The vacuum source for transmission control comes off the brake booster vacuum supply line (black plastic going from vac pump to brake booster).
The source is one of two tees off the line. These tees are metered holes. They are designed to supply vacuum in small enough portions that if they both were wide open the vacuum strength in the brake vacuum source wouldn't be threatened.
The way it works is the vacuum going from the line to the modulator is teed through the microswitch on the valve cover to a proportioned leak (vac proportioning valve) on the back side of the injection pump.
At idle, full vacuum goes to the modulator (should be around 20in). As the throttle is taken up (before pump movement) the microswitch opens the line to the proportined leak and the vacuum is immediately dropped to around 10-15in. As the throttle continues to rotate to full, the leak should become so sufficient that the vacuum is dissapated to zero.
Since the size of the leak is proportional and FIXED, the volume of vacuum at the source is critical. Too much (a supply hole reamed out) and the vacuum is always high. Too small (oriface is maybe only 0.020in and it cloggs) and the vacuum is too low or non-existant.
Before chasing the vacuum too much, a new vac hose with proper oriface size is a must for those lines with broken tees (don't reglue them).
That is basically what happens. There are multiple adjustments that can change the range of vacuum, the onset of proportioning, the lever of leak, and the result of it all at the modulator. A multitude of contrary commands can confuse some of those hard shifters to play fair, but thats another story.
[Edited by stevebfl on 04-19-2001 at 08:28 AM]
Bosch Master, ASE Master, L1
33 years MB technician