Tranny not shifting when cold

[JTodd]

I changed my tranny fluid about 4 weeks ago, and have driven the Sub infrequently since. The last week or so I noticed that it does not shift from 2 to 3 until the engine temperature has warmed up (approx 5 minutes of run time). 1 to 2 shift happens normally. I have only run the engine up to about 2k rpm, so I don't know if will finally shift above that, but I have no desire to run a cold engine up there to find out. Once warmed up, shifting and OD seam to be normal. (BTW 'Cold' in this case means 70* outside temp and engine sitting overnight.)

Any ideas, or is this 'just one of those things'

[Marty Lau]

The are a couple of sensors on the out side of the tranny on the drivers side. One is in the tail peice and the other is near the fron there is a plug on each and a single bolt holding in place. You can change them out there are about $30 each or you might try taking them out and cleaning them up there is a little screen in them if they get plugged up it will affect them. Now if that doesn't do it there are 2 solenoids in the tranny and acessible when you drop the pan.
one is the 1/2 shift the other is the 3/4 shift they also run about $30-35 each. If you change them I also who change out Transmission Fluid Pressure Switch Assembly it's right in the middle of the tranny when you drop the pan and it's not too bad I think about $40. I was having my truck not wanting to shift out of 2nd when cold I changed all those out and the tranny worked great after that.

Copy of Technical Service Bulletin:

A/T - Shift Solenoid Update

File In Section: 7 Transmission

Bulletin No.: 37-71-48A

Date: November, 1996

INFORMATION

Subject:
Transmission Product Updates
(New Shift Solenoids)

Models:
1991-97 Chevrolet and GMC C/K, P, G Models
1991 Chevrolet and GMC R/V Models with HYDRA-MATIC 4L8O-E Transmission (RPO MT1)

This bulletin is being revised to update the part numbers and to include 1995-97 model years. Please discard Corporate Bulletin Number 377148 (Section 7 - Transmission).
hope this helps.

[ 09-19-2005, 06:41 AM: Message edited by: 16ga SxS ]