I have only had a few automatics fail by losing reverse,and a few other problems that some of which I was able to fix without yanking them out..so far I've yet to grenade one..
One TH350 in my 77 GMC K2500 worked perfect until one day I went to back up after plowing some snow into a pile,I heard a buzzing noise like a bee's nest for a few seconds,then nothing--it wouldn't back up,didn't make any noise,it did still drive forward and shift normally into all 3 gears though..
The truck ended up being parted and scrapped,I kept the 400 SB I put in it and gutted the interior,the axles and cab & bed,frame,all went to the scrapyard..
I regretted junking it,it only needed the transmission fixed,but the frame was getting thin in places and it had been patched all over (body rot)..I bet I could have got several more years use out of it had I put another trans in it..
My 81 G10 van's TH350 with a lockup converter had only first gear and reverse when I bought it for $500 in 1990..guy thought the transmission was junk,but all it needed was a govenor gear...
I drove it another 35,000 miles after I put another govenor from a junkyard trans in it..it has 178,000 on it now..when I first got the van,the stoplight switch would not let the converter lock up,and driving it nearly 100 highway miles got it hot enough to peuke some ATF out of the dipstick tube,but after that was fixed it didn't seem to bother it..
The 85 K10 diesel Burb I bought 10 years ago with a 700R4 loses reverse after it was driven some distance,and wont shift in or stay in overdrive ,I was told by the previous owner--since I never drove it on the road,I don't know if that's true,it has had reverse seem fine driving it around the yard,but I don't doubt the trans is dying,since its not the "best" year for a 700R4..it has 155K on it..
The former owner told me after it started losing reverse he had a mechanic drop the pan on it and said what was in it for fluid looked like coal tar..they sprayed it clean with brake cleaner,put the pan back on,filled it back up with a quart of Lucas in the fluid and said it only refused to back up a few times after that..but the overdrive still wouldn't stay engaged..
This was the main reason my plans to start using the Burb never happened,I didn't have the money for another trans & the install,plus what it'd cost to put it on the road..
The 72 K5 I had with a TH350 refused to go in forward gears every morning after a cold start,or if it sat more than a few hours in cold winter days below 30 degrees--once the engine started to warm up,and you left it in drive,you'd feel the clutches start to engage,and it would finally go in gear and you could drive the truck 500 miles ,it would shift perfectly,never slipped,but the next day it was the same BS all over again..
I tried every "transmission repair in a bottle" fix in that TH350,one mechanic said Lestiol worked many times for him in similar cases,so I even put a bottle of that in it..nothing made a bit of difference,but didn't seem to do any harm either..other trans shops told me the "lip seals" were hardened up and it'd require an overhaul..
I decided to swap in a SM465 from a wrecked K5 instead,a job I regretted starting,it sucked from beginning to end,especially putting the pedals in it--and I was not happy with the gear spacing & 3:08's,it was dog,and the SM465 ended up having worn splines on the output shaft and it stripped one coupler out in 10,000 miles,the new one I put in with some locktite only stayed quiet for maybe a month,then began rattling again..
I don't think GM has ever had a really "good" automatic with the exception of the TH400's,but those fail too,they just seem to hold up the longest..their manuals all seem to have poor gear ratio spacing,but some are pretty rugged,like the SM465..
I have zero experience with the newer electronically controlled automatics like the 4L60 or 4L80E,or Allisons,but I've heard just about all overdrive automatics dont hold up as well as the old 3 speeds ones,and not just GM,but Fords and Mopars too..
Out of all of GM's automatics I'd say the Allison is the beefiest..
I did "break" one automatic but it was when I helped a friend install one at his shop,in an older Ford Crown Vic..he was struggling to get it in place and lined up with the engine,and I was able to get a bolt started ,then he could wiggle it around and get it lined up right..when we started to crank the bolts in,I said "I don't think this is a good idea,it should be flush with the engine before we snug the bolts down"..
"I just want the dam thing IN and over with!" he said..
He started tightening the bolts nearest to the dowel pins,and we heard a noise like a "snap" followed by a crackling noise,like someone crumpled up a bag of potato chips..we saw a large crack appear at the bottom of the bell housing,and watched in horror as it slowly spread almost all the way around to the other side..

..
My friend was quite pissed and upset,this was a customers car!..and it had fought him tooth and nail all the way..he looked at it and said "There is still about 6" of bell housing with no crack"...
We pulled it back off and dropped it down..
He drilled a hole at the end of the crack and made his first attempt at mig welding aluminum ,he was able to blob it back together..
He told the customer the used tranny from the salvage yard "must have had a crack,and it grew as he tightened the bolts,so he tried to weld it"...the guy was pretty clueless about cars in general,he just said "As long as it works,I dont care--I'm selling this soon or junking it anyways"...my friend said he should have scrapped it before ,when he brought it in with a dead transmission,but he insisted on fixing it..
Funny thing is,that car drove fine for a few years after that,until the lower control arm ripped away from the frame..
