w126 tranny questions - Page 2 - PeachParts Mercedes-Benz Forum

hangit · #16 10-24-2007, 03:20 PM

Quote:

Originally Posted by Brian Carlton

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.

I could only draw 14" of vacuum which decreased to zero over 42 seconds.

John

hangit · #17 10-24-2007, 04:50 PM

I read on a different thread on the same topic that the modulator leak could be caused by the rubber cap on it. Mine is cracked around the edges, new one is on its way.

John

Brian Carlton · #18 10-24-2007, 05:06 PM

Quote:

Originally Posted by hangit

I could only draw 14" of vacuum which decreased to zero over 42 seconds.

John

You've got a slight leak at the modulator...........nothing serious.

That's not your major problem, however. The 14" of vacuum is too high.

There is a small domed cap on the side of the VCV. Pry this cap off carefully and you'll find a slotted screw beneath it. Some have a locknut.......some don't.

In any case, you wish to turn the screw about five degrees..........NOT MORE.

Connect the Mityvac and pump it up to 14" and turn the screw very slightly. Pump it up again and observe the gauge. If it's now 12"........you're moving in the right direction. Turn it a bit more until it reads 10" when you pump it up...........then STOP. Return the cover to the VCV and take it for a test drive.

If you don't know what you're doing...........DO NOT randomly move the adjustment screw or you'll destroy the VCV.

hangit · #19 10-24-2007, 05:49 PM

Brian;

My VCV had a locknut for adjustment. I adjusted it so now my vacuum reads 10" to the modulator. Test drive resulted in firm 1st to 2nd shift, 2nd-3rd ok, but 3rd to 4th flared until I backed off the pedal.

John

Brian Carlton · #20 10-24-2007, 06:03 PM

Quote:

Originally Posted by hangit

Brian;

My VCV had a locknut for adjustment. I adjusted it so now my vacuum reads 10" to the modulator. Test drive resulted in firm 1st to 2nd shift, 2nd-3rd ok, but 3rd to 4th flared until I backed off the pedal.

John

Old transmission with internal issues.

Only hope is to adjust it down to 7-8" and hope the 1-2 doesn't start to bang.

Another possibility is a modulator adjustment, but, that's a last resort.

hangit · #21 10-24-2007, 06:08 PM

Quote:

Originally Posted by Brian Carlton

Old transmission with internal issues.

Only hope is to adjust it down to 7-8" and hope the 1-2 doesn't start to bang.

Another possibility is a modulator adjustment, but, that's a last resort.

What modulator adjustment would be called for? I haven't bypassed the 3/2 valves yet, would this make any difference?

I'll run out quick and try the 7-8" vacuum before it gets dark. Thanks.

John

Brian Carlton · #22 10-24-2007, 06:12 PM

Quote:

Originally Posted by hangit

What modulator adjustment would be called for? I haven't bypassed the 3/2 valves yet, would this make any difference?

I'll run out quick and try the 7-8" vacuum before it gets dark. Thanks.

John

The modulator pressure is adjusted via a T handle on the transmission. With more modulator pressure, all shifts become firmer and faster. In you situation, that might be desirable for the 3-4 but could create havoc on the 1-2.

It's going to be a compromise........no matter what we do with it.

hangit · #23 10-24-2007, 06:25 PM

Brian;

Vacuum to modulator now a 7". 1st-2nd not too hard, 2nd-3rd better, flare between 3rd-4th much less. Now what?

Thanks again.

John

Brian Carlton · #24 10-24-2007, 06:27 PM

Quote:

Originally Posted by hangit

Brian;

Vacuum to modulator now a 7". 1st-2nd not too hard, 2nd-3rd better, flare between 3rd-4th much less. Now what?

Thanks again.

John

I'd drive it like that to get some additional data. If it's still a problem, reduce vacuum to 5". It's going to start to bang on downshifts somewhere along this process............

hangit · #25 10-24-2007, 06:34 PM

Quote:

Originally Posted by Brian Carlton

I'd drive it like that to get some additional data. If it's still a problem, reduce vacuum to 5". It's going to start to bang on downshifts somewhere along this process............

What other data would you like me to collect? I'll try 5-6" vacuum tomorrow. You mentioned internal issues. Complete rebuild?

John

Brian Carlton · #26 10-24-2007, 06:38 PM

Quote:

Originally Posted by hangit

What other data would you like me to collect? I'll try 5-6" vacuum tomorrow. You mentioned internal issues. Complete rebuild?

John

Just see how it drives.

How's the 3-4 upshift?

How's the 3-2 and 2-1 downshifts? Any banging?

Clearly, if we cannot get to a satisfactory compromise, the clutches and bands need replacement. But, usually, with proper tweaking, they can go for awhile.

tarbe · #27 10-24-2007, 07:02 PM

Quote:

Originally Posted by Brian Carlton

Old transmission with internal issues.

Another possibility is a modulator adjustment, but, that's a last resort.

Brian

Why is modulator adjustment a last resort? I thought it was part of the overall strategy, used in combination with the other tricks?

Tim

hangit · #28 10-24-2007, 07:03 PM

Quote:

Originally Posted by Brian Carlton

Just see how it drives.

How's the 3-4 upshift?

How's the 3-2 and 2-1 downshifts? Any banging?

Clearly, if we cannot get to a satisfactory compromise, the clutches and bands need replacement. But, usually, with proper tweaking, they can go for awhile.

Ok. I'll be driving it tonight and will report back tomorrow. Thanks.

John

Brian Carlton · #29 10-24-2007, 07:05 PM

Quote:

Originally Posted by tarbe

Brian

Why is modulator adjustment a last resort? I thought it was part of the overall strategy, used in combination with the other tricks?

Tim

I consider it a last resort because if it's used as an adjustment tool, without regard for modulator pressure, the transmission is now set to a value outside the manual specification. While the performance may be adequate, I have no knowledge of the long term ramifications, if any, of this adjustment.

Therefore, I don't recommend it unless we've exhausted all vacuum remedies.

hangit · #30 10-24-2007, 10:04 PM

Brian;

Since my modulator has a slow leak, will replacing it make any difference in my shifting? If I do replace it, what are the pressure settings for an 83 300SD?

Driving it tonight, downshifts are smooth, 1-2 is firm, 2-3 is ok, and 3-4 is marginal if I don't lead foot it.

John

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode