if you watch a video made by precision transmission in amarillo , tx , he shows you exactly how to do that.https://www.youtube.com/watch?v=bs_ugK5aCgs

 

the electrical solenoids are removed easy .. pan drop ..
if the truck has high miles and uses dexron III over many years could also be the valve body wear .. more work to drop the valve body if the codes pop up again.

use Dexron VI .. GM got bankrupt because of the dex III fluid turns to acid at 20K miles.

 

1999 Chevy Express Conversion Van 1500 5.7 liter Vortec. 95000 original miles.
Similar problem here, need further help. Check engine light came on with no symptoms. Code for faulty B shift solenoid. Then light went off two days later. A couple of days later started noticing difficulty with downshifting, only when accelerating to increase speed rapidly, such as taking off from stop or getting on freeway. Dropped pan, B solenoid was obviously broken, loose and twisted in its slot. Replaced it along with transmission filter, fluid and new gasket. Fluid level fine. Replaced pan, reset ECM. Problem not resolved. No check engine light. Tried shifting manually, no problems. Shifting problem only occurs when in Overdrive and between 3/4th gear and when accelerating rapidly. No I did not replace the A solenoid.

 

are you the original owner ? when did you buy it ?
many do have Dexron III issues .. the a-b valves wear and the torque converter gets debris in it ..
did the trans pan have debris in it ? filter dirty ? well you have to purge out all the transmission fluid.. old transmissions have to use a inline filter when you did the fluid / filter work .. new fluid will loosen up debris and cause torque converter erratic .
your info on the solenoid b valve a person did cause a stupid work on your truck.

 

are you the original owner ? when did you buy it ?
many do have Dexron III issues .. the a-b valves wear and the torque converter gets debris in it ..
did the trans pan have debris in it ? filter dirty ? well you have to purge out all the transmission fluid.. old transmissions have to use a inline filter when you did the fluid / filter work .. new fluid will loosen up debris and cause torque converter erratic .
your info on the solenoid b valve a person did cause a stupid work on your truck.

I am not the original owner. Vehicle sat for years until i bought it. I bought it at 65000 miles in August of 2019.
pan did not have debris in it and it was very clean along with fluid.
I'm not sure what your last sentence implies. I did the work.

 

put a transmedic fluid to restore the internal seals and rubber parts. put one half the bottle in the transmission..
if the transmission you worked on was clean ....then use this additive ..

 

Did the code(s) go away?

In 3rd and in 4th gear, the B solenoid is operated and stays operated, the A is released for 3rd gear and operates for 4th gear.
I would say if your experiencing 3 to 4 and 4 to 3 shift problems you should look at the A solenoid, or the wiring to the solenoid.

 

code went away prior to replacing the B solenoid and hasn't returned. downshifting problem only occurs when van is in Overdrive. Shifts fine manually and in 3rd gear. Never got a code for the A solenoid. Thinking of dropping the pan again and replacing it anyway. Wondering what else I can do while i have the pan off.

 

Inspect the wiring connector and wiring.
I'm surprised they are not sold together, like u-joints the big labour is getting to the parts that need changing, sell them as a matched pair and do the job once

Anyway, I ramble, good luck.

 

When in overdrive, the transmission control module commands the shift points based on VSS output.
These are the failure modes for shift solenoid B. Solenoid can either be open or shorted. If it is open, it will be in the OFF state all the time. If it is shorted to ground, it will be in the ON state all the time. 12V is supplied to all solenoids on one leg and the other leg is supplied a ground path by the TCM.

These are the A & B states when the TCM commands both solenoids in each gear.
Gear 1, A is ON, B is OFF
Gear 2, A is OFF, B is OFF
Gear 3, A is OFF, B is ON
Gear 4, A is ON, B is ON

When solenoid B coil is open, it will always be OFF since the ground path will be open.
Gear A B
1 ON OFF
2 OFF OFF
2 OFF OFF
1 ON OFF

As you can see, there is only first and second gear available when solenoid B is open.
Open means, the solenoid itself is bad with an open coil or the ground path from the TCM is not present due to a loose connection or a broken wire from the transmission connector to the TCM or a break in the internal wire harness inside the transmission.

Good luck finding it.