Automatic Transmission Vacuum Modulator Adjusting??? - Mercedes-Benz Forum

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post #1 of 6 (permalink) Old 05-13-2009, 02:17 PM Thread Starter
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Question Automatic Transmission Vacuum Modulator Adjusting???

According to Gregs300CD (thanks for the info, BTW) the automatic transmission vacuum modulator has to be tested for proper vacuum before adjustments can be made. Is this really necessary for a pin adjustment with the car off and then test driving to see if shifting is any better?

I'm wondering if I can just adjust the pin clockwise slightly to increase pressure so I won't be having all my shifts occurring before 20MPH (early shifts) like shown here: 722.118 Transmission Adjustments. Three shifts before 20MPH is making my car really sluggish and would like to be a little closer to the max shifting points on the speedometer.
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post #2 of 6 (permalink) Old 05-13-2009, 07:01 PM
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A 722.1 is not the same as the 722.3 you have. Vacuum controls shift firmness, not timing. If you are shifting into 4th before 20mph your bowden cable on the valvecover is disconnected or broken.
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post #3 of 6 (permalink) Old 05-13-2009, 07:09 PM
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post #4 of 6 (permalink) Old 06-23-2011, 11:19 AM
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My 300D with 722.1 with only a vacuum line (no rod or cable) used to flare a lot from 2-3. I tested all the normal thigns -- the vacuum connections, the filter was changed, fluid checked, etc. We brought it to a place in town that specializes in transmissions on german cars (it's called German Transmission Company) and he kept it for a few days, put it up and down the lift, adjusted the modulator, and told us he got it as good as he could without rebuilding anything, and charged us $60 (nice deal). It is much better, but he had to set it in such a way that when the car is cold, it revs very high before shifting. Once it warms up, it shifts beautifully with only a bit of flare/slip from 2-3. Much better than before.

I'm trying to figure out if there's a way to get it even better. It revs really high -- all the way to 3500 or 4000 or so -- before shifting into 2nd and from 2nd to 3rd when it's cold. Also, the shift from 1-2 is very hard, to the point of getting a slight tire chirp. I am sure this is not good for it.

I was thinking of using some vacuum tricks to get the shift to be smoother when it's cold -- maybe I could use the EGR temp valve to stop the vacuum leak while it's cold to provide more vacuum to the transmission till it warms up?

Are there any other adjustments I can make?

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post #5 of 6 (permalink) Old 05-11-2012, 08:29 AM
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sshanky - did you ever attempt using the EGR temp sensor to control vacuum at the tranny when cold? Interesting idea. I'm having similar issues with cold hard shifts that become very smooth once the engine is warm. I've got my vacuum dialed in pretty good with a 13 to 0 drop under full throttle and I've adjusted the modulator as much as I can, but still the hard cold shifts.

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post #6 of 6 (permalink) Old 05-13-2012, 11:32 AM
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I tried a few things but it's not easy to get it right. I think that I've decided, at least for now, to settle for a little bit of flare/slipping between 2 and 3. It's really easy to control while driving -- I just let up on the accelerator for a few seconds at the right time. The transmission takes a bit of time to shift from 2 to 3 but, once it's in there, it's fine and I can floor it without slipping.

I think it was a good idea in theory, but realistically it's hard to know when those valves are going to open or close compared to when the transmission is warmed up enough.
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