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Old 10-04-2003, 12:44 AM
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Join Date: Sep 2003
Location: S.F. Bayarea
Posts: 196
Dynamics of the vaccum control valve?

Question for the trans guru:

Inserted a vaccum gage to measure the vaccum going to the transmission. At idle (aroud 750 RPM) the gage reads 13 in. , at 1000 rpm, the vaccum drops very fast to around 3 in. At WOT (over 4000 RPM), the vaccum drops to zero.

Is the normal behavior of the vaccum control valve so "non-linear"? Intuitively, I thought a linear vaccum drop proportional to the throttle position would make sense. Maybe I need a better gage.

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Old 10-04-2003, 10:43 AM
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Join Date: May 2002
Location: Central Oregon
Posts: 1,004
Just my theory here...

Non-linear should be accurate enough as when accelerating you do not need fully hard shifts unless you have that baby floored and want to get out of dodge quickly.

Partial throttle (let's say halfway) would not generate enough pulling power to warrant a really hard shift. You have to remember that it is not the speed of the engine that controls vacuum to the tranny but is the position of the acelerator pedal (at least on my 1980) and therefore is not truly related to engine output as measured by RPMs. If you floor it off the line it will have essentially zero vacuum throughout the shift range. If you lightly accelerate it wil have full vacuum all the way through the shift range. I played with multiple fittings to reduce vacuum at a standing idle just to see how the car would react with the adjustments made to the tranny and got some pretty interesting results but finally went back to the original configuration.

You can of course adjust the bleed valve to suit your needs. My wife likes them soft all through the spectrum of throttle but for myself I like to make sure there is no slippage except when accelerating just above idle.

I'm still adjusting to suit both our likes...
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
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Old 10-04-2003, 11:14 AM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
Should be more or less linear with throttle position, but this WON'T be liniear with RPM under no-load conditions -- you will get 3000 rpm with a small throttle opening in neutral.

Sounds just about right to me. If low speed shifts are too sloppy you can adjust the vac modulator a little, but don't get to enthusiastic with it.


1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
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