6L80 Put Itself in Park at 70MPH

[dromey]

Since adding TU/TD to my swap, I have had a transmission gremlin show up intermittently and nearly blow my 6L80 to shreds.

I followed this thread to add TU/TD to my 6L80 through pin 7 of the transmission plug: https://forum.hptuners.com/showthrea...p-shift-no-BCM

Everything was working perfectly until I went to drive on the freeway. All of a sudden, about 15 minutes up the highway on an incline, I lose drive and the RPMs drop. Next, the RPMs shoot up as the trans started selecting whatever gear it felt it wanted to.

I pull over and check the log, and even though the selector was in D6, 'Transmission current gear' went into Park, then Neutral, then 3, then 1 (between 11:51:30 and 11:51:33 of the attached log). Immediately after the hiccup, 'Trans Shift Mode' started reading 'TUTD WOT' and PRNDL Status was 'Invalid' despite the selector remaining in D6 the entire time this was happening. No matter which detent was selected, 'Invalid' remained.

EDIT: I also checked the TUTD WOT tables, which may be causing this problem. In the speed table, I currently have all upshifts set for 300MPH and all downshifts set for 0MPH. I did this to prevent TUTD mode from shifting up or down at all depending on vehicle speed, which I don't want. Can anyone confirm my reasoning here? I may have it backwards. Still, I'm not sure that this would be the cause of the transmission randomly showing 'Park' for a second while in D6, especially when TUTD is only active in the D5 range AND requires a request to activate.

I was able to get home by going into the tune and changing TUTD to 'Disabled'. It hasn't acted up since, but now I'm hesitant to reenable it for further diagnosis in fear of the transmission putting itself in Park while going 70mph again.

Anyone have any ideas as to where I should look first?

6L80 strange issue.hpl
trans tune step 2 tutd v2.hpt

[dromey]

Bumping this; could any TUTD experts peek at the Shift Speed TUTD WOT table?

[ns158sl]

I am no expert, but the trans OS might not be capable for TUTD. Might need to ask around/search how to determine what OS could work with your ECM/TCM.

[dromey]

Thanks for the response. I poured over any forum posts I could find relating to OS problems as they relate to TUTD, and I’m decently confident that it’s not OS-related (though it could be and it’s just not documented anywhere). Most folks with unsupported OS versions seem to have an issue where TUTD will not engage at all; but in my case, it works perfectly fine when D5 is selected.

Im going to try and set the TUTD WOT values back to the stock values and go on a bunch of test drives around town to try and get it to act back up.

Are there any additional channels I should load into VCM scanner to try and get more info? I wish there was a “commanded gear” channel that I could compare to “current gear” or something like that...

[dromey]

Well, I don't think that it's the TUTD settings that are causing me grief... my issue is that the TCM loses communication intermittently.

In the log attached to this post, it happens between 56 and 62 seconds. Note the speed spike up to 158mph and the trans temp drop to 32 degrees.

My first thought was that something is loose or melted. It would make sense since the engine temp was 216F and the trans temp was 180F (going up a winding steep canyon road). I took apart the harness from the E38 to the transmission plug and didn't find anything melted. I reassembled everything, ensured the connections were tight, and added some heat shielding around the loom where the exhaust passes the transmission just for good measure (exhaust is also fully wrapped).

I thought that I had fixed it, but it happened again on my drive home when engine temp was 200F and trans temp was 135F. I don't have a log of this most recent occurrence, though.

Any ideas? These intermittent lapses in comms make it completely undrivable above 35mph, since I live in constant fear of the 6L80 throwing itself into 1st at high speed.

6L80 communication hiccup.hpl

[relicdew]

Put your transmission tables for tap shift back to stock. I've seen truck tunes where folks try enabling tap shift put the vehicle back to 1st gear on a down shift request. Your transmission Range is setup to D5 and tap (one gear below drive) is actually D4. No clue what the heck D5 even is. Ignore tap shift for the time being since it's showing invalid data and TUTD WOT in the tune. I'm assuming none of this happened until you started messing with tap shift? I've always used a Corvette tune in the APPROPRIATE ECM and reprogram the TCM for swaps. You can TRY using a 2010 Camaro tune in the transmission for tap shift to function. You have to use the CORRECT 2010 Camaro TCM file if you wanted to try a hybrid file since some of the 2010 Camaro's did use a 2009 TCM. Your 2007 TCM should take a 2009 TCM reprogram just fine. I don't experiment a lot with hybrid tunes and normally make sure the ECM and TCM match to avoid potential drivability issues.

[dromey]

Thanks for the response. The truck was driving with the tune posted above; TUTD was disabled at the time of driving. On my 6L80, D6 is normal drive and D5 is the position below it. I have heard that different years of 6L80 will display differently for some folks in HPTuners, though.

You are correct that this did begin happening when I first added and enabled TUTD in the truck, but I also changed how the TCM was powered at the same time the truck was down. I used to have it where the power wire to the trans was directly to switched (and fused) power. The E38 now grounds a relay that provides power to the 6L80 because I was having issues with intermittent transmission limp mode.

I think that the next step in the process is to put everything TUTD-related back to stock and drive with an oscilloscope hooked up to make sure that the relay doesn?t momentarily freak out or that the E38 intermittently doesn?t ground the relay.

[dromey]

Figured it out! It wasn't a software issue. I don't mind humiliating myself online, so here's what happened: TL,DR; The starter power wire welded itself to the crankshaft pulley.

I had the oscilloscope hooked up as