Horrible Chevy Tahoe shifting problems-UPDATE FIXED!


#1

All right guys I’m reaching out to the experts here for help with my transmission problems. Luckily, I have some inclination as to what might be causing it but I’m not positive so I really need some help. The issue is that my 2012 4wd Tahoe is having really weird shifting points, especially in the low gears and it jerks a lot when taking off. Worst of all, when I’m braking and coming to a stop, the car sometimes bucks and lunges forward as if it were a manual and I hadn’t pressed down the clutch. I’ve heard of other people having these problems with other Chevy modes including Impala, etc. and they said that they found the problem was with the transmission pressure control solenoid (pcs) or even the body control module which I guess sends the information to that solenoid. Here’s what I found out: First of all there are no error codes or check engine light coming on my car which kind of troubles me because I’ve already taken it to the Chevy dealer and had several things fixed so far to try and fix this problem. They replaced the fuel pump and 02 sensors but neither has corrected the problem. So last night I went ahead and removed the body control module fuse. Doing so COMPLETELY has fixed the transmission issues and it now shifts perfectly and doesn’t buck when I’m coming to a stop! However, obviously I can’t ride around like this permanently because my turn signals and other stuff doesn’t work when this fuse is out, plus when the fuse is out if gives me the message to service my 4 wheel drive on the dash. This leads me to my question of do you think the problem is the Transmission pressure control solenoid or the BCM? It leads me to think it might be the module because how can the transmission sensor works so perfectly when the BCM essentially isn’t able to send any information to the transmission? I’m not exactly sure how these two communicate and what information they send, but how can the transmission now works so perfectly when I remove the BCM if the pcs is the actual problem? Do you guys think it’s the pcs or a bad BCM thanks so much. I really need your help before I go to the dealership again and get taken for a ride to get something else fixed that’s not going to correct the problem!


#2

The problem might very well be with the Body Control Module.

The BCM is the gatekeeper for the Controller Area Network that allows computers and modules to communicate with each other.

http://www.nxp.com/files/abstract/overview_applications/CANDESIGNCHLG.html

So in order for the transmission to shift properly, it needs know such things as what position the transmission range switch is in, the RPM’s of the engine, if the brake/gas pedal is being applied, etc…

If the BCM garbles the signals between these computers/modules it can cause erratic function of a component.

The dealer should be able to monitor the CAN operation using a factory scan tool to see if the BCM is causing the problem.

And if it’s found the BCM is causing the problem, it’s covered by the 8 year/80,000 mile emission control warranty.

Tester


#3

Thanks. Question: can my current bcm be reprogrammed? I ask bc the dealer may say it’s not really malfunctioning since all the electronics is working and it’s not throwing any codes. So if they don’t want to necessarily replace it, can a bcm be reprogrammed and possibly cure my problem?


#4

The only thing I can tell you is GM has two labor claim codes.

N9999= Electrical malfunction requiring replacement of component under warranty.

J9999= Power train control anomaly where a computer/module requires reprogramming.

That’s all I can find for your vehicle.

Tester


#5

I think there is an electronic connection to the transmission, my guess you live in a climate that uses salt on the roads in winter, and hope a cleaning of the connections to the tranny solves the problem.


#6

It looks like you did the troubleshooting yourself; you are to be commended. I’m not pleased with your dealer’s “service” department. It’s situations like this that give dealer “service” departments a bad reputation. And it’s another bad mark against Chevys.


#7

@Barkydog I definitely live in a salty climate, Miami Florida. So I think you might be onto something. I took it to the dealer and had the mechanic drive with me this morning and while he did say that the shifting points felt within spec to him he said he’d be happy to diagnose the body control module and everything and see if it spits out any codes. He also said the BCM doesn’t really communicate w the transmission at all, that’s more of a powetrain control module thing. Is there any specific electric connection I should have him look at? The guy seemed pretty reasonable and wanting to help


#8

Try googleing the trans connector harness for your vehicle. If you have an auto position on your 4wd use that only. Gotten me through snow, towing boats out on a sand launch etc. 4w hi or lo are gilding on the lily.99% of the time 2wd wlll be fine imho,


#9

Yea I’ve tried that. Still does the same in auto. It’s something electrical related to how the BCM modifies or effects the transmissions actions. I can say w 100% certainty that removing the BCM fuse eliminates the prob. I think I’m going t have to resort to getting a tune by black bear performance. I’ve read several other with 6 spd Chevy and gm transmissions and no one has found any satisfaction with the dealer or anyone except for retuning the car to shift diff. Sad. It’s as if the trans was designed to tow a heavy load. The car acts like a tug boat or dump truck w it’s shifting. I’ve towed a boat behind it before and the Stock shift pattern(not even in trailer mode) works AMAZING for pulling and controlling a heavy load. But when it’s w the car by itself it acts ridiculous.


