping tire with whiplash Harsh
4 4700 82 Chirping tire with whiplash Harsh

I have the Bowden cable and the tranny modulator to still try to adjust, but I haven't messed with either yet. With little or no boost, the car drives and shifts great. With anything more then 25% throttle I find myself trying to anticpate the tranny shifting so I can let off the gas at just the right time to try and soften the shift through pedal manipulation, only 50% of the time I'm successful. This has created some bad driving habits for me in my other cars. The really weird thing about what I see above is that the 50% pedal series shifts good and in control from 2-3 but not 1-2nd or 3-4th. I’m also curious to know if the Mighty vac readings can tell me anything. Should I keep digging through Vac components? I’m not sure how to tell if each one is working or not. I think the next thing to try is messing with the tranny modulator to see how it softens the harder shifting. Is there another way to check for Vac leaks that I’m missing? Sorry for the long post. Hopefully it will help others who use the “search feature” in the future!

Attached Thumbnails

 

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1990 300D 2.5 Turbo 150k miles
1999 F250 7.3L Powerstroke 225k miles 22mpg
Intro Thread

For some reason I can't get the values above to display in rows and columns, Hopefully you guys can read what I'm trying to show.

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1990 300D 2.5 Turbo 150k miles
1999 F250 7.3L Powerstroke 225k miles 22mpg
Intro Thread

Do you know if anyone has screwed with the trans. vac modulator...?

vac readings are what you would reasonably expect

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Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....

I agree. I would soften the modulator on the transmission itself. CW is firmer, CCW is softer. Remove the cap, pull the metal "T" up enough to turn it, and back it out 1/2 turn at a time. Test drive after each adjustment. If you go too far soft, you'll get flared/slipping shifts - that means you went too far, and need to go back a bit. You want it as soft as possible, but with no flare or slipping for any shift.

It's possible that you'll need to adjust both the tranny modulator AND the VCV (again). You should be able to set the VCV per the factory procedure and obtain proper shifting. Since you had to tweak the VCV out of the factory spec, I think it's compensating for an over-firm modulator.

Final thought - have you tested the modulator line to make sure it's holding vacuum? It could also be a faulty modulator. Another thing is that you can bypass the BFS entirely... have a vac signal into the VCV, and route the output directly to the transmission, bypassing all the BFS junk. I did this on my '87 and it works great.

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Good to know the Vac readings look good, The PO was an older couple who had the car sitting for a long time, sat more then she drove.

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1990 300D 2.5 Turbo 150k miles
1999 F250 7.3L Powerstroke 225k miles 22mpg
Intro Thread

Heading out to give this a try right after typeing this.

I'm assuming for this I will just got draw a vacum on the line from the VCV to modulator and see if it holds? Is ther another way to test the modulator? So the thought is that after I bring the Modulator to a reasonable shift, I then retest the VCV by loosening the 2 10mm nuts, with the engine off pull the throttle to Wide open, and turn clockwise until I feel the spring start to resist right?

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1990 300D 2.5 Turbo 150k miles
1999 F250 7.3L Powerstroke 225k miles 22mpg
Intro Thread

So I pulled the line to the transmission modulator from where it comes out of the Vacuum amplifier on the drivers side fender. I pulled a vacuum and it didn't hold anything, not even a second. I got under the car, removed the modulator end and pulled it out to inspect and found a split in the line. I replaced the line and its rubber boot ends and was excited to go for a test drive when i though I would hook the mighty vac up directly to the Modulator input. With 4 quick squeezes, I was reading 15" hg, but it would drain down to 0" after about 5-10 seconds. Is this right, or should it hold constant vacum once its applied? I know that the VCV obviously gives the modulator variable Vacuum depending on the engine load, but I don't know if the modulator then vents the vacuum, or if it should be a closed, not leaking dead end. I looked through GSXRs manuals under transmission section to see if there was a way to test the modulator, but didn't find anything. Any tips? Does this sound normal? I took the car out for a test drive and there was no improvement. Grrr.

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1990 300D 2.5 Turbo 150k miles
1999 F250 7.3L Powerstroke 225k miles 22mpg
Intro Thread

I also looked through the files on Accelorator issues and found one that talks about Bowman cable adjustment, but all it really shows is that it should be just slack when there is 0 throttle pressure on the pedal. I'll look some more for VCV and tranny modulator procedures but they don't seem to be in the most obvious of places.

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1990 300D 2.5 Turbo 150k miles
1999 F250 7.3L Powerstroke 225k miles 22mpg
Intro Thread

I would expect the VCV to 'leak' as you describe. Basically the VCV is a 'Variable Leak' to alter the vac to the modulator.

