Vacuum adjustments 1995 W124 E320

[ksing44]

After searching and reading through many posts on the forums, I am pretty convinced that I must have a problem with too much vacuum. The symptoms include a shift flair with 3rd to 4th gear shifts and a slapping air vent on start-up. It all started after I had my transmission flushed, all seals replaced, and had the Bowden cable replaced.

My real problem is that the dealer seemed to be ignorant about how to fix the issue, even though they seemed to have caused it with their repair. To their credit, or harm, the shop foreman did come out to talk to me and he admitted that they don’t really have many or any mechanics that are really very knowledgeable about my car. He said that many of the new mechanics were more oriented toward understanding the electronics of the new cars. This problem, and a couple more, along with the foreman’s comments, was what had me thinking I should just ditch the car, but I just couldn’t give it up. I love the thing!

So what do I do? How can I get this fixed before it actually hurts my transmission or my air handling system? I think my Indy might be able to figure it out, well maybe one of their fathers that started the business, but I think it would help if I could point the way. I am hoping to get some expert tips about the things to adjust and/or replace.

I did find quite a bit of discussion about vacuum modulators down on the transmission and also some things about a K1 kit for inside the transmission, but the conversations were virtually all in reference to diesels. In fact it seemed like some of the stuff may have been specific to only diesels.

The actual questions:
1. Could the 3rd to 4th shift flair and the slapping vent both be caused from too much vacuum?
2. How can I adjust the vacuum?
3. Does a 1995 E320 gas engine have a vacuum modulator like the diesels?
4. If I have a vacuum modulator, does it control the vacuum for the entire system, or just the transmission?
5. Do you guys have any other ideas?

Thank you

[stevebfl]

1. Could the 3rd to 4th shift flair and the slapping vent both be caused from too much vacuum?
2. How can I adjust the vacuum?
3. Does a 1995 E320 gas engine have a vacuum modulator like the diesels?
4. If I have a vacuum modulator, does it control the vacuum for the entire system, or just the transmission?


1 - Not sure what you mean about the slapping and there is no such thing as too much vacuum in a gas motor... the more the better!

2 - you can't adjust vacuum directly. Vacuum is the byproduct of proper running. The better it runs the more vacuum at any throttle position.

3 - The 95 E320 does have a vac modulator like a diesel.

4 - the vacuum modulator never controls vacuum it responds to vac. It is the transmissions way to interpret engine load.

based on your stated series of events I would first adjust the control pressure cable (bowden cable) to achieve earlier shifts! If that didn't achieve the desired effect I would turn the modulator in a turn.

[ksing44]

Thank you Steve,

I think I do understand what you said that I should start with the Bowden cable adjustment and then try turning the modulator in a turn, but I think there is a chance that I am still confused about whether my problem is too much vacuum or not enough vacuum. My shift flair does not occur if I accelerate briskly.

I found this using bowden and pressure as keywords:
“The Bowden cable increases throttle pressure inside the transmission and thereby causes the shifts to occur at higher rpm's. The tighter the cable, the greater the rpm required for the shift.

The vacuum modulator increases the modulator pressure inside the transmission and thereby increases the speed of the shift. The more pressure, the faster and firmer the shift occurs. This adjustment can also be done with the vacuum control valve to increase or reduce the supply vacuum to the modulator.”

It sounds like I could use a little tighter Bowden and maybe a little increase in modulator pressure too. It also sounds like these adjustments might be more of an art than a science. Do you think I need a Mercedes specialist or would it be good to go to a transmission specialist? Is this a Mercedes thing or is this a similar thing for all transmissions? I am also thinking about just going out to my garage and trying to tighten the Bowden myself. From some pictures I found during my searches, it looked like it might be very accessible right under the hood. Could this be a simple do-it-yourself kind of thing for a non-mechanic like me? Any tips would be very welcome.