#10

WOW WOW WOW, before I get into this, I can’t say how much my world has changed in the past 45 minutes. I wanted to love this car so much, but its shifting and jerkiness had made it impossible. Well this has resolved and the car is shifting like a new car!!! I’m so freaking happy I can’;t even describe it. So an hour ago I read an article online about how to reset ur transmission control modules of ur car is shifting at the wrong points, jerking, or just not accelerating properly but is NOT currently spitting out any codes(read: dealer tells u nothing is wrong even though u can tell it is). I saw this and I was like hmm, this is interesting, I should read on. Quite simply, it said go into your fuse box and you’ll see 2 transmission control module fuses, one regular and one labled TCM Battery, that battery one is keeping a constant 12v to hold the memory in the TCM of the shift program that its currently running off of which MAY somehow, be suddenly than the original story setting, don’t ask me how. Well, I followed the directions and took that fuse out (and of course put back in the BCM fuse that i’ve been fiddling with [Ive had a feeling this shifting problem was an electrical-communication issue between the cars computer and the transmission, not something wrong within the the transmission necessarily]) so after I took out the TCM battery fuse, I turned the key to accessory, NOT on, it shot out trans error codes bc there was no tcm battery fuse (a 12v constant to save its shifting program)hence is deleted its current shift program and RESTORED back to factory, let it sit 5 min to make sure, turned key off, and then put the fuse back in and and HOLY SXXX@!!! No codes and it shifts at all the right places just like a new car! In fact, I would even go far as to say it clicks into gears!!! I NEVER thought i would use that word to describe the shifting of this car!!! The bucking at low speeds is gone, the jerk in acceleration is gone!!! I just pray it stays like this, but I can say 100% that the shifting is completely perfect since I’ve done this. Again, I just hope this doesn’t wear off. I drove around town for the past 45 min, excited to stop at stoplights without the embarrassment of the car bucking and jerking!! Moral of the story is if you’re having a transmission issue and your car is not spitting out any codes so therefore nothing is likely mechanically wrong, before you try anything else, take out this fuse and put it back in. Ok that’s it for now, keep u guys updated


#11

Great!


#12

Interesting that the nice tech said your “terrible, horrible, jerking” shifting was Normal or in spec. Do all new Chevy trucks shift terrible? And tech knows this? U said it went from awful to sweet. Well, your perception is one thing. U seemed to be real happy so I assume something did change? Maybe tech really has no option or fix. U pulled fuse. Why not just disconnect battery? Pulling the Tcm fuse disconnects it from vehicle power circuit. No battery means vehicle and tcm has no power. Same result.


#13

This really sounds like a “Windows 98” computer solution. Get the Blue Screen of Death Lockup…unplug the power to the PC and reboot! Sounds like the learning routine in the TCM got “lost” and initiating a restore to the starting point solves it.


#14

The idea that it simply went off the tracks and got lost is far more concerning to me than if one of the inputs it uses to make decisions is off whack and caused it to react that way. It will be interesting to see if it returns or not…


#15

This is a somewhat common issue actually… Wish I saw this earlier. Many times you need to unplug and re-seat the trans or engine ECU connector…and or remove power completely. It forces hard resets on any and every computer in the vehicle if you remove power completely for ten minutes or so.

The files used in the Trans or any other ECU may have gotten corrupted by the vehicle sitting a long time…and the battery running down to unacceptable levels…Sort of like a “Brown Out” to the cars computers. Not to mention what could happen from repeated Jump starts. There’s a lot of ways the ecu’s can get a little “wonky” and need a hard reset. With cars these days…it never hurts to do a hard reset.
Glad you are up and running. A problem like that can really make you nervous.

