TH400 tranny issues

Cheekster96 · January 13, 2005

hello everybody. i have an 87 3/4 ton 4x4 (8600 GVW) with the TH400 and 350 TBI motor.

just recently, the tranny has started to act up. It has done this once before but not as bad and was told to replace the Throttle Position Sensor and did so, that seemed to do the trick. Now, the problems are back and a bit worse. I changed out the TPS again but no luck.

The tranny stays in gear for a very long time, no matter if the truck is warm or cold or throttle position. Each time it shifts it jerks the truck real bad. Is a concern because when the truck is cold, i dont like to rev it that high in gear and with icy roads, makes the rear end squirly. There is no shift kit installed.

I did a fluid and filter change last spring and keep on top of all fluids; the tranny fluids is still a nice cherry red and smells like tranny fluid, not burnt.

I added a bottle of that Slick 50 additive for higher mileage trannys and will see what that brings.

What other troubleshooting steps should i take before i take the truck in?

Any advice or recommendations?

thanks for all the help.

Lee

96Z 383

Haulin' 8.1 · January 14, 2005

Lee that transmission uses a vacuum modulator to regulate shifts. If it fails or looses the vacuum signal the tranny will act exactly as you have described.

There will be a small metal tubing line with rubber ends going from the lower throttle body or intake back down to the rear of the tranny. Check that the line is connected, air tight, and not restricted. If the line is there and has vacuum at the tranny where it connects to the vacuum modulator then replace the modulator. It's very easy to do and inexpensive. Chances are very good it will fix your problem. They are availiable where ever auto parts are sold.

Vernon

Cheekster96 · January 15, 2005

ROCK ON! something that sounds quite possible that can fix it. i will most definately try that out and see what i come up with. Wont be till monday or tuesday though, working this weekend.

thanks again.

Lee

Cheekster96 · January 17, 2005

Hey Haulin!! thanks for your help. your advice did the trick. i found that metal connection on the intake and traced it down to the modulator. sure enough, the rubber sleeve on the intake side was corroded and not even attached. was also the reason why my idle was high.

i cut a new piece of rubber hose and attached it to the intake and the metal line, started the truck and immeadiately noticed a smoother lower idle.

went down to get gas and test drive the truck and didnt even notice the gear changes! that smooth. even now when the temp is 5F, the tranny shifted nicely!

thanks again for yours and others advice!