The transmission didn’t flair at all until after they replaced the Bowden cable, flushed the transmission fluid, and replaced all of the seals. I just had my engine light come on and they said it was the kickdown actuator, but I had no symptoms. They fixed some things and I ended up with a problem that they couldn’t fix. I sure hope it is really just a simple adjustment. If only somebody knew how to fix it.

The fact that my flair is not there with full throttle is what made me think that it is not caused by simple wear and tear on my aging transmission. It seems that the transmission should slip worse under heavy load, if it was a problem with wear. Is that true?

[softconsult]

Only a note of a similar behavior. My '92 300E has had what sounds like a similar thing to your slapping vent. In mine it sounds like a vent flapper closing in the rear of the car. It only happens once and has caused no problems.

Steve

[ksing44]

Originally Posted by softconsult

Only a note of a similar behavior. My '92 300E has had what sounds like a similar thing to your slapping vent. In mine it sounds like a vent flapper closing in the rear of the car. It only happens once and has caused no problems.

Steve

I was planning to come back to questions about the vent after I sorted out the transmission thing. I just thought that maybe they were related. I still think it is possible that they are related, but I was focusing on the transmission.

My slap is at start-up. There is an air vent that slaps shut when I start the car when cold. It is in the center of the dash. I figured it was probably controlled by vacuum pressure and that maybe I just had too much or too little vacuum. It started after the transmission was serviced, so I though it could somehow be related.

[stevebfl]

I have a hard time deciding whether a real knowledge of vacuum will be of any help to your quest. It is more likely that just accepting a couple issues as fact and proceeding to suggested solutions might give greater results.

But, I always liked knowing what was going on, so let me give you a little basic knowledge. Vacuum occurs in a gas motor (some gas motors, it doesn't exactly occur in my BMW 545 and I'll leave it up to the group to explain why) because the pistons are moving down and air pressure is trying to fill the space behind. This can get complicated if one uses the absolute pressure description. I'll just stay with the atmosperic modle that includes concept such as sucking.

So the piston goes down and sucks air in. If one wishes to control the speed of the motor one restricts the flow of air into it. This is absolutely different in a diesel and may be causing this quantity of vacuum concept in your mind.

Idle speed is the lowest controlled speed. At that speed the throttle is basically closed, the piston's suck is greatest and thus one gets the highest vacuum. Actually one can achieve a higher vacuum by raising the engine speed to about 2000rpms and holding it. Anyway as the throttle opens the vacuum goes down. It would help here to use the absolute pressure description but dream of it as less suck because the restriction (throttle is being removed.

To simplify this now think about full throttle. There is no difference in the manifold or the atmosphere so this is no vacuum. Maybe an example would be better. Think of a straw with a hole in the side. Suck! No fluid can be lifted. Put a finger over the hole and gradually cover the hole. If done slowly one can actually view the difference in vacuum quantity created by watching how high the fluid rises. If you want real numbers try placing a 30 foot straw into a 30 foot deep well. Nothing but a perfect vacuum will rise the water.

Anyway, the engineers understand that vacuum is a function of throttle position and load. Thus it is used through a vacuum modulator to modulate the system pressure of the fluid used to apply clutches and bands in the trans. (It is more complicated than that but lets just leave it at that).

So, by changing the shift point with a cable adjustment one now has the shifting they expect happening at a different throttle position (or different vacuum position). This isn't exactly right or wrong! But if results cause issues as is the case for you, then one must modify the conditions to solve the symptoms. One other point is that usually this all occurs because there is wear in the trans of either seals or clutches that changes the shift timing cushion (an even longer story, I'm not getting into).

So, the solution is to get the shift to occur before you let off the gas (increasing vacuum) or to increase the pressure the vacuum causes (modulator adjustment. Since the logic on your story is that the cable has a different adjustment I would start there.

Make the car shift earlier!