Blackbird


#16

@cavell I can’t believe that either. It may not have been “atrocious” to the point where it didn’t drive but it certainly wasn’t normal. The brakings were very abrupt and the starts jerky, I can say that for sure, so it is disconcerting that he was ok with that and didn’t simply recommend the fix. Also as part of the fix I think pulling the fuse and then turning on to acc is important as it lets the rest of the car realize the tcm has had memory erased, letting it spit out tcm codes, and then forces tcm to start and refresh new. I don’t know if just disconnecting batt and then reconnecting would have done the same. It may but I’m not sure.
@Mustangman great analogy!! That’s literally what the article I read compared it to, a pc restart.
@TwinTurbo I feel you. I’m going to keep a close eye on it and hopefully it will be ok from now on.
@“Honda Blackbird” that’s great to hear how it’s possible that it manifested short of the car being crap and more likely from battery dying in the past, etc. I’m just so happy it’s working now


#17
Many times you need to unplug and re-seat the trans or engine ECU connector...and or remove power completely. It forces hard resets on any and every computer in the vehicle if you remove power completely for ten minutes or so.

On a desktop PC, this is only an inconvenience. In a vehicle, this is completely unacceptable from a hazard and risk assessment perspective IMO. And the solution is simple and not costly making it even more egregious if allowed to occur.

This is why I said I felt it was most concerning if the computer just “lost it’s mind” versus reacting to bad input information. Because if they couldn’t be bothered to implement brown out protection on the memory, did they even bother to institute limits on the control outputs? Data being corrupted is one issue…corrupt data that results in very dangerous operation is another level up in the culpability of the designers…

Safety and mission critical design is one of my fortes. Too many times I have run across existing designs that had intermittent faults due to various oversights by the original designers. One of the most unforgivable is the non-use of built in power supply protection mechanisms in most modern day microcontrollers/microprocessors. Back in the day, you had to design your own. Then along came accessory chips to perform this function and nowadays, this is ubiquitous in the native hardware. All you have to do is hook it up.

I have not run into this in my limited experience of my own cars. If this is truly a widespread issue, then shame on them. It is either sheer laziness or ignorance, neither of which belongs in a critical application…


#18

Good fix! I like the fuse only solution, too many people I have read on trailvoy forum actually have issues after a battery disconnect, usually ac and blend door related. I am not sure if a memory saver, which I use for every battery disconnect would save the trans memory, but use one religiously. If a shop is doing something that the battery will get disconnected I set all the climate controls to off.


#19

Maybe I shouldn’t have said the plug disconnect is common it sort of is for me because I get vehicles that 2 dealerships and 3 independents all say they cannot figure out and its unfixable. So I think my repair approach of leaving no stone unturned forces me to play with plugs on ECU’s and such. This is after I go through normal troubleshooting myself to the best of my ability and computer interfaces will allow. I should have said its common for me not the “industry”

Because before I replace an ECU on a vehicle I do play with every connection prior. I also lack the 20K and up computer diagnostics available at the dealerships that could or should be able to see file corruption on the chips of these ECU’s… There should be Checksums run as the normal part of reboot on these units in the vehicle.

So I suppose I check everything a little sooner than most people would…if ever because I am told that nobody can repair this vehicle before I ever get it. Maybe I shouldn’t take anyone elses word on repairs…its funny sometimes I do…and sometimes I totally ignore them. I cant point out the deciding factor…I think it just comes to me in the conversation about what has been done or looked at…and if it makes no sense to me I think…they just dont know whats going on…but if I respect the opinion of the people in the “Problem Description” I proceed more cautiously. The result however is always the same…I figure it out and give it back to the owners.

What protections DO vehicles have for when the battery does a slow drain past 12 vdc? If the files are hard on the chip and the data that is pulled is sort of at risk. The worse that should happen is that you lose the last bit of “living data” not what is hard programmed on an EEPROM…Right? I really dont know what kind of protections are on current vehicle ECU"s at all if any. There must be some kind of protection…because they seem to be pretty resilient… Overcharging from a bat Alternator…Some guy who hooks up jumper cables backward…a battery draining down to zero volts slowly over time… These units have to put up with A LOT when in a vehicle… I wonder how they deal with this?

Blackbird


#20

Hobda: very good point, we need someone expert in auto computers to weigh in. When the voltage drops below the specified minimum, just about anything can happen.

Ideally a detector would sense the lowering voltage and shut everything down soon enough to prevent strange things occurring, but does this protection actually there ??

I do remember from my IC testing days that auto ICs are tested for a high voltage surge of 40 volts or so, caused by the battery not connected while the alternator is delivering current.