Archived transmission sometimes won't downshift

[tgrfan2]

I know there was a topic on this , but I could not find it with a search. My 98 12V will fail to downshift to first when rolling to a stop sometimes. I can pull the shifter to 1 and it will shift in OK. Getting ready to service it again at 110K and I think I remember there being a switch or valve that controls this. Also want to raise the shift points a little while I am in the pan. read Bill K's post on this. Wondering who else has done this.

 

[inarush76]

downshift

It is not uncommon for a truck to not down shift on a rolling stop.

Governor pressure has to drop to near zero before it will down shift. The PCM watches the output speed sensor for an input to control governor pressure. If the PCM doesn't see "0" on speed it doesn't know to adjust pressure to zero. Does the truck go to low gear everytime if you come to a complete stop?

Check fluid to assure it is full.

If trans shifts to first on full stops, the problem may be driver induced. (sorry)

Before you adjust the stop on the valvebody for later shifts, confirm that TV cable to trans is adjusted properly. Adjusting for later light throttle shifts can lead to a 1-2,2-1,1-2 shuttle shift at real light throttle. HTH

James

 

[KB0OU]

Pressure too high

My 96 did that when I turned up the pressure on my transmission until I got the right amount of resistance in the pressure sensor input to the cumputer.

 

[tgrfan2]

Fluid level is good. The first time I noticed this was soon after I serviced the transmission and I thought then that I might have left it a little low, but level was OK then also. It only fails to downshift every once in a while. The pressure sensor may be the part that was discussed in the previous thread. I will do a search for that and see if the thread comes up. Thanks for your help.

 

[lmills]

The parts you are referring to are the governor pressure sensor and the govenor solenoid. They are around $200 and are pretty easy to replce.