Automotive Forums  Where Car Enthusiasts Go to Discuss Cars Online

have a 93' gmc c1500 which drops out of 3rd or 4th gear(not exactly sure, because it drops out at lower and lower speeds  as the truck warms up) Otherwise everything seems ok, but I just barely bought the truck. I am far from a mechanic, and wondering if possibly a sonnax bore plug (maybe 77754-21) would fix this problem and spare me the expense of rebuilding or replaceing my 4l60e.  Thanks.  

Hows the fluid look? Does it smell burnt? From what you said I would suspect its the 3-4 clutch pack being wiped out, very common. But if the fluid smells o.k. then maybe, but have the speed sensor checked too.  

I changed the filter and fluid over a week ago, and I dont remember the smell, but I think the oil was dark. I'll check the speed sensor. Can I test it with a scanner? Thanks.  

Yes, and see what it says while it is acting up. While your at it, look at the trans range sensor and throttle position too. Any one of them could fail. If you can, watch the gear shift command or which solonoids are on/off to see if the trans is doing what the computer is commanding.
It sure helps when you have a scanner.  

The bore plug is more related to lock up, if possible drop the valve body and look carefully at the holes where the check ball seat on the separator plate, the #7 check ball may be sticking or allowing fluid to pass, to correct the problem ether get a new sep. plate or replace the ball with a little bigger one. Check all holes where check balls seat carefully!