Testing is very precise here and specifications are necessay.

When cold the controller switches the switchover valve to direct vacuum to the vacuum element. This must be tested in atleast three ways. First take the vacuum line to the element and pull a vacuum on it. I usually do this by sticking it in my mouth and suck. Does it hold vacuum (this is the most common problem - a bad diaphram).

Second, does the vacuum appear at the element side of the switchover valve when it is supposed to. I'm not sure of all the conditions for all cars so I look up that criteria. I then hook in a gauge to the line in question and see if vacuum appears when starting the car cold. If it has vacuum then and not later when warm it is probably working properly.

Third, does the shift point change when vacuum is applied. Hook a line to the line going to the trans vacuum element and apply vacuum. Check the shift point, release the vacuum recheck. Are the shift points the same or different? Just being different is not enough, the vacuum effect is adjustable and there is a criteria for the rpm at the shift point. It's not likely that an old unit would be out of adjustment but it is a problem after installation of a new vacuum element.

Everything is exactly measurable so the answer is as easy as 1,2,3.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician