Posting this for a buddy that's not online yet, he's got a '79 with the 351-M and guessing a C-6 (not sure about the tranny) it is lifted and riding on big tires, 39's I think he said. He lost all forward power coming home from work, trailered it home but then couldn't make it up the driveway even after cooling off. Happened a couple times in the past and when eng is shut off an turned back on it's regained forward power. Not so lucky today, it took more than a few tries and he barely got up the driveway. Okay, so here's the question, is this a tranny valve body issue or should he look at the torque converter first. Sorry, but theres a time zone issue so can't get any more info right now. He did have some water in the tranny awhile ago and drained pepto bismal out of it, but not sure how long ago that was either. Are there any tell tale signs to look for on the tq converter once it's out, like it smells burnt or is discolored from heat? Don't really know much about the guts of a C-6 or C-4 transmission and so turning to you guys for some help. Thanks!
I'm not a tranny expert, but would guess that the problem isn't the torque converter, but rather a worn tranny. He could change the fluid and add some Lucas tranny stuff and see if he can limp a little more out of it, but it is probably close to toast. I did the Lucas on mine when it wouldn't shift into 3rd and adjusted the front band and it is working again, but I know I only bought a temporary reprieve.
I'm no expert but I've burned up my fair share. Your friend has a bad transmission. The dead give away is the pepto portion of the story. For right now (since he still has some forward movement) have him tighten the band. The C-6 only has one band and simply tightening it may help. Here are the directions:
Raise the truck on a hoist or jackstands.
Clean all dirt away from the band adjusting screw. Remove and discard the locknut.
Install a new locknut and tighten the adjusting screw to 10 ft. lbs. (13 Nm).
Back off the adjusting screw exactly 11/2 turns.
Hold the adjusting screw from turning and tighten the locknut to 35-40 ft. lbs. (47-54 Nm).
Remove the jackstands and lower the vehicle
If he still has no forward power, then it will be time to replace the transmission. Tell your buddy that when he replaces the transmission, install a transmission cooler. Also, he may want to snorkle the vent tube on the transmission up above the body so when he goes into deep water, he doesn't suck the water into the transmission.
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