Its possible you may have an issue with the Vac Amp....

I have no experience of the Vac. Amp, (Never had one...) maybe a member who knows them can shed some light....

Might be worth just connecting the trans. line direct to the VCV and see what differences in operation are...

__________________
http://i190.photobucket.com/albums/z...0TDnoplate.jpg

Alastair AKA H.C.II South Wales, U.K. based member

W123, 1985 300TD Wagon, 256K,
-Most recent M.B. purchase, Cost-a-plenty, Gulps BioDiesel extravagantly, and I love it like an old dog.

W114, 1975 280E Custard Yellow,
-Great above decks needs chassis welding--Really will do it this year....

The modulator should hold vacuum and not leak, but if it's a slow bleed off, that shouldn't affect shifting drastically. The old type modulator has a rubber cap, the new type has a plastic cap with an O-ring. If the rubber cap is cracked, or O-ring isn't sealing on the plastic cap, this could account for a slow loss of vacuum. What you describe sounds like a moderate leak if it takes 10 seconds to leak to zero (double-check all the fittings, including at the Mity-Vac itself).

With this cracked line fixed, your shifting should improve significantly. Next step would be to replace the modulator itself so you have a modulator that holds vacuum properly, then set the VCV per FSM, and tweak the modulator slightly (if needed). I would also try (for grins) bypassing the BFS entirely and connecting the modulator directly to the output of the VCV. Oh, and yes, you can't test the VCV with a Mity-Vac, since it's a controlled bleed... you have to test that with a constant vac source, but your previous testing pretty much proved that it's working normally (12" at idle, zero at WOT, variable in between).

The Bowden cable only affects part-throttle shift RPM. If you're happy with the shift points, leave it alone.

The blue flying saucer on the fender is actually my vacuum amplifier. The VCV output goes to a seperate "in"line on the vacuum amp, then through a "damper (#72) and then to the tranny. I would say the bleed down on the modulator is probably more like 5 seconds then 10 when it is sucked down to 15" Hg.

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1990 300D 2.5 Turbo 150k miles
1999 F250 7.3L Powerstroke 225k miles 22mpg
Intro Thread

Correct. I'm suggesting to bypass the blue flying saucer entirely, and connect the VCV output directly to the tube that goes to the transmission modulator. A number of people have done this with good results. I'm not sure what MB was doing with that vac amplifier, but it was deleted starting with the 1992 model year.



That sounds a bit excessive to me. Might be time for a new modulator - they're not that expensive. This is a medium PITA job, as it's hard to see well enough to get the new modulator in place with the O-ring properly seated (so you don't have an ATF leak when you're done). There may be better clearance on the 2.5 with the smaller 722.4 tranny though - I've only done it on an M119/722.3 where there isn't even room for the hands of a small child. So, maybe it's not too difficult on your car. The modulator should hold a steady 15" vacuum until you release it by flipping the valve on the Mity-Vac.

Interesting they got rid of the amplifier for the 92 model year... It seems to work as indented on my car.. firmer shifts when on boost.

-Jason

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

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2013 Fiat 500E.

The OES replacement modulator units look like they are only $35, so I'll order one today. Dave, I looked on your site for some modulator R&R but couldn't find it under the tranmission section, would it be somewhere else? It is tight in the tunnel, but until I know what I need to get it out I'm not sure how nasty it will be.

I'm assuming I'll have to drop the pan and do something on the back side as well?

As far as playing with removing the Vac Amp, I'll wait to "experiment" until after I get the car working on the stock config, and then look at "tweaking"

__________________
1990 300D 2.5 Turbo 150k miles
1999 F250 7.3L Powerstroke 225k miles 22mpg
Intro Thread

Yep, but it's also interesting that even when removed on my '87, the tranny still shifts fine either on boost or off. I think it was an over-complicated solution to a problem that existed in the early/mid 80's where they had the tranny valve body programmed to shift excessively firm, and the BFS was an attempt to soften it. Maybe it worked better with new transmissions, but with our old/used ones, the BFS appears to be unnecessary. Further proof is that the transmission, and valve body, is the same for 90-93. That seems to indicate that MB finally decided the BFS simply wasn't needed anymore, without any change to the trans.



The modulator replacement is not in the FSM, at least not in the CD-ROM version. It's in the WIS, but the procedure is painfully simple (see attached).



Nope, the pan can stay attached. I vaguely recall that some fluid would dribble out so have some rags or paper towels handy. It's not necessary to drain the pan.



Probably a good idea, but since it's so simple to just swap some tubes, you could test both before & after if you want...

Attached Files

722_vac_modulator_R+R.pdf (22.7 KB, 647 views)