Vacuum adjustments 1995 W124 E320 - PeachParts Mercedes-Benz Forum

ksing44 · #1 03-17-2007, 02:31 PM

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 · #2 03-17-2007, 02:52 PM

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 · #3 03-18-2007, 08:34 AM

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 · #4 03-18-2007, 09:36 AM

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 · #5 03-18-2007, 09:46 AM

Quote:

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 · #6 03-18-2007, 11:54 AM

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 · #7 03-18-2007, 12:25 PM

I think I pretty much fixed the 3rd to 4th shift flair with the Bowden cable adjustments. I just turned it a little bit at a time and went for test drives. I turned and drove and turned and drove. Actually, I am surprised how little turning it takes to make a difference. I think I was turning the plastic screw thing out away from the connection, thus making the cable slightly tighter. It got to the point that I couldn’t really get into 4th gear and then I turned it back in a little until I thought it was shifting properly. I think it is fixed! I think it is fixed! Thank you! Thank you! Thank you!

The car is also a bit more responsive to the gas pedal now. I think it feels like it did before they replaced the cable. Looking back on things now, I remember thinking my car was a little less responsive and slower on strong acceleration after the dealer “fixed” my transmission. Now I think it seems more like the way it always did. To think that all they had to do was turn that screw a bit and my problem would have been solved. I guess I am glad they didn’t, since now I am done with them!

I also think what I did may have affected my brake pedal feel. Is that possible? I have always had some play in the brake pedal that seemed to respond to pumping. It seemed to be related to the vacuum boost, rather than the hydraulic system. My brakes and brake lines have always been very well maintained, but I still would have the occasional slop. It typically would be best when I drove fast, as if pushing the system increased something in the vacuum boost. Please forgive me if this all sounds ridiculous. I am afraid that I am real bozo about how my car works. Even so, I think I may have fixed my problem with the transmission shift flair.

Now maybe I can find out more about the air vent slapping at cold start-up.

Thanks again!

Also, thank you for trying to explain vacuum to me.
I definitely understood the analogy of a straw with a hole.

ksing44 · #8 03-18-2007, 05:14 PM

Well, I took the car out again after it cooled down. It seems it isn’t fixed. I would say that it has improved, but it isn’t fixed. The slightly later shift seems to improve the firmness of the shift, so there seems to be less flair, but it is still there.

Now I wonder if the vacuum modulator adjustment might help. I don’t think that one will be as easy for me to get to. I am going to ask my indy when I go to have my snow tires removed. Hopefully they will have had some experience with this kind of thing and they will be able to fine-tune the shifts. It does seem like it might be an art of trial and error to get it just right.

Perhaps the fluid flush really did dislodge some grit that was keeping things shifting properly, as I have read other s comment about in these forums. Perhaps I really do have some wear and no amount of adjustment will completely fix the problem. It is really not so terrible, so I may be able to drive like this for years. It may be just like the head gasket and timing cover leaks that have been OK for over a year.

One of these days I am going to fix the leaks. I just wanted to wait until I hadn’t already dumped a bunch of money into the car for some other things. When I first had the leaks diagnosed, I had just paid for a new water pump and the kickdown actuator stuff with the transmission. Just recently it was a new exhaust and I also have to do the brakes, since the brake light came on last week. I am trying to keep my car on a budget of about $2000.00 per year, so the head gasket, timing cover gasket, and wiring harness will probably shoot the load for any given year.

Thanks again for your help.

96C280 · #9 03-19-2007, 10:58 AM

Steve,

Thanks for that description of how vacuum works with the engine and transmission. I'm wondering on my car, there are instances where the car is responsive to throttle input and other times when it is extremely sluggish (throttle position changed a few inches = slow buildup of speed). What are the possible causes in the variation of responsiveness? (Probably hundreds of things). I have tried to determine whether it is ambient or engine temp related but it's inconclusive in my nonprofessional judgement. Perhaps a little more responsive when cold. Are there any things that should be focused on? Just a tired transmission or engine? Bad vacuum modulator? Leaks? I guess the confusing part is the somewhat varying behavior, i.e., leak would be all the time, modulator bad all the time, engine tired all the time. Thanks for any input.

david s poole · #10 03-19-2007, 11:14 AM

if this is a 96 c280 as your handle suggests you may find that the mass air flow meter is dying or the pressure regulator at the front end of the fuel rail needs replacing.incidently the better performance when cold is because the temp sensor allows for richer mixture when cold and hence my ideas above.

96C280 · #11 03-20-2007, 12:23 AM

Thanks David,

it is a 96 c280. The MAS was changed for a fluctuating idle a few years ago. Is the fuel pressure regulator a somewhat common performance culprit on this car? Do you have a part #? I looked up how a pressure regulator works. It says that it opens wide the fuel return valve when decelerating. Is a defective pressure regulator also the cause of a somewhat surging of the engine when coming to a complete stop? It seems to blip a couple of times as it comes to a complete stop. I posted this question before and others apparently have this problem as well. The website, of course, says this part is very simple and thus "rarely, if ever, goes bad." Finally, I have had a whining sound from the engine area when driving at freeways speeds.
After a while, when releasing the throttle and pushing the throttle there is a pitched whine upon applying and releasing the throttle. Could ALL my problems be a pressure regulator!

Oh how I wish. Maybe it's responsible for my bumpy ride.... just kidding. Thanks.

david s poole · #12 03-20-2007, 10:49 AM

if you take off the cover and expose the injectors you will see a fuel rail that has a bulge in the front.that is the fuel pressure regulator.i think it's held in by a strong clip on the top.i used to replace a lot of them.

ksing44 · #13 03-31-2007, 01:02 PM

Quote:

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.

Quote:

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 · #14 04-01-2007, 08:24 PM

Quote:

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!!!