w126 tranny questions - PeachParts Mercedes-Benz Forum

hangit · #1 10-23-2007, 11:23 AM

My recently purchased 83 300SD, 146k miles, is flaring and double shifting between 3rd and 4th gear. The tranny was just flushed/filter changed, fluid level is correct. My vacuum reads 15-20 at the brake booster line, 12-15 past the tranny dashpot. The vacuum drops toward zero as accelerator advanced at the vacuum relief valve.

The plastic levers on the vacuum valves on top of the valve cover are completely gone, new ones are on the way. Could this be the main reason for poor shifting? I have yet to draw a vacuum on the modulator to test for leaks. That will be my next test after the new levers are installed.

BTW, I have read Brotherton's article, Peter Schmidts as well as several other DIY articles for diagnosing these problems. Thanks for any advice.

John

rrgrassi · #2 10-23-2007, 11:35 AM

The 3-2 valves can be bypassed. Brian Carlton did a great write up. The 3-2 valves are a known vacuum leak source.

hangit · #3 10-23-2007, 11:39 AM

Quote:

Originally Posted by rrgrassi

The 3-2 valves can be bypassed. Brian Carlton did a great write up. The 3-2 valves are a known vacuum leak source.

I'll search for the thread. These valves aren't necessary for smooth shifting?

John

rrgrassi · #4 10-23-2007, 11:48 AM

No, they are not. Proper vacuum and bowden cable adjustments are. A vacuum leak elsewhere in the system can cause hard shifts. Flaring is usually caused by too much vacuum, mal adjusted or failing modulator, or worn clutch packs. Sometimes slipping and flaring are mistaken for each other.

johntksr · #5 10-23-2007, 12:48 PM

Quote:

Originally Posted by hangit

I'll search for the thread. These valves aren't necessary for smooth shifting?

John

The valves are not necessary if you disconnect the EGR valve and plug associated vacuum lines. I had the same tranny flare and shot plastic levers. I "temporarily" disconnected the EGR and am very happy with the results.Tranny shifting is smooth and also seem to have a little more punch off the line. However, If you desire to keep the EGR functional, you must replace the plastic levers. If you have no vacuum leaks or other related vacuum issues...the plastic levers will fix your problem. FYI, after replacing my levers, I still had tranny flare. I believe the valve is shot. At that point, I decided to disconnect the EGR. Very happy I did.

hangit · #6 10-23-2007, 01:16 PM

Quote:

Originally Posted by johntksr

If you have no vacuum leaks or other related vacuum issues...the plastic levers will fix your problem. FYI, after replacing my levers, I still had tranny flare. I believe the valve is shot. At that point, I decided to disconnect the EGR. Very happy I did.

I'll post my results after replacing the levers.

As a quick follow-up, I drove the car this morning and if I am very light on the pedal it shifts appropriately. If I depress moderately or hard, the problems occur. Any leads I should follow?

John

Brian Carlton · #7 10-23-2007, 01:16 PM

Quote:

Originally Posted by hangit

My recently purchased 83 300SD, 146k miles, is flaring and double shifting between 3rd and 4th gear. The tranny was just flushed/filter changed, fluid level is correct. My vacuum reads 15-20 at the brake booster line, 12-15 past the tranny dashpot. The vacuum drops toward zero as accelerator advanced at the vacuum relief valve.

The plastic levers on the vacuum valves on top of the valve cover are completely gone, new ones are on the way. Could this be the main reason for poor shifting? I have yet to draw a vacuum on the modulator to test for leaks. That will be my next test after the new levers are installed.

BTW, I have read Brotherton's article, Peter Schmidts as well as several other DIY articles for diagnosing these problems. Thanks for any advice.

John

The first piece of data required is the vacuum level at idle on the way to the transmission. Is it 12"........or is it 15".........or is it some value between the two?

The plastic levers on the top of the valve cover are not required. Leaking of these devices would create hard shifts.........just the opposite of your situation.

It may be possible to reduce vacuum to the transmission to solve your issue, but, what you have is not uncommon in older machines.

hangit · #8 10-23-2007, 01:20 PM

Quote:

Originally Posted by Brian Carlton

The first piece of data required is the vacuum level at idle on the way to the transmission. Is it 12"........or is it 15".........or is it some value between the two?

I'll have to check it later today and then I will post the definitive readings for your expert opinion. Thanks in advance.

John

johntksr · #9 10-23-2007, 01:42 PM

Quote:

Originally Posted by hangit

I'll post my results after replacing the levers.

As a quick follow-up, I drove the car this morning and if I am very light on the pedal it shifts appropriately. If I depress moderately or hard, the problems occur. Any leads I should follow?

John

Prior to my disconnecting the EGR, I was also able to reduce flaring by feathering the pedal. However, if I did not feather the pedal, I had tranny flare. The harder I accelerated the more pronounced the flare.

rrgrassi · #10 10-23-2007, 03:07 PM

The tranny is getting too much vacuum. Check with a mity vac. VCV or modulator may need adjusting or replacment.

hangit · #11 10-24-2007, 12:55 PM

Here are my vacuum readings at idle.

20" at T fitting on main brake booster line.
20" before green dashpot.
20" after green dashpot.
15" on line to modulator dropping to 5" at 2000rpm.

When I applied a vacuum to the modulator line, it did not hold. I have isolated the leak at the modulator. It's not supposed to have any leak is it?

Me thinks I have found my problem.

John

tarbe · #12 10-24-2007, 01:43 PM

Quote:

Originally Posted by hangit

Here are my vacuum readings at idle.

20" at T fitting on main brake booster line.
20" before green dashpot.
20" after green dashpot.
15" on line to modulator dropping to 5" at 2000rpm.

When I applied a vacuum to the modulator line, it did not hold. I have isolated the leak at the modulator. It's not supposed to have any leak is it?

Me thinks I have found my problem.

John

Vac leaks are never good....

My transmission flared and double-shifted when I got my car. Sounds like our cars are related!

I dropped the vac at the T to 10", tightened the Bowden cable and turned my modulator key a full turn counter-clockwise. It has shifted fine ever since.

I also replaced the K-1 springs and plunger and switched over to Mobil 1, but I could not tell the difference from those changes. Dropping the vac, tightening the Bowden cable and turning the modulator key really made a difference!

Get your blaze orange on John, less than 4 weeks until deer season opener!

Tim

Brian Carlton · #13 10-24-2007, 01:53 PM

Quote:

Originally Posted by hangit

Here are my vacuum readings at idle.

20" at T fitting on main brake booster line.
20" before green dashpot.
20" after green dashpot.
15" on line to modulator dropping to 5" at 2000rpm.

When I applied a vacuum to the modulator line, it did not hold. I have isolated the leak at the modulator. It's not supposed to have any leak is it?

Me thinks I have found my problem.

John

Do you own a Mityvac?

hangit · #14 10-24-2007, 02:01 PM

Yes, I have the metal Mityvac.

John

Brian Carlton · #15 10-24-2007, 03:08 PM

Quote:

Originally Posted by hangit

Yes, I have the metal Mityvac.

John

Remove the supply vacuum from the point where it connects to the T above the green damper. Connect the Mityvac to this point and pump it up. Read the gauge before it starts to fall. If you cannot pump it up to above 12", report back with that result.

When you let go of the handle and watch the gauge, estimate the time it takes for the gauge to fall to zero.

Do not move the rack at all during this procedure.