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  #1  
Old 01-31-2008, 08:42 AM
katchung's Avatar
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Should the green transmission damper vacuum dashpot have a slow leak?

300cdt, 1982...Just wondering. i left it for a while with the mityvac on it at about 12. I came back in a few hours and there was no vacuum left on it. Is it supposed to blead off or should I replace it?

I'm determined to finish this vacuum system repair and move on to other issues.

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  #2  
Old 01-31-2008, 09:07 AM
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What was connected to the opposite end of the dashpot?

Was it properly installed in the vehicle at the time of the test?
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  #3  
Old 02-04-2008, 03:47 PM
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Yes Brian. Installed properly. Mityvac was connected where vacuum line was going in. Was a slow leak on mityvac. should the vacuum seal on the dampener to the transmission be airtight?
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  #4  
Old 02-04-2008, 03:50 PM
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Originally Posted by katchung View Post
Yes Brian. Installed properly. Mityvac was connected where vacuum line was going in. Was a slow leak on mityvac. should the vacuum seal on the dampener to the transmission be airtight?
Was the transmission modulator line connected to the source vacuum during the test? Were you testing both the VCV and the modulator simultaneously?
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  #5  
Old 02-06-2008, 01:26 PM
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No. Engine was off. I pulled the source vacuum hose off and applied vacuum to the damper still attached to car. It slowly leaked. Please forgive my cluelessness. I'm totally new to car repair and in over my head.
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  #6  
Old 02-06-2008, 07:00 PM
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You pulled a vacuum and then let it sit for a few hours. Have you tested your MityVac tool itself? Will it hold a vacuum for a few hours?

I think the stuff I read said something about the modulator being able to hold a vacuum for a few minutes. If it couldn't, it was time to replace. Make sure you're testing the modulator itself and not the vacuum line connections to it -- mine didn't hold even that long the first time I tried, but it led me to a leak in the hardline-to-modulator connection.
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  #7  
Old 02-06-2008, 07:26 PM
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Okay..cool... it should stay sealed like the door lock vacuum.
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  #8  
Old 02-06-2008, 08:25 PM
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Originally Posted by katchung View Post
No. Engine was off. I pulled the source vacuum hose off and applied vacuum to the damper still attached to car. It slowly leaked. Please forgive my cluelessness. I'm totally new to car repair and in over my head.
OK, well that test is not completely valid. To properly test the modulator........which is the device that is probably leaking.........find the T above the vacuum damper that you just connected to.

On the T are three hoses:

1) Vacuum supply from the engine
2) Vacuum to the modulator (on the transmission)
3) Vacuum to the vacuum control valve (VCV) via the damper.

Pull off the line to the modulator and connect it to the Mityvac. Pump up the Mityvac to 18" and count how many seconds until the vacuum is down to 2". Report back with results.
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  #9  
Old 02-06-2008, 08:52 PM
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I mityvac tested the transmission modulator...results below

Brian,

I tested the transmission modulator today. To begin, it had been disconnected completely. Once I reconnected the line...had to jack the car up...modulator is green on a 300cdt 1982 right? I set the mityvac at 18 and it took 20 mins to get to 2. Does that sound right?
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Last edited by katchung; 02-07-2008 at 01:42 PM.
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  #10  
Old 02-07-2008, 06:24 PM
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W124 transmission harsh upshift

Quote:
Originally Posted by Brian Carlton View Post
....Pull off the line to the modulator and connect it to the Mityvac. Pump up the Mityvac to 18" and count how many seconds until the vacuum is down to 2". Report back with results.
I have a similar problem with my 1987 W124 300D-T transmission (722.357 is the number on the transmission although EPC says it should be a 722.317). If one is very light on the throttle, the transmission shifts nicely but at low engine speeds. If the footfeed is pressed down farther, the transmission stays in whatever gear it is in longer, allowing the engine to wind up above 4000 RPM, but then shifts with a jerk. Warm or cold (machine and/or environment) does not affect this much, if at all. Downshifts are not harsh.

My 1985 W123 300D California version has a similar transmission (722.416) and the '96 210 has a 722.438. Both shift normally, giving me a little idea of what the 124 should be like, more or less.

I tested the modulator according to Brian's suggestion, removing the vacuum line to the transmission after the green damper. After pumping the Mityvac to 18" Hg, I allowed it to sit for 18 minutes, by which time the vacuum had dropped to 12" Hg and I got tired of waiting so I disconnected.

This would seem to indicate that the modulator is OK, not leaking, and the problem is elsewhere. I have not adjusted the Bowden cable at all -- from my understanding, it affects only the RPM at which the transmission shifts, right?

I plan to go through the vacuum system and check all of the components, then run a line into the passenger compartment and watch the vacuum at the modulator line while driving the car. I understand that the vacuum is supposed to be lower with more throttle, mimicking the way a gasser engine works.

