I was driving down the interstate last week at about 70 mph in my 1988 K5 blazer. Coming down a hill the truck starts bucking up and down. So I pull to the side of the road. The thumping remains until I put the truck in park. I tried to move it again and you could hear something knocking around, the sound was coming out of the tranny hump. It will still move but had to be towed to prevent further damage. Some people tell me it is a flex plate others say it is a torque converter. I just dont know what the problem is. Any help would be much appreciated. BTW 350 engine with a TH700-R4 auto tranny. Truck has 98500 miles.
If the truck runs fine in park or neutral, I would have a very hard time believing the flex plate is briken. So that would leave it to be something in the tranny. The pre'88 700-r4 transmissions are known by GM to be very weak. During that model year they made significant improvements to the whole thing. My guess is that it is something internal in the transmission broke. The only way to know for sure is to drop it out and look. You could replace it yourself for well under a grand. Just make sure you get a newer one to swap it with. Did you check the U-Joints?
It runs fine in park but thumps in neutral. I checked the u joints that was my first idea. Thanks for your reply.
torque converter problem. pcm sees excessive slippage, increases line pressure to compensate, harsh shifts and bucking are the result. shut vehicle down, system resets. problem does not reappear until next extended highway trip. usually occurs at higher speeds and loads (like towing)
stupid me. disregard that last post, I don't know what I was thinking. Yes, the 700 is a very weak trans. You may want to look into an upgrade kit that will let you put a newer version of the TH400 in there. Find youself a good tranny guy who can give you the details.
Thanks Dave, I think I will end up rebuilding the tranny. It will help in the long run when I get the 33in mtrs for the blazer. I was hoping it a simple problem but this is what I expected.
