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.

 

[ilikemikes]

Money says you are running a 3.42 gear ratio. What size tires are you running? I'm thinking smaller tires or a larger ratio, like 3.73.

I am looking into gears now...
Ran my VIN on this and its says 3.08 min to 3.42 max. Mech and powertrain specs say 3.08.

 

[S33k3r]

3.73 gears might be the way to go then. And/or have your transmission tuned. Are you using tow/haul mode?

 

[ilikemikes]

3.73 gears might be the way to go then. And/or have your transmission tuned. Are you using tow/haul mode?

As I mentioned before, I think I'm going to have to rebuild my transmission. Pretty good bump switching into Drive. Smoother after it warms up. Had to buy tires recently and went 1 size down to 32" from 33". Yeah I just started using tow/haul mode on hills when towing the boat. The trans shop does differential/transfer case work as well so might get some gears. As a part of the trans rebuild they will tune to tire size and towing so might hold off the gears. I was thinking gears because of another post of someone adding mods to get 500 hp and that sounds fun!!

 

[S33k3r]

Higher (numerically) gears will multiple the torque you are producing so you get more work out of it. However, at highway speeds, this works against you by reducing your gas mileage; the engine has to rev higher at those speeds.

In a nasally, head up my a** kind of way: 500hp doesn't really matter. It is your foot-pounds of torque that move you down the road.

 

[Pacfanweb]

Hey Nick,
16 Tahoe 6L80. Was rebuilt by a GM tech 2 years ago. Had an issue not going back to 1st when you come to a stop. Intermittent. Also didn't always engage in reverse, also intermittent, but reasonably often. Also, after it was rebuilt, it always shifted funny...almost like a stick from 1-2, it stop accelerating almost like you were shifting a stick.

I'm a dealership vendor, water leaks/sunroof, etc. Had a former GM tech that now works at a Ford dealer look at it. Changed fluid/filter. There was a bit of glitter on the magnet, not awful. He removed and cleaned out the TCM and reset it.
Worked perfectly for a good long while. Didn't do that shifting glitch anymore, and still doesn't. But now doing the not downshifting into 1st at stops thing and slow to engage in reverse. Intermittent as before. Works fine otherwise.

It was rebuilt with what I assume was stock replacement stuff from Transtar. I replaced the same trans in my 15 truck 6 months ago with one from a buddy's tranny shop. It is beefed internals, billet converter, so I guess all improved stuff that normally fails.
Thinking I may have to do the same with the Tahoe, or let it go be someone else's, lol. But we like it.

Would you concur that it's on the road to ruin once again?

 

[Tahoe14]

@NickTransmissions

 

[NickTransmissions]

Hey Nick,
16 Tahoe 6L80. Was rebuilt by a GM tech 2 years ago. Had an issue not going back to 1st when you come to a stop. Intermittent. Also didn't always engage in reverse, also intermittent, but reasonably often. Also, after it was rebuilt, it always shifted funny...almost like a stick from 1-2, it stop accelerating almost like you were shifting a stick.

I'm a dealership vendor, water leaks/sunroof, etc. Had a former GM tech that now works at a Ford dealer look at it. Changed fluid/filter. There was a bit of glitter on the magnet, not awful. He removed and cleaned out the TCM and reset it.
Worked perfectly for a good long while. Didn't do that shifting glitch anymore, and still doesn't. But now doing the not downshifting into 1st at stops thing and slow to engage in reverse. Intermittent as before. Works fine otherwise.

It was rebuilt with what I assume was stock replacement stuff from Transtar. I replaced the same trans in my 15 truck 6 months ago with one from a buddy's tranny shop. It is beefed internals, billet converter, so I guess all improved stuff that normally fails.
Thinking I may have to do the same with the Tahoe, or let it go be someone else's, lol. But we like it.

Would you concur that it's on the road to ruin once again?

These kinds of symptoms have causes that can be one or more of the following:

- TEHCM
- Valve Body
- Torque Converter
- Internal Case Components

If it were me, the first things I'd do is drop the control body assembly, separate and test the TEHCM (or test the TEHCM for mechanical function using the Bosch Test Block while the vehicle was still present) then pull apart, clean and reassemble the valve body...Once each half has been reassembled, I'd test them both on the Sonnax vacuum test machine to see if there's wear/weakness in any of the valve trains. What I find when evaluating those two things would dictate my next moves.

 

[YukCCR]

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!

Got it back from the dealer today after having the manual/trailer switch assembly replaced. Will pull the trailer around tomorrow to verify that it's working correctly. If so, it's boat shopping time!

 

[89Suburban]

Got it back from the dealer today after having the manual/trailer switch assembly replaced. Will pull the trailer around tomorrow to verify that it's working correctly. If so, it's boat shopping time!

What kind of boat?

 

[CTX-SLPR]

One thing I've noticed is the wide variety of prices on converters, even one claiming "billet covers"
- The big names like Yank, Circle-D, etc. seem to want about $1300 for a billet converter from their websites and they are all higher stall than stock
- There are some more obvious names playing in the high $100's like Monster and ProTorque that seem to hold to stock stall if you want
- Then there's this section down around $500 like the reman on RockAuto or a RECON from Transendence

Not all of these list specs to look for the lockup clutch thicknesses as Nick suggested. How does one go about shopping for something that's a definite step up from the stock JMBX in terms of durability without going all in on increased power handling like a 2600rpm stall Circle-D?