Anything else I should check?

Jeremy
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Total. . . . . . . . . . . . 722,685
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  #11  
Old 02-07-2008, 06:39 PM
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Quote:
Originally Posted by Jeremy5848 View Post
I have a similar problem with my 1987 W124 300D-T transmission (722.357 is the number on the transmission although EPC says it should be a 722.317). If one is very light on the throttle, the transmission shifts nicely but at low engine speeds. If the footfeed is pressed down farther, the transmission stays in whatever gear it is in longer, allowing the engine to wind up above 4000 RPM, but then shifts with a jerk. Warm or cold (machine and/or environment) does not affect this much, if at all. Downshifts are not harsh.

My 1985 W123 300D California version has a similar transmission (722.416) and the '96 210 has a 722.438. Both shift normally, giving me a little idea of what the 124 should be like, more or less.

I tested the modulator according to Brian's suggestion, removing the vacuum line to the transmission after the green damper. After pumping the Mityvac to 18" Hg, I allowed it to sit for 18 minutes, by which time the vacuum had dropped to 12" Hg and I got tired of waiting so I disconnected.

This would seem to indicate that the modulator is OK, not leaking, and the problem is elsewhere. I have not adjusted the Bowden cable at all -- from my understanding, it affects only the RPM at which the transmission shifts, right?

I plan to go through the vacuum system and check all of the components, then run a line into the passenger compartment and watch the vacuum at the modulator line while driving the car. I understand that the vacuum is supposed to be lower with more throttle, mimicking the way a gasser engine works.

Anything else I should check?

Jeremy
The engine/transmission is performing properly. You may not like the setup and can tune it to your preference by slackening the Bowden cable slightly. This will cause the engine to shift up sooner and it won't go to 4000 rpm as easily.

The adjustment should be on the left side of the engine just ahead of the oil cooler..........and down near the bottom of the cooler. There will be a white plastic adjustment knob on the cable. Turn the knob counterclockwise (when facing forward) to loosen the cable. Try it one turn at a time.
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  #12  
Old 02-07-2008, 07:55 PM
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Quote:
Originally Posted by Brian Carlton View Post
The engine/transmission is performing properly....
Thanks for the adjustment instructions, Brian. I will try some adjustments and see what happens.

I should have been more specific in my complaint, I guess....the shift is currently rough, much harder than either of my other two diesel Benzes. (It reminds me of the hard shifting that the 210 had when I accidentally disconnected the vacuum line to the modulator in that car's transmission.)

It's harsh enough that a mechanic who was inspecting the car for a prospective buyer thought the car had a transmission problem. I knew it was not that; the point to me was that the shifts were harsh enough to attract his attention. I'm looking for a problem in the vacuum system that is causing the modulator to have too much or too little vacuum.

Jeremy
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"Buster" in the '95

Our all-biodiesel family
1995 E300D (W124) . .239,000 miles My car
1996 E300D (W210) . .313,000 miles Wife's car
Santa Rosa population 170,685 (2012)
Total. . . . . . . . . . . . 722,685
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970
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  #13  
Old 02-07-2008, 08:24 PM
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Quote:
Originally Posted by Jeremy5848 View Post
It's harsh enough that a mechanic who was inspecting the car for a prospective buyer thought the car had a transmission problem. I knew it was not that; the point to me was that the shifts were harsh enough to attract his attention. I'm looking for a problem in the vacuum system that is causing the modulator to have too much or too little vacuum.

Jeremy
T a vacuum gauge into the modulator line and route the hose into the cabin. Drive the vehicle and observe the gauge under different operating conditions.

It should read 10" or so for most driving until you get more than 1/2 way into the pedal.........then it should start dropping toward zero.
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  #14  
Old 02-07-2008, 09:36 PM
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Thanks Brian

Speaking of rough shifting. I've plugged all of the leaks in the vacuum system, and reconnected the line to the tranny modulator. The modulator took twenty minutes to go from 18 to 2 on the mityvac. I took it out for a spin and shifting is much more smooth. One difference I've noticed, however, is that when I slow down to stop at a light, there's a distinct sound...kind of a "klunk"...almost feels like something has dropped...as the car comes to a stop. Anything to do about it?
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  #15  
Old 02-07-2008, 11:08 PM
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Quote:
Originally Posted by katchung View Post
Speaking of rough shifting. I've plugged all of the leaks in the vacuum system, and reconnected the line to the tranny modulator. The modulator took twenty minutes to go from 18 to 2 on the mityvac. I took it out for a spin and shifting is much more smooth. One difference I've noticed, however, is that when I slow down to stop at a light, there's a distinct sound...kind of a "klunk"...almost feels like something has dropped...as the car comes to a stop. Anything to do about it?
Vacuum could still be on the low side. Connect up a vacuum gauge to the transmission line with a T and see what you get at idle.

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