6 speed suddenly rough shifting

[1976gmc20]

There is a steep winding hill going down into the west side of town. This morning after about 50 highway miles we turned on the road down the hill and after getting rolling again I pulled the lever down into M which holds it in third gear because that's where it is by then (about 20 mph). I always do this going down this hill.

At the bottom of the hill there is a stop for a traffic light then another block through a 25 mph residential area to a second traffic light. After that it levels out to a main 35 mph city street.

So coming to a stop at the second light I shift back into D. Upon the light turning green and trying to go, the transmission jerks like it wasn't in gear at first. From that point on it seems to be shifting rougher than normal. It isn't really bad but just noticeable because normally you can't even feel it shift just notice the rpm drop.

Transmission fluid is full. Haven't been towing or doing anything unusual lately, just a trip to town and back every couple weeks, and a couple of short drives locally. I had stopped at the little park about a half mile before the turn down the steep hill and it seemed perfectly normal accelerating back out on the highway. Truck only has 76K miles.

[EVRLET]

Have you attempted to pull codes? From your facts, going to M put you in sports mode and it may have temporarily got stuck in this mode, technically referred to as driver shift control quadrant. You would have more aggressive shifts when in this mode. The 6L is so heavy with electronics that I would start with codes. If you don’t have any, try going back into M mode and back into standard to see if the electronics can get you back to normal.

[1976gmc20]

Originally Posted by EVRLET

Have you attempted to pull codes? From your facts, going to M put you in sports mode and it may have temporarily got stuck in this mode, technically referred to as driver shift control quadrant. You would have more aggressive shifts when in this mode. The 6L is so heavy with electronics that I would start with codes. If you don’t have any, try going back into M mode and back into standard to see if the electronics can get you back to normal.

No, there were no lights on. All I have is a simple little code reader.

The truck was shut off and re started a bunch of times in town and it kept acting about the same.

I always use M in this truck going down that steep hill into town. I don't use the +/- buttons. I just pull it down after getting going out of the roundabout and by that time it is in 3rd gear per the display so it just stays there all the way down.

I haven't driven it since we got back except to move it 50' to its usual parking spot after we unloaded. It often sits a week or two at a time.

If it doesn't clear up I will probably schedule an oil change with the dealer in a couple weeks and see what they think about it. We've only put about 3000 miles on it since we got it but it's been almost a year.

[EVRLET]

Trans codes don’t often trigger a check engine light. You need a scanner that can read into the trans controller and pull stored codes. A simple code reader usually won’t cut it. If you’re going to take it to the dealership, they’ll scan and more than likely pull a code. Let us know if there was a code and what the code is.

[1976gmc20]

Originally Posted by EVRLET

Trans codes don’t often trigger a check engine light. You need a scanner that can read into the trans controller and pull stored codes. A simple code reader usually won’t cut it. If you’re going to take it to the dealership, they’ll scan and more than likely pull a code. Let us know if there was a code and what the code is.

Okay, thanks. No use trying to read codes then.

It will probably be a couple weeks or more. We're planning to take the 4runner to town next week and I don't like to go to the city more than once in a week. Heck we used to only go once or twice a month, but life seems to be more complicated now.

[LEEVON]

I'm not a transmission expert, at all. But I have found the 6 speeds love to have clean fluid. We've had several and they start to get jerky/rough after about 40k. You're past that point anyway, can't hurt to do a fluid service.

[1976gmc20]

Originally Posted by LEEVON

I'm not a transmission expert, at all. But I have found the 6 speeds love to have clean fluid. We've had several and they start to get jerky/rough after about 40k. You're past that point anyway, can't hurt to do a fluid service.

The dealer claimed they had done everything before we bought it. We've only driven it about 3000 miles.

The fluid looks very clean. In fact it is hard to read on the stick (must be this new Dexron VI or whatever is more pale than the older ATF). I will press them about the fluid when I take it in.

This was a 4yo truck originally bought at that dealer and then traded in for a new 2020. We bought it in April 2020.

[1976gmc20]

I have an oil change appointment at the dealer next Wednesday (2-3-21) and getting them to check out the transmission also.

After sitting for almost two weeks it will probably be just fine by then

[1976gmc20]

We have fully diagnosed your transmission concern. We found a broken planetary gear internal in the transmission has caused the failure. There are chunks of metal in the transmission pan and this metal has done damage to other components. The cost to rebuild this transmission unfortunately exceeds the price to replace it. The price to replace the transmission is $5,806.00. You will receive a 3 year/100,000 mile warranty on the replacement transmission.

Well, that sucks! I guess I should have spent a little more and ordered a brand new 2020 since now between the pickup and the topper and the new transmission I will be about $30K into this truck.

[1976gmc20]

Well, I got it back today finally. It drives great

Though now I may have a leaky tire Or it could be just the extreme sub zero weather?

[special-K]

Did they show you the parts? That sucks. A broken planetary gear? I wonder how that could be.

[Palf70Step]

Not the first time I have heard of that Tim. It seems to be pretty common in the 6 speeds.

[1976gmc20]

Originally Posted by special-K

Did they show you the parts? That sucks. A broken planetary gear? I wonder how that could be.

They did not rebuild it. I guess they could have shown me the oil pan full of broken pieces but I never asked. We were back home by then anyway.

Last few blocks going there it wouldn't shift out of first. According the the papers, when they tried to drive it the transmission quit entirely but they shut it off and back on and got it to engage long enough to get back to the shop.

[1976gmc20]

Originally Posted by Palf70Step

Not the first time I have heard of that Tim. It seems to be pretty common in the 6 speeds.

Yeah ... now I hear about that

Any idea what precipitates that? I thought these 6L90's were supposed to be super strong?

The first owner reportedly had a tool bed on it but the hitch receiver looked unused. All we did was tow a 6x12 U-haul about 700 miles, and highway driving to town and back.

Funny, despite all my complaints I never had a TH350 totally quit me. They would just get weaker and weaker. That's the main reason I traded in my 73 K20 years ago. It leaked fluid back into the transfer case, and one morning it wouldn't go, but I added a couple quarts of Dexron and away we went. I drove it several more years after that before I traded it.

[special-K]

That's why I'm scared of owning a newer vehicle. That transmission replacement cost twice what I paid for my '95 Suburban that I have driven and worked for six relatively trouble-free years. I just can't grasp that comparison. 4L80Es are electronic overdrive transmissions with a great reputation but they had to replace it with a craptastic inferior unit to gain an unneeded extra gear to keep up with marketing to yuppies is how I see it. TH350s? I had one rebuilt and that was a perfectly operating transmission ('72 in the '80s) that I wanted beefed up for mud bogging. The first guy didn't know what he was doing. He heard the hi-performance part but he must have thought I said I had a Camaro drag car. That transmission build lasted a few hundred miles before blowing up. Then I found someone better who did it right and it took all the abuse I gave it. I'm sorry to hear about your transmission trouble. If it were mine it would be sitting in the driveway waiting for me to figure out how I'd be able to pay for the repair.

[1976gmc20]

Originally Posted by special-K

That's why I'm scared of owning a newer vehicle. That transmission replacement cost twice what I paid for my '95 Suburban that I have driven and worked for six relatively trouble-free years. I just can't grasp that comparison. 4L80Es are electronic overdrive transmissions with a great reputation but they had to replace it with a craptastic inferior unit to gain an unneeded extra gear to keep up with marketing to yuppies is how I see it. TH350s? I had one rebuilt and that was a perfectly operating transmission ('72 in the '80s) that I wanted beefed up for mud bogging. The first guy didn't know what he was doing. He heard the hi-performance part but he must have thought I said I had a Camaro drag car. That transmission build lasted a few hundred miles before blowing up. Then I found someone better who did it right and it took all the abuse I gave it. I'm sorry to hear about your transmission trouble. If it were mine it would be sitting in the driveway waiting for me to figure out how I'd be able to pay for the repair.

I said I never would, but when I went shopping for the configuration I wanted I couldn't find anything that wasn't 200K++ miles and pretty beat up. Then I looked on the dealer site just for comparison and found this one just what I was looking for. I figured it would cost about the same in the long run to fix up an old one plus a year or more in time and dealing with various shops as I wouldn't do it all myself. I don't even have a decent place to work on anything so I'd have to spend several $10K building a shop first

So then they still had it after we had the money available, plus by then they had fixed a tear in the driver seat. I opted out of the service plan because we don't carry collision on our older vehicles worth <$10K and it's nothing but an insurance policy anyway. I figured at 73K miles nothing big would happen for several years and at least another 75K miles. It's all a big crap shoot anyway as "Ned Ryerson" would say

Hard to figure how a "90" transmission is craptastic compared an 80 or even the 60 that I had in the old Suburban. When it works it does work really nice, even almost up to the end. If I had realized that these weird symptoms we started having last summer were the transmission instead of the tires, then maybe I could have had it checked out sooner and saved it from total failure.

I'm just glad that it didn't fail while we were going to Colorado and back last summer! That would have been a real sh1t sandwich on top of a very strenuous expedition.

[special-K]

Yeah, you were lucky to be "local" when you discovered the problem. I'm sure as time goes by I will own a vehicle from this era. The old keeps getting older and so does the new. I've been saying for years, since vehicles went all electronic, complicated, and expensive to repair, that vehicles are made for those who can afford new and screw the people who have to buy used. That's how it's always been, but a person used to be able to figure out and fix their own old vehicles and the repairs were affordable. Meaning not a cost that rivals the price of a decent used vehicle. I've seen good not so old vehicles get junked because they needed a transmission. I couldn't believe it when I never paid more than a few hundred to get a TH350 done. I know it costs more now, but not thousands of dollars. Just think if the transmission goes up ten years from now. That repair cost will be higher and the value of the truck much less. You get hundreds of thousands of miles out of them now, but some just won't have the funds to pay for that, or not willing to dump it into an old truck.

[1976gmc20]

Well, it's got a three year warranty now. We'll see what happens.

This was going to be my "last truck" considering the way I usually keep them.

Since I'm not all that happy with my old K1500 x-cab, maybe I will keep a lookout for another GMT400, K2500HD regular cab 5.7/4500, then just gradually fix it up until it's in pretty good shape. You always need a spare vehicle living out here, and when your spare breaks down then you need a spare for that!

When we go somewhere together then we would be stuck with having to call someone to come get us if we broke down and couldn't get home, but whenever she goes shopping or to church I make a point of having to have something dependable that I can get started and go rescue her if necessary.

I'm still retired/reserve with the VFD, but in the winter that is a joke because I don't keep a pickup plugged in 24/7 to respond. I don't do structure stuff anyway though I could run a pump, but I am not going to leave her here alone without a vehicle that's ready to start and drive somewhere.

Same thing with keeping things gassed up. We always fill up the tank when we are in town even if it only needs a few gallons.

[1976gmc20]

Just for reference:

https://67-72chevytrucks.com/vboard/...d.php?t=804196

Not complaining, but no one ever said anything bad about this truck.