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post #1 of 6 (permalink) Old 04-09-2013, 01:07 PM Thread Starter
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I have a 1979 450SL with about 160k miles. Transmission shifting troubles.

I replaced the vacuum modulator, and took the car to a German car shop to have the modulator adjusted. Prior to installing the new modulator, the car had shifted fine; I chose to replace the old modulator at the time I had the car apart while replacing flex discs, trans. mounts, and carrier bearing.

The shop charged me $250.00 for the job. When I started to drive it home, it shifted extremely hard from first to second at about 25MPH. If I was "easy on the gas, it didn't ever shift into 3rd even at 45-50 MPH unless I took my foot off of the pedal just a bit. I took the car back to the shop immediately. A technician gave it a test drive with me, and - even after the tires "chirped" while shifting from 2nd to 3rd - said, "That's the way it's supposed to shift."

I find that hard to believe. The shop is going to readjust the modulator at no charge, but they were upset that I asked them to make it shift softer and sooner. They cautioned me that if they adjusted it, they couldn't be sure how it was going to shift. I replied that that was why I hired them; to adjust the modulator so it shifted like a quality car (as it shifted prior), not a dragster. I also felt that shifting so hard would be difficult on components such as the flex dics, and engine and trans. mounts.

Am I out of line, or is it the shop? This is the first 450 I've owned, and I've had it almost 15 years. I've never known a transmission to shift so hard and late. Is the way the shop claims the way a 450 is really supposed to shift?
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post #2 of 6 (permalink) Old 04-09-2013, 03:02 PM Thread Starter
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The shop just called and said the car develops a rough idle when it warms up, and that's why it's not shifiting well. They claim they can't get a good fix on the vacuum modulator.

Does that sound reasonable?
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post #3 of 6 (permalink) Old 04-09-2013, 06:04 PM
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i would put the old modulator to see .if there was nothing wrong with the old one , why replace it . the modulator is spring loaded , you can pre adjust the new one to same tension , by feeling by hand .the price seems high to me .
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post #4 of 6 (permalink) Old 04-10-2013, 01:51 PM
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Vacuum isn't the issue, it's the adjust. In '06, I had my transmission rebuilt by Aamco, it shifted VERY softly. I took it back and they adjusted something so it shifted normally. I *think* your tranny and mine are identical...
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post #5 of 6 (permalink) Old 04-10-2013, 02:56 PM
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I think you would be better served down in the R/C107 area where your car is discussed continually. However the hard shift and rough idle are indications of a vacuum problem. Perhaps the transmission folks didn't get the vacuum line to the transmission hooked up right or they re-used the old rubber pieces and it has developed a leak.
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post #6 of 6 (permalink) Old 04-10-2013, 05:55 PM Thread Starter
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Thank you all for your input. I've moved the original two posts to the R/C 107 forum as Nosilw suggested.

I might try the AAMCO thing if I can't get it figured out.

Thanks again.
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