Truck: 1986 GMC Sierra 3500 6.2 Jasper motor TH400

I have been going through the whole broken starter bolts, chunked out flywheel teeth process since April when I got it. Sunday I replaced the flywheel and shimmed the starter 1/32 on the outboard side to bring it closer to the flywheel. It seems to start correctly now but it won't shift properly into third gear. When changing the flywheel I knew that I shouldn't remove the torque converter, because it has to stay on the transmission to stay lined up. I spun both the torque converter and the flywheel several times during the job but I put them back together in the same place. I don't know if that matters or not. I disconnected the vacuum hose and the shifting linkage, but replaced them without interfering with their adjustment as far as I know. The fluid is topped up. It shifted like a Cadillac before the operation, but the last trip was hauling a 6000 pound trailer. Could I have just bumped something or could it have shifted its last?

Sounds like a vacuum line off the transmission to me.

Brian

I'll try messing with that line again but I won't get a chance till Friday

Thank You

Sounds like you kick down cable is out of adjustment. Also have you been putting the bracket that goes on the starter that bolts to the block? If not that is why you are breaking bolts.

A couple of notes. It does not hurt anything to remove the torque converter. In fact, if the transmission has been in service for a number of years, it is a good time to change the front seal.

You said: "The fluid is topped up". I hope you didn't fill the transmission over the "fill to" mark on the dip stick. An overfill can cause the oil to foam and interfere with the shifting functions.

Thank You for the help. I filled the fluid to the line when hot. I already put it back together, so its a bad time to change the seal. I DO have the bracket. It stopped breaking bolts when I put that in but the flywheel was gone in no time. For a couple months I had a mark on the bottom of the pulley where the bad spot was. I would check it before starting and if it had landed in that spot I would roll it over with an 11 R (clamp) This strategy was a real loser because before long I had 4 bad spots.

Another note on the no shifting: 1st to 2nd was still good. 2nd to 3rd was not happening. I took what was supposed to be a victory lap until I couldn't get into third. I found that by goosing the throttle I got it to go into third on 2 out of maybe 8 attempts. This is while I was working my way home hoping it would get me there.

O.K. so when you say kickdown cable, is that drivers side, mechanical linkage with a nut to lock it in place? I didn't mess with the nut, just pulled the little cotter pin out and disconnected then reconnected afterwards. Also, the vacuum hose: It slid right off to move the transmission. Afterwards, I slid it back on. When it was malfunctioning I slid it back off with the truck running and could hear suction. It looked a little different than before and now I have this feeling I might have bent it, but I also have a feeling my mind is playing tricks on me.

No kickdown cable on a THM400, downshifts are controlled by a switch above the throttle pedal. Probably a vacuum related problem. Check vacuum signal at the modulator end, if it's low or below 11" hg, it will cause a harsh/late shift.

O.K. so Friday, I'm going to get Vacuum gauge, Start my truck, pull the hose off (at the transmission?) and check for 11"?

Check at the transmission first. Then check on the passenger side of the injection pump where the two vacuum hoses attach. Lower one feeds from the vacuum pump; upper one sends variable vacuum to the modulator on the transmission.

As stated earlier, most likely the line is pinched, dislodged, or bent somewhere by the transmission, but this test will rule out a vacuum pump or vacuum valve issue... good luck.

These are good suggestions and possibilities, but these are not to say that there isn't a problem with the modulator itself. Confirm the vacuum supply first.

This issue is resolved

I didn't even get to check the vacuum on anything. I did bend the line on Sunday. I just tweaked it around, made sure that it was on there nice and square, fired it up, and it worked perfectly. Thank You everyone. I know this turned out to be a simple problem, but I was going to park this truck for the winter without a place to turn for good advice.