Before I drop the pan and inspect the valve body and downshift solenoid, I am looking for additional guidance with an issue that started without warning in a turbo 400 transmission. The transmission was rebuilt about 5000 miles ago and has a shift kit that shifts firm indicating good pump pressure. The issue is when in 3rd gear it acts like it goes into neutral on its own. When this occurs I can come to stop and accelerate normally and it shifts fine through all 3 gears again or manually pull the shifter into 2nd gear and accelerate normally it shifts fine back into 3rd gear. What has me puzzled is I can disconnect the 12VDC wire that supplies voltage to the downshift solenoid and still get an occasional downshift from 3rd to 2nd with moderate acceleration. I have replaced the vacuum modulator hoping it was a freak vacuum issue, however it still does a random downshift and acts like it goes into neutral on its own. What should I be looking for other than a bad downshift solenoid or sticky 2-3 shift valve? Any input is appreciated!
Posts: 102 | Location: Ga | Registered: July 28, 2007
Follow up with a resolution to the above problem. The power brake booster shared a manifold vacuum terminal with the transmission modulator valve. The brake booster diaphragm has begun to leak allowing fluctuating vacuum to the transmission modulator. Swapped the transmission vacuum line to a separate carburetor base vacuum port and all is well now. Just an FYI to any other do it yourself'rs that stumble across this same issue...
Posts: 102 | Location: Ga | Registered: July 28, 2007
Bracket Talk 
