3rd gear acting funny

[Craz]

Talking about a 93 GMC Jimmy here...

Recently changed the tranny out, everything worked fine for at least 600 miles... but heres the issue now...

Never thought for once I babied my truck, but I guess I do now that this problem has arisen. If I try to just drive my truck, like cruising on the beach and looking at women type cruising, I start out in 1st, it shifts to 2nd fine, but then I feel it kick down to 3rd, but it doesnt pull. Motor revs up, but no pull. Not sure if its slipping either tho, no smell of clutches or anything. Seems like its going into neutral. But wait... theres more...

Stop at a stop sign or just on the side of the road and smack the steering wheel and scream out... WTF now ??? :mad: :confused: OK... I start out again with that bit of anger in me. Obviously more heavy on the gas.... shifts to 1st fine, shift kit gives my 31's a bit of bark into 2nd, here comes 3rd, tires try to bark again.... all is fine... everything shifted fine this time.....

So I stop again, and try to drive like grandpa again... and 3rd slips or goes into neutral... whatever the hell its doing...

I put a scanner on the truck and get the following codes...

#28 - Manifold Pressure Switch
#38 - Brake Switch OFF

This is a new tranny and would hate to think the manifold switch is bad, but at least its not an internal part, and I know how electronics can be. Brake switch, yea ok.. I can change it, but thought I would get some advice before I just started swapping parts... I hate just swapping without trouble shooting first and knowing for sure.

Any advice on determining this problem (hopefully someone has had this issue before) would be appreciated. This is on a 4L60E obviously.

Thanks guys/gals !!

[Mikz86TA]

Its a E tranny so the only thing that comes to mind is something the computer is sending signal. Since you dont have the issue on heavy throttle its not likely the pump, servo or any other major internal part The MAP switch is on the intake manifold. The signal from it could be malfunctioning at low speeds/pressure, thus sending bad info the the ECM. Only other thing that hits me right off is asticking governor. DO THIS : Pull the dipstick and wipe it on a rag. Is there any gray smudge particles (and they dont need to be clumps or shavings just grayish smear) with the red on the rag. If so, pull the governor out, clean it in mineral spirits, inspect it to be sure it operates freely and the valve inside is sliding, and blow it dry B4 reinstalling it.

[Craz]

Well, havent had the chance to stop and check the governor. Fluid and stick looked good tho...

Havent driven it much lately due to the problem, but wife had to take it somewhere the other day. After she got back, I tried to drive it around the block and it doesnt have 3rd or 4th now. Weird thing is clutch fan is also NOT disengaging. I can hear the fan like crazy... ALL THE time now. Weird that I lost 3rd and 4th simultaneously with the clutch fan not disengaging. Clutch fan is new too.. straight from GM... $350 bucks worth ? Any further suggestions ??

[Mikz86TA]

I couldnt find it this morning, but I saw somewhere a toipc where a guy had a motorhome and his clutch fan wasnt disengaging. He said it did have shift troubles. Try to google under that topic.

[Craz]

Havent really found much on the matter. I have no clue why its giving me such problems lately.... hoping I could figure out what it was without having to remove the entire tranny... AGAIN !