1992 w124 Vacuum Control Valve Operation - PeachParts Mercedes-Benz Forum

JasonC · #1 12-12-2006, 11:22 AM

Hello,

Sorry but I couldn't find a post on a similar year W124 to confirm my numbers. I have some hard shifts and I checked the vacuum going to the transmission. I was getting 12.5 at idle 9 at 1/4 throttle 6 at 1/2 throttle and 3 at 3/4 throttle and 0 at full. Has anyone else measured theirs on a similar model? Does this sound good or low?

Does anyone know how to adjust the ALDA on this year W124?

Thanks,
Jay

JasonC · #2 12-14-2006, 11:01 AM

It's doesn't look like anyone has measured it for a 90-93 W124. Does anyone know where I can find the specs online for the vacuum going to my transmission from the vcv? If not online where?

Thanks,
Jay

gsxr · #3 12-14-2006, 02:12 PM

Those numbers sound correct... I don't think that's your problem. When was the last time the fluid & filter was changed, and is the level correct? And have you checked for vacuum leaks? The vac pump puts out about 25 inches of vacuum but this is modulated down to 0-15" (approx) to the tranny, based on throttle position.

JasonC · #4 12-15-2006, 02:22 PM

The trans was rebuilt 6000 miles ago so that shouldn't be the issue. The vacuum modulator on the transmission holds pressure. The car shifts hard from 1-2 mostly when I'm running on grease. Shifts a little hard but okay on diesel. I have a grease conversion kit (two tank). It seems like there is slightly less power running on grease. The problems seems to be equivilent to the people who have alda problems which do not allow enough fuel through creating a hard shift. I was thinkinig about adjusting my alda. Do you have any other suggestions. Would the bowden cable be an option?

Thanks,
Jay

gsxr · #5 12-15-2006, 03:18 PM

The Bowden cable only controls shift points (speed/RPM), not shift firmness. So that won't help.

The shift firmness is almost entirely based on throttle position. If you have your foot deep into the throttle without much power being produced, it may provide a harsh shift. This is often the case on older diesels with lean ALDA adjustments. However, your car has an electronically controlled transmission, in addition to the vacuum signal. I'm not sure how much the electronics affect things.

If your ALDA is like mine (the round steel can, not the square aluminum type) I don't think you can adjust it with the set screw. I tried on mine, and it seemed welded to the case. I didn't force it. Instead I inserted a 1.0mm shim underneath (roughly equal to about 1.5 turns richer). This helped my throttle response off the line by a decent amount.

JasonC · #6 12-16-2006, 05:31 PM

Quote:

Originally Posted by gsxr

The Bowden cable only controls shift points (speed/RPM), not shift firmness. So that won't help.

The shift firmness is almost entirely based on throttle position. If you have your foot deep into the throttle without much power being produced, it may provide a harsh shift. This is often the case on older diesels with lean ALDA adjustments. However, your car has an electronically controlled transmission, in addition to the vacuum signal. I'm not sure how much the electronics affect things.

If your ALDA is like mine (the round steel can, not the square aluminum type) I don't think you can adjust it with the set screw. I tried on mine, and it seemed welded to the case. I didn't force it. Instead I inserted a 1.0mm shim underneath (roughly equal to about 1.5 turns richer). This helped my throttle response off the line by a decent amount.

Hey GSXR thanks for the help.

Can you describe how you shimmed it? Did you remove it or did you loosen it and insert the shim then re-tighten it? What was the shim shaped like? What car do yo have?

Has anyone else adjusted an alda on a 92 w124 or similar car without a shim?

Thanks,
Jay

pmckechnie · #7 12-16-2006, 07:15 PM

Iwill describe the procedures and problems I had with hard shifts on my 91 300D 2.5. I believe they are the same or very close. I had hard shifts and poor performance at one time. I found that I had a boost problem. The boost problem caused poor performance which required more accelerator to get the car going. More accelerator caused less vacuum to trans which caused hard shifts. I worked on the boost problem first and once it was fixed, the trans shifted great. The ALDA is adjustable. Remove the protective cap, loosen the lock nut with a 8mm wrench, and turn the screw CCW 1 turn to start with. Lock the nut down and throw the cap away. Just DONT FORCE anything. With the lock nut loose the screw should turn EASY. If not, stop. If you work carefully this can be done without removing the intake.
Good luck

JasonC · #8 12-17-2006, 09:27 AM

Quote:

Originally Posted by pmckechnie

Iwill describe the procedures and problems I had with hard shifts on my 91 300D 2.5. I believe they are the same or very close. I had hard shifts and poor performance at one time. I found that I had a boost problem. The boost problem caused poor performance which required more accelerator to get the car going. More accelerator caused less vacuum to trans which caused hard shifts. I worked on the boost problem first and once it was fixed, the trans shifted great. The ALDA is adjustable. Remove the protective cap, loosen the lock nut with a 8mm wrench, and turn the screw CCW 1 turn to start with. Lock the nut down and throw the cap away. Just DONT FORCE anything. With the lock nut loose the screw should turn EASY. If not, stop. If you work carefully this can be done without removing the intake.
Good luck

Hello pmckechnie,

Thanks for the great info.
Since you had a boost problem with a similar car can you give me some tips on evaluating mine to see if there is a problem also? If I measure the pressure what readings should I get? Where is the best place to measure it?

Thanks,
Jay

pmckechnie · #9 12-17-2006, 10:49 AM

You need to check the boost pressure where it goes into the ALDA. You will need a T fitting and the necessary rubber hoses to get the boost guage hooked up. My car runs about 14 psi. Probably anything over 12 will be ok. If you find a problem with the boost then let us know and we will go from there. In these cars there is a lot that can cause low boost. By the way, you didn't unhook the EGR valve did you. That is the first thing recommended but it won't work on these cars unless a lot of modifications are done.

gsxr · #10 12-18-2006, 12:18 PM

Boost pressure spec for this model is 0.75-0.85 bar, which is roughly 10-12psi. I just checked mine yesterday (on the 1993 300D) it it peaks at 12-13psi. Anything over 10psi is fine though. Make sure the fitting at the manifold is clean... mine was partially blocked with a nice oil+carbon goop. Cleaning that out improved part-throttle performance a little. If the EGR has been unplugged or otherwise disabled, try re-enabling that first. I've been told that the computer senses this and kills boost, although I haven't yet experimented with this on my car yet. After seeing the carbon goop I do intend to disable the EGR eventually but it may require more effort (there's an old thread on this forum detailing how to do it - electronics were required.)

ALDA shim size: I can't find the post with the specs, but I can measure a spare I have at home. Here's a tidbit I found in another post:

The large copper washer that comes with the oil filter had the perfect inside diameter, but I had to grind the outside down to get it to fit. I think this was between .6 and .9 mm thick.