Vacuum Modulator help?

[Mikeandwendy74]

Hello again another question for you transmission specialists :) What are the symptoms of a faulty vacuum modulator? I know my vacuum pump works good as it has good strong vacuum at the modulator(where it connects). I just bought a new modulator with the little tiny screw inside to adjust the shift points. I'm going to install it tomorrow but I was wondering if any of you have this type for your turbo 400 and how the adjustable modulator works...good or bad thanks for any help.

[Robyn]

A faulty modulator can do a couple things but generally if the diaphram breaks they will allow tranny oil to suck through.
If thre modulator is not reacting it usually will make late hard shifts, BUTTTTTT it can work the other way too if things are hinky.

The screw is to fine tune the shift points up or down..

Be sure your vacuum is high with light throttle and drops off as the throttle is depressed. If not your VRV on the IP is fouled up or the lines are crossed up.

Vacuum at the pump should be about 25"+

[Mikeandwendy74]

After installing the new adjustable modulator here's the results: Good tight shift in first to secondaround 25, nice 2nd gear to 3rd around 35 quick but smooth shift. The old symptoms of my bad vacuum modulator: When cold, tranny shifted smooth 1-2, when hot a very harsh but tight shift from 1-2, with an immediate unnoticable shift into 3rd gear, basically I had no 2nd gear. Tell ya what, after my first rebuild of this tranny I sure am glad the funny shifting was related to a $12.00 modulator on the OUTSIDE of the turbo400 :)

[bbbear]

After installing the new adjustable modulator here's the results: Good tight shift in first to secondaround 25, nice 2nd gear to 3rd around 35 quick but smooth shift. The old symptoms of my bad vacuum modulator: When cold, tranny shifted smooth 1-2, when hot a very harsh but tight shift from 1-2, with an immediate unnoticable shift into 3rd gear, basically I had no 2nd gear. Tell ya what, after my first rebuild of this tranny I sure am glad the funny shifting was related to a $12.00 modulator on the OUTSIDE of the turbo400 :)

I'm having simular problems on a 1986 GMC 6.2 L bought just a little over a week ago... Hot or cold, it shifts hard into first and 2nd and smooth into 3rd. I have the service manual, but I'm no mechanic... Where is the easyest place to hook up the mityvac to check the vacuum... Can I hook it up on the vacuum line at the transmission? Or must I remove the aircleaner? If I have to remove the aircleaner, how is it done? Also, is the modulator only for turbo's? Is there anyplace to adjust the tranny on the 1986?
thanks,
charlie