Originally Posted by ihatebarkingdogs

I've done about a dozen 4L60 and 60e's over the years, I'm not an expert by any stretch, but I'm past the point where I make mistakes from inexperience. . I've never seen this before. I want to be sure I won't overlook a root cause.

I "patched" this transmission for a friend a couple of years ago, after a sprag failure that destroyed everything in the input drum and the drum itself. I used parts from my stash, and it worked pretty well when we were done. He drove it probably 20,000 miles since. It was a complete used sprag assembly from a working transmission. It is the double-cage late sprag.

He said he was going pretty fast (better than 80, which in this truck is about 3,000 RPM) and it "downshifted" he thinks to 3rd, and sent out a huge smokescreen. He was able to drive it home. All it had was 1st, 3rd, and reverse.

The band and drum are destroyed. Total melt-down, with metal transfer to the drum from the band. The forward sprag wasn't seized, but was really tight. The flippers are ground down on the inside. Both inner and outer races are brinelled and scored. It looks like the sprag ran dry and seized. If the sprag seizes in 4th, it will force the band to slip. Hence the band burn-up and all the smoke. The servo looks fine, I didn't assemble it incorrectly. The 3-4, forward, and lo-reverse clutches all look fine. I don't think it was a TV problem.

Am I on the right track? Did the sprag seize at the high speed, and the band was collateral damage? Did the sprag seize because it was "used", although it checked out fine? The ATSG book says to replace the sprag if there arfe any signs of distress on the ears of the shell or drum, even if the sprag checks and feels Ok.


Or is there something else I should be looking for?

Thanks in advance.

Originally Posted by ihatebarkingdogs

arbee:

I've seen a lot of broken and damaged sprags. None like this one. Almost like cold-weld damage. It didn't turn in the free direction very well at all, but was holding in the hold direction. Because the guy knew it was a patch-job (he was there watching / learning) he said he drove it gently. Most of the time.

There was a TON of smoke. Looking at the band and drum, its easy to deduce that this is where the smoke was generated from. Nothing else is burned. The 3-4 was still the clearance we had when we put it together.

AFAIK, the 4th circuit is pretty simple. Not much to leak other than the servo seal, and the pin bore. Please correct me if I'm wrong. One thought is that I had too much clearance on the band. The servo pin was too short. I don't have the special checking tool. I was taught to check the band by "wiggle" with a screwdriver after assembly, and that the output shaft must turn backwards. Perhaps the band was too loose, and that would of course hasten its demise. Any thoughts on how to check the clearance without the checking tool?

I still think the sprag damage is related to the smoke-generation (sprag seized with the band applied, forcing 3rd, forcing the band to slip) but thanks to your input, I'm going to pay attention to the band clearance when it goes back together.

Thanks. Much appreciated.