[ksing44]

Originally Posted by stevebfl

based on your stated series of events I would first adjust the control pressure cable (bowden cable) to achieve earlier shifts! If that didn't achieve the desired effect I would turn the modulator in a turn.

Originally Posted by stevebfl

Idle speed is the lowest controlled speed. At that speed the throttle is basically closed, the piston's suck is greatest and thus one gets the highest vacuum. Actually one can achieve a higher vacuum by raising the engine speed to about 2000rpms and holding it. Anyway as the throttle opens the vacuum goes down. It would help here to use the absolute pressure description but dream of it as less suck because the restriction (throttle) is being removed.

Hi again Steve,

I’m back. When I first read your post, I thought you must have mistyped the thing about adjusting the Bowden cable to “achieve earlier shifts”. I thought you must have meant later shifts, like when I am accelerating hard and I don’t have the slip. Well, it looks like I was totally wrong! It still seems a bit counterintuitive to me, even after rereading your posts, but adjusting the Bowden cable to “achieve earlier shifts” seems to be helping to almost eliminate my shift flair.

I am going to reserve my final judgment until after I have driven for a while and made some more adjustments, but I really think I am on the right path now. As I said above, that is the path you originally recommended. Earlier shifts it is!

So right now the car shifts a little earlier going into 4th gear and it doesn’t seem to slip. I can’t help but think it may not slip partly because there really isn’t much force being applied to the trans at the lower speed and throttle, but based on rereading some of the things from your post, it is probably really because there is more vacuum at lower throttle, just as you explained. In addition, when I really stomp and go again there seems to be no flair. I suspect that is because even though the throttle is open there is still considerable vacuum from the higher engine speed. Am I making sense?

You know, I am having so much fun doing this myself I may have to try to find a way under my car to go after the vacuum modulator next. My car is a bit lower than normal, so there isn’t even enough room for your head to fit under my car. I don’t have jack stands or ramps or anything, but I think they may be in my future. I have read a little about stands, so I already understand that I should get the ones with pins that go right through the supports, as opposed to the scary ratchet things. I also liked the ramps I saw that had extended run-on areas that would hold the ramps in place with the weight of that car as you drive up and on. I guess the jack stands are nicer, since you then have better access to the wheels and tires, although ramps seem like a nice quick way to get the car up in the air.

Thanks again for your guidance!

[ksing44]

Originally Posted by ksing44

I am going to reserve my final judgment until after I have driven for a while and made some more adjustments, but I really think I am on the right path now. As I said above, that is the path you originally recommended. Earlier shifts it is!

No flairs all day, except just a very little bit when I first started the car and it was cold this morning. You know how Mercedes has that absolutely ridiculous thing of the delayed shift when cold, which I absolutely hate, well that may have messed me up a little this morning when it was cold. For the rest of the day, however, NO FLAIR!!! I did turn it in just a bit more too, so maybe tomorrow morning will be better. I will have to wait and see.

All in all, it is a fantastic improvement so far!!

I am thrilled!!

Next, I am definitely going to find a way to adjust the modulator to get an even firmer shift. That may help prolong the life of my trans. It just can’t be good to slide into gear, even though it is sooo much better already.

I am just so grateful to Steve for helping me and I still can’t believe that the Mercedes dealer was ignorant about such a little adjustment that can do so much good.

Anyway, thanks again Steve!!!

[stevebfl]

I'm glad it's working for you. I imagine a small modulator adjustment will tighten it right up.

[ksing44]

I am going under the car to adjust the vacuum modulator. I purchased what I think is a pretty decent jack and some pretty good jack stands. I found them for a good price at Pep Boys and they seem to be heavy-duty tools for a very reasonable price.

3-Ton Torin Big Red Jack, Model# T83002 - $49.00
3-TonTorin Big Red SUV Jack Stands, Model# T43006 - $29.00

My car shifts pretty good when warm, but I still have some flair when it is cold, depending on my driving style. If I am taking it very slow through the neighborhoods on my way to work, then I do get nice shifts. If I go very fast, I also get good shifts. With intermediate acceleration, however, I still have a shift flair when my car is cold. I am hoping that adjusting the vacuum modulator will complete my repair and eliminate all of the flair. If not, then I guess I will just be doing an experiment to see how long I can drive with a flair before my transmission completely fails.

It is a shame, if I can’t get this fixed easily. I didn’t have a flair until the dealer replaced the kickdown actuator and Bowden cable and also replaced all of the seals on the transmission “while they were in there”. I wish I had just ignored the stupid engine light, since I had absolutely no symptoms before the dealer did the “repair”. Another mechanic told me that sometimes you get erroneous codes that cause you to fix things that aren’t really broken. This time it seems an erroneous code caused me to break something that wasn’t broken.

[brewtoo]

Originally Posted by ksing44

You know how Mercedes has that absolutely ridiculous thing of the delayed shift when cold, which I absolutely hate, well that may have messed me up a little this morning when it was cold.

This is easy to disable. Do a search on it.

Also, when you say they "replaced all the seals," what do you mean exactly? Did they remove the transmsission? What seals were replaced?

[hamrt]

I'm not understanding what you mean be a shift "flair". I have a 1991 300CE that is not shifting up into 4th. Is that what your car is doing or did I miss something? And what is the deal that german cars don't start in 1st gear? I have a 2000 BMW that starts in 1st but at it's own discretion, starts in 2nd. I put it in Sport Mode and it will start in 1st everytime. Strange, a car with a mind of it's own!

But anyway, back to Mercedes... Now, if there is something wrong INSIDE the transmission, would this cause some of these same problems. I had a '97 GMC truck that blew a servo inside and I lost 1st, 4th and reverse because they were all controled by that internal servo. My car (the Mercedes) has no slippage or clunking in the tranny, it just won't shift. Am I to understand that if the bowden or pressure cable is too tight, it might start in 1st gear and not go past 3rd? The shift quality is okay, smooth, that is, but does seem to be a bit late at moderate throttle positions. Tranny has always been serviced and has 112,000 miles.

I'm not an automatic tranny expert, I only know the basics but I am hoping with help I can figure this out.

P.S. A picture of a typical bowden cable arrangement at the engine and tranny on a late 80s/early 90s 300 6cyl would be helpful.

[ksing44]

When I was playing with my Bowden cable I did tighten to a point where it basically would not upshift to 4th gear, but it was similarly delayed shifting into every gear. The tight cable made my car think I had the throttle floored, even when I was only giving a little gas.

The "flair" is a kind of slipping as it shifts to the next gear. As a result of the slipping in the transmission the engine rpms go up, or flair, in between the shift.

Bye the way, just last week I turned the modulator back, i.e., counterclockwise, by a half turn. I was worried that all the shifts were just a little too firm. I do have the slightest flair from 3rd to 4th, sometimes, but the rest of the shifts are just about right. I think I am going to stay with the half turn change. Last week they also dicsovered a leak in my evaporator, so I hoping the transmision hangs in there for another year or two.

[brewtoo]

Originally Posted by hamrt

Am I to understand that if the bowden or pressure cable is too tight, it might start in 1st gear and not go past 3rd?

The simple answer is No. Probably not what you wanted to hear.

[babymog]

Originally Posted by hamrt

Am I to understand that if the bowden or pressure cable is too tight, it might start in 1st gear and not go past 3rd?

Yes, that is possible. If the bowden cable is too tight or bound-up and unable to return to the low-throttle position, the transmission will shift as if the car is floored all of the time so you'll have to exceed the 3rd-gear maximum speed to reach 4th gear.

There is also a downshift switch under the gas pedal, unplug that to see if it changes things (sometimes stick).

Also, AFAIK only the 103-powered 124s did the 2nd-gear start, the diesel and M104-powered cars start in first. I don't know about the V-8s.