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Old 01-02-2002, 04:17 PM
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Join Date: Jan 2002
Location: Northern Calif
Posts: 116
300TD Vacuum switch function?

Hello All,

My car is a 300TD '84 grey market. I've been working on the vacuum system. So far I've found one bad lock actuator. The climate control puple line is also leaking...both these have been plugged until there is time to fix them. The priority item to fix is the hard shifting. There is '0' vacuum on the modulator black line when the throttle is just cracked open. I disconnected the vacuum regulator linkage and verified the vacuum is variable and correct at the '10mm from stop' setting. When hooked up again, there is no variable action with the throttle. The two plastic vacuum switches on the valve cover seem to be bleeding off all vacuum when the throttle is opened. This particular car only has two lines (white and black) going to these switches (no EGR). I've pugged the connector where the white line have variable vacuum, and the car shifts well.

Can someone explain to me the purpose of the valve cover vacuum switches? With the vacuum pump hooked to the white line, vacuum can be established only with the throttle either opened or closed.

Thanks for your help!

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Old 01-02-2002, 10:22 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
The white vaccuum line is supply from the vaccuum pump. The two "switches" vent the vaccum from the line to the transmission so that the vaccuum signal mimics a gasoline engine, more or less.

What should happen is that the vaccuum applied to the modulator should be 14-16" with throttle closed, drop to 12" as soon as the throttle opens, then gradually drop to 0 or so at wide open throttle.

There was a posting a month or so ago with the correct diagram included, but I don't remember what it was, you will have to search -- try "vaccuum diagram".

I think your lines are wrong, too -- if I remember correctly, the white line has a Y in it, one end to the switches, the other to the tranny. There should be a little yellow plastic restriction in the line where it conncects to the main vaccuum supply. The black line is a vent line to the interior of the car (to prevent dirt from getting into the vaccuum pump). If you route the vaccuum supply through the switches, you may not get enough at the modulator. Check the diagram posted and see if it matches yours.

1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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Old 01-05-2002, 08:50 PM
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Join Date: Jan 2002
Location: Northern Calif
Posts: 116
Vacuum switch

Thanks Peter,

I have a copy of the vacuum diagrams posted here. Mine seems to match one of them (except for the egr hookup). The car seems to shift pretty well with the switches bypassed. I took another look at them..the actuator ends are badly worn, and they seem to leak badly..think I'll order new ones, then worry about it. It seems that the variable action of the vacuum modulator alone should be enough to cause the proper shift action (hard or soft).
Perhaps it is not fast enough. If one is forcing a down shift by pushing the accelerator to the want a hard shift to avoid clutch slipping in that situation. (vacuum =0) I'll let you know how it works out.


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