GM 6L80 Transmission Information Thread

[NickTransmissions]

To clarify the history - the shifting problems began around the first of this year, and the manual shift mode stopped working at the same time. The two shops I took it to prior to the dealer both said it need a transmission rebuild or replace. They did not say that the manual shift mode, which I noted in my complaints, was a separate issue that would not be solved by the transmission replacement. I went with the dealer trans because of the warranty.

began

It can be a problem with the shifter itself not communicating to the PCM/TEHCM once you place it into manual mode...The only way to know for sure is to put a scan tool with live data capabilities...Plug the scan tool in, bring up 'Transmission' data then fire the engine up...Note the range being displayed on the scanner (it should say 'Park' or 'P' if you're in park)...Then, with foot on the break, shift it into M1 - see if the scan tool shows 'M1' or 'Manual' displayed...If not, it is likely a shifter problem, or at least the Internal Mode Switch isn't seeing the change or the IMS itself is bad (rare, not likely) or the TEHCM is no good (i.e. they sent a DOA TEHCM - not likely either).

If M1 is displayed, then you'll need to determine if the TEHCM is bad (i.e. shift solenoids stuck off, CPC solenoids not functioning as they should, etc...Here's a diagram showing all of the TEHCM's variable force solenoids and their function patterns based on each gear:


Let us know once you have had a chance to diagnose with the scan tool.

 

[YukCCR]

I guess it's time to get a scan tool. I have Dodge, Chrysler and Acura vehicles as well. What's a good one for a beginner that will work with these different makes?

 

[NickTransmissions]

I guess it's time to get a scan tool. I have Dodge, Chrysler and Acura vehicles as well. What's a good one for a beginner that will work with these different makes?

I'm familiar with Snap-On but Autel, Bosch and SPX are also popular. There's various levels of sophistication but bi-directional communication, code reading (including ABS, Chassis codes) and adaptation resets is usually sufficient for most DYIers. Some have wifi connectivity - those are nice to have when trying to research problems during diagnosis. A few have integrated lab scopes but you can get by with a good DVOM and power probe in most cases.

 

[YukCCR]

I'm familiar with Snap-On but Autel, Bosch and SPX are also popular. There's various levels of sophistication but bi-directional communication, code reading (including ABS, Chassis codes) and adaptation resets is usually sufficient for most DYIers. Some have wifi connectivity - those are nice to have when trying to research problems during diagnosis. A few have integrated lab scopes but you can get by with a good DVOM and power probe in most cases.

O'Reilly scan says "p0700 TCM Requested MIL Illlumination". Also just drove it in trailer mode, more than 5 times it would not shift up getting to almost 5k RPM at 55mph after slowing for 30mph corners. Have to lift to get it to shift. Didn't do it when going from a stop. VERY frustrating!

 

[NickTransmissions]

O'Reilly scan says "p0700 TCM Requested MIL Illlumination". Also just drove it in trailer mode, more than 5 times it would not shift up getting to almost 5k RPM at 55mph after slowing for 30mph corners. Have to lift to get it to shift. Didn't do it when going from a stop. VERY frustrating!

Yea, unfortunately you won't get anywhere near the full picture of what's happening and why with just a code grab. P0700 is a generic TCM code that's informing something is wrong...You need a scan tool with live data and two-way comms capability to properly trouble shoot...I'd start watching YRT vids on the various scan tools available, look up their pros/cons and purchase one that meets your needs, keeping in mind you have vehicles from a few different manufactures.

I will say that having to let off to shift indicates a line pressure or applied element problem in most cases...So I'd also purchase a line pressure gauge (Here's one example) and do a test to confirm your pressures at idle and when accelerating are appropriate for the amount of acceleration taking place/throttle angles...Ideally you'll want your scanner plugged in at the same time to view both tools' readouts.

 

[azk19]

Hello everyone, I have a question and would appreciate your input.
I own a 2015 Tahoe PPV with 145k miles. For the past few months, I've noticed that it shifts hard between 1st and 2nd gears, and between 2nd and 3rd gears, especially when accelerating aggressively.
I dropped the pan and replaced the fluid and filter; there was no metal on the magnet, and the fluid wasn't too bad.
Unfortunately, this didn't resolve the hard shifting issue. I've also started noticing occasional vibrations around 28-30 and 46-50 mph, but not consistently.
There are no codes, RPM fluctuations, or noticeable slippage.
Could this be related to the torque converter?
What are your thoughts? Do these symptoms point to a bad torque converter, or could it be something else?

 

[NickTransmissions]

Hello everyone, I have a question and would appreciate your input.
I own a 2015 Tahoe PPV with 145k miles. For the past few months, I've noticed that it shifts hard between 1st and 2nd gears, and between 2nd and 3rd gears, especially when accelerating aggressively.
I dropped the pan and replaced the fluid and filter; there was no metal on the magnet, and the fluid wasn't too bad.
Unfortunately, this didn't resolve the hard shifting issue. I've also started noticing occasional vibrations around 28-30 and 46-50 mph, but not consistently.
There are no codes, RPM fluctuations, or noticeable slippage.
Could this be related to the torque converter?
What are your thoughts? Do these symptoms point to a bad torque converter, or could it be something else?

Converter is breaking bad on you.

Plug in a bidirectional scan tool and monitor trans and converter data, particularly shift speeds and tcc duty cycle in combo with tcc slip speed...tcc slip > 10 rpm or so at high duty cycle (40+ mph, especially) indicates slipping clutch or stator...Either way, trans will need to come out and at 145k, I'd overhaul it per my build strategy as posted on page 1. Install an upgraded converter and you should be good to go...

 

[azk19]

Thank you for your detailed response, Nick.
Transmission was replaced at 90k miles, do you think a full overhaul is still necessary, or could I get by with just replacing the torque converter? Unfortunately, I don't have another vehicle, so leaving the car in the shop for an entire week isn't really an option for me at the moment.

 

[NickTransmissions]

Thank you for your detailed response, Nick.
Transmission was replaced at 90k miles, do you think a full overhaul is still necessary, or could I get by with just replacing the torque converter? Unfortunately, I don't have another vehicle, so leaving the car in the shop for an entire week isn't really an option for me at the moment.

You're welcome. If you get it diagnosed and swapped out quickly you might get lucky but it's not something count on or do. Id overhaul it and install an upgraded converter.

Once the converter starts shuddering/slipping, especially on a unit with 90k, it's usually due to wear elsehwere or the prev overhaul included a stock level converter with the same POS stock converter clutch. Either way, it sounds like it's time to revisit.

You can buy a GM reman and have it installed (or install it yourself). It should only take a few days once the trans arrives at the shop or dealer (can be done in one day if you do it). Pay for it in advance, keep driving your truck until the shop/dealer has it (or when it arrives to you) then bring it in (or stab it and go). The GM remans come with a 3 yr/100k mile warranty and dont require any programming or fast adapt relearn.

 

[azk19]

Thank you for getting back to me so quickly. It looks like I'll have to begin exploring it.