Quote:
Originally Posted by 300DFarmer
Problem is, now I have no vacuum reading at all where the vacuum line comes out of the vacuum control valve to the modulator. I am not sure if this is a result of my recent changes, or somthing else, but I do know I had vacuum at this point last year.
|
The operation of the VCV is quite confusing. On the surface, it appears as though vacuum enters the top of the VCV and comes out the bottom hose.
However, this is totally incorrect.
The top line feeds the VCV and also sends vacuum to the transmission. There is a critical orifice in the line to the VCV and only a small amount of flow can pass across the orifice. Then, the VCV starts its "leaking" of vacuum. As the VCV "leaks" more vacuum, the vacuum level to the transmission drops proportionally to the leak. Eventually, at maximum throttle, the leak is so huge, that the vacuum to the transmission is zero.
So, if you wish to check vacuum to the transmission, put a Mityvac or a vacuum gauge in the line to the trans and read what you have. On the older vehicles, you can disconnect the supply vacuum and substitute the Mityvac for the vacuum pump. Pump up the system and then open the rack and see if the reading falls proportionally. This won't work on any '85 or newer vehicle, however.
The line on the side of the VCV is a vent line........it goes into the cabin to obtain "clean" air for the VCV. Remember, the VCV is always drawing air from the outside to "kill" the vacuum level.