Just chasing down a leak... Is the transmission vacuum valve suppose to hold vacuum? Mine doesn't. The car doesn't shift hard at all though. Also, if I blow or suck on the green vacuum dashpot, there's no resistance either way. Is that normal?

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1984 300td euro turbo

The modulator should hold vacuum, for a while anyway.

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83 SD

84 CD

Doesn't hold at all. It quickly goes back to zero. Very Strange. Guess I'll purchase one then.

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1984 300td euro turbo

are you talking about the modulator at the transmission or the vacuum control valve (VCV) at the IP? the VCV will not hold vacuum, it just controls the amount going to the modulator.

The modulator might just need a new rubber cap (126 277 0081).

Sixto
87 300D^2

I'm talking about this thing... MERCEDES W123 300D W126 300SD Transmission Vacuum Valve W 123 126 TRANS 300TD | eBay

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1984 300td euro turbo

How about this little green vacuum dashpot? This lets air in both directions. Is this correct?

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1984 300td euro turbo

Post #6 is the VCV, it's adjustable.

Above is a dashpot and it does allow air flow both ways.

Use a mity vac on the black line going to the modulator, it branches off from the dashpot.

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83 SD

84 CD

Are you talking about the vacuum tube circled in red? There's also a black line that seems to go behind then under the engine.

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1984 300td euro turbo

the line that goes behind and under the engine is going to the transmission

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1981 NA 300D 310k miles

The other side, the line coming out of the top of VCV and the green dashpot.

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83 SD

84 CD

No, that device won't "hold" vacuum. It's designed to leak "vacuum".

If you want to test it, you need a large vacuum supply...........such as the vacuum pump on the engine. You start the engine and you connect the Mityvac downstream of the T that is above the device (the line to the modulator).

The Mityvac will then read the vacuum being supplied to the modulator.

It is preferable to run a length of hose into the cabin and read the Mityvac under various pedal positions.

The valve shown can be tuned to match the driver's shift preferences and/or the transmission's flare preferences.