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Old 09-10-2015, 07:10 PM
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300SEL/M103 smoking like crazy, line to vacuum return modulator full of tranny fluid

Well the title of the post pretty much says it all. I was on the freeway and started to notice billowing clouds of gray smoke following me. It got worse every time I touched the gas pedal. My first thought was - oh crap - head gasket or valve seal issues, but after reading some threads here, I took a look at the vacuum line going to the transmission modulator, and sure enough it's full of red transmission fluid.

Am I correct to assume that replacing the modulator and changing the fluid is what's needed, or is there anything else I should be on the lookout for? And what's a good source for a new modulator?

Big thanks in advance!
ThePhoenix

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Old 09-10-2015, 08:24 PM
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Interesting. I thought only the diesels had a modulator at the tranny.
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Old 09-11-2015, 10:06 AM
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I think you got the modulator and diesel thing mixed up, the unique item in your MB diesel that can control the modulator on the transmission is the vacuum control valve on the injection pump. It bleeds off vacuum gently as you press the accelerator pedal.

This mimics the function of the intake manifold losing of a gasoline engine losing vacuum as the throttle plate is opened up.

The problem this user is having is not new, modulators are known to go bad in this way. There are dozens of such incidents on the internet, and usually identified by gasoline engine benz owners because of the smoke that happens.
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Old 09-11-2015, 10:09 AM
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Got it; thanks. And the modulator makes it go from a very soft shift to a firm shift the more clicks you turn it - I think - counterclockwise?
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Old 09-12-2015, 12:10 PM
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anyone have a diagram of the part? My 103 gas is using some ATF recently.

Thanks.
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Old 09-17-2015, 05:47 PM
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Quote:
Originally Posted by krjt View Post
anyone have a diagram of the part? My 103 gas is using some ATF recently.

Thanks.


It is right on the driver side of the transmission with a vacuum line running to it. About the size of a golf ball.

They can also fail causing full line pressure shifts at low throttle input, BTDT.

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