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wow. this one is a mess.
so so many things this can be, and it all doesn't really sound related to me.
you really need a tech 2 for these trucks. the big thing is the freeze frame data. when they set a code, they capture a snap shot of everything happening at that time. if you have a scanner that can pull up that data, that will got a long way. most high end scanners can do it. the clone tech 2 being the cheapest at about 300$. and then a monthly subscription to repair software, like alldata or prodamand will help digging thru how all the systems work together and tests to run for each code.
sadly you're going to become a troubleshooting mechanic to keep it. shops are all terrible with these trucks, well any real electrical troubleshooting at all, they mostly just empty your wallet and leave you with a still broken truck.
just off the top of my head, my guess is the stalling 2 years ago was probably a hybrid cell voltage over volt. a freeze frame would have shown forsure, but as the hybrid battery gets weak, the cell voltage is less stable and swings higher and lower. if during regen any cells go above 20.8v I believe it is, it opens the main contactor in the hybrid battery. this basically kills the entire truck. except some 12v accessories. that's my guess. hybrid battery really only last 100k or so in these. by 130k they are acting funky. bucking jerking and all kinds of stuff. the number of guys that got sold a new tranny when it was a weak hybrid battery is just depressing.
the new stuff, few different things come to mind.. the can-bus crashing from that rear system module getting corroded out is common in all up north trucks, ours just rely on the network more than normal trucks. youtube channel "south main auto" has a good video on trouble shooting and bypassing that module if you can find it.
next is wiring harnesses rubbing on things, there's a few different gm technical service bulletins about different spots to check. but down by the tranny there's a big plug going into the tranny that can rub on a heat shield next to it and short out wires. the other, and I think it was fixed by your year truck. but easy to check. there's a thick harness that is attached to a bracket about where the normal truck's alternator would be. the bracket is metal and sometimes the harness can wear on that bracket and short out stuff.
watch out for these trucks weird cranking. they don't have a starter. they use the tranny front motor to spin the engine. they will spin it over so fast a lot of people think the engine is started and then stalls, but it's never started. Just being spin over at like 1000rpm for a while till it times out.
limp home mode you saw is a weird one, I've never seen anyone post about that or experience that. my guess is that was just the normal v8 engine limp mode, usually something like a throttle body or throttle pedal not agreeing with each other will throw gm drive by wire vehicles into limp mode. so that again sounds like harness issues, not really hybrid specific issue.
for now if it's running and driving, you pretty much need a good high level scanner to work with thru issues. if it's not running that south main auto youtube has some great videos about checking the can-bus network. you kinda need that up and running so your scanner can talk to everything or you're just blind guessing.
oh the tapping under the truck could be lots from things. even a torn motor mount. all these Gen trucks tear motor mounts all the time. there's a huge sticky thread about it. mine had a weird sound at idle in neutral that went away in drive or under load. kinda sounded like timing chain slap. turned out it was just a bad motor mount setting metal to metal.
good luck with it. it's a bit of a deep rabbit hole. one guy on another board had just remote started his truck to warm up after a ski trip. ran for a bit and then shut off. took him 6 months, tons of ideas from everyone. he even ended up buying a 2nd truck he found cheap that needed a hybrid battery and rebuilt that himself and then bam, he finally found it. Just before he bought it, a reman engine was put in it over the afm lifters failing. well turns out the shop didn't know you can't use standard pag ac oil in a electric compressor ac. pag gets conductive over time and the hybrid module saw voltage where it shouldn't have and opened the main contactors for safely.
I only tell you that cause that's the kinda persistence you need sometimes. no shop mechanic is going to find that for you. that's an extreme case, and a eazy fix once found. flush the ac system and refill. but yeah, these have a lot of systems and safetys all running thru the standard horrible gm wiring. battery cables tend to rot from the inside out and catch everyone out.
I listed all the codes that it stored.this has been posted around a few times. but it might help.
if you get any good codes from it. this will give you can idea about what was needed to set those codes.
Thanks @j91z28d1
I am a little confused by your response. From this one event the other day where the truck was driving along at 50mph and suddenly out of nowhere either engaged the electric drive motor or applied the brakes on its own, you think it could be any one of those things that you mentioned? I replaced the Hybrid battery just about 2 years ago. It also has a new 12v battery and the connections are clean and solid. I love this truck. So quiet, so comfortable. Sure I miss the towing capacity of my 8.1 Suburban but aside from that this Yukon has been such a pleasure to own. I'd liked to repair this issue and keep it on the road.
My gut tells me it may be the electric drive motor that suddenly engaged mainly because of the noise it now makes while just idling in the driveway. I can't think of anything that would cause this sound related to it being a communication error due to voltage flux or absence etc...
I think there is a chance that the sudden braking or violent jerking episode could be related to the active suspension module shorting out and it shutting the system down but I don't think that would cause the rattling noise at idle after like it has now. I already sourced a genuine GM part for the active suspension module and will have that installed but I could really use some direction regarding what else this might be. As of today the truck is angry and does not want to run. It will start and then turn off 2-3 seconds later. Giving an Engine Damage warning Video of sound now
I will post the codes and scanned datawhat I was meaning is I think it shutting off on you a year or so ago could be something different than happened this time. I've never seen an engine damage warning. is that on the dash display? makes me wonder what triggers that warning and how the tranny would make it think the engine is damaged.
man that does sound pretty bad. I might have missed it but how many miles? these transmission seem pretty common to go 250k or more miles.
These trannys are bit of a mystery and no one really rebuilds them. I talked to a retired gm tech that said he went to class for these when they came out. they spent 2 days on the transmission and then said but don't worry about any of that, you won't be rebuilding these. if they are bad gm engineers want them back and will send new ones in.
A used unit seems to be the only real way to get one these days. I've seen some on Facebook market for as low as 600$. they do have 2 electric motors in them, but they run kinda all the time doing different things. I honestly do not know if one could have a issue and lock up the tires like that. the transmission control module is inside the tranny and the shift linked is a cable. so I don't know if a can bus failure could cause this or not, but I'd hazard a guess that it shouldn't be able to. I guess it could have also engaged a lower gear at the wrong time too, like say downshift to 1st while doing 50. but I just don't know enough to say.
These don't have a torque converter like a normal tranny but there is a flex plate type of thing where a fly wheel would go that transfer power from crankshaft to transmission input shaft. maybe it broke that?
if you pull the dipstick and put it on a clean towel, do you see any metal? you could pull the pan and look for metal on the magnet inside too. Just be careful 14qt of fluid all come running out. makes a mess if you're not ready for the amount.
so for the codes, I guess you tried clearing them? did they come back? if so which ones? are you able to communicate with the modules with your scanner right now? what scanner do you have? does it show any live data from the transmission?
sorry, all I have is questions, no real answers. for the short time it's running, have you tried putting it in gear and see if it moves?
I fully understand wanting to keep it. I like mine a lot and since I'm basically a mechanic by trade. I hope I can fix whatever to keep mine going too. and I think I can fix most things, as I've read tons of posts and threads about issues to try and understand what all can go wrong. mine has been pretty solid thankfully, just the normal stuff for this generation and I'm Bata testing a different hybrid battery type right now that seems to be going well so far.
the transmission being the one thing I worry about that I can't fix. I've kept my eye out for a used one local, just to pick up as a spare if the day comes. I have read of some wearing out and slipping, but never once have I seen anyone post anything about one exploding internal like yours might have done.
