tranny shifting when not supose to

[aprice]

a friend of mine has an 87 3/4 chevy with the 350 and th400. he uses the truck for a backup daily driver, all of around sort of a truck. He also pulls in the local truck pulls, which brings me to my point. he has pulled about 15-16 times with the truck, but the last 2 times the tranny wants to shift out of low 1 even though the shifter is in low gear. he has checked the linkage and thats dead on. last night it didnt shift but then he unpluged the vacum line for the tranny. the tranny otherwise is in top shape, any ideas why it shifts when its not supose to, other than a weak spring in the valve body?

[1976Scottsdale]

I have heard, but I am not certain that an automatic will override the linkage when the pressure gets too high and will engage the next gear regardless of the shifter position.

[Mahugh]

A 400 turbo has a fluid bypass system that is when the trans fliud reaches a certain pressure it will shift into the next gear on its own. When the fluid pressure goes down the trans will go back into the lower gear again. tell your buddy that he needs to let the trans cool off a little betmeen pulls then this will stop.

[aprice]

so if he were to pull with a cold tranny after idling the truck a couple minutes it wouldnt shift like that. also is there a way to change what pressure it shifts at when shifteds in to first?

[Mahugh]

Yes he should definately make the pull when the trans is cold. the best way to modify the pressure by pass is with a shift kit.

[aprice]

my friend pulled this weekend. it was a 15 minute drive and the high for todsay was like 74. he showed up about 3 hours early to watch the atv pulls. the truck still shifted though. my friend is considering rebuilding the tranny and puting a shift kit in but not prolly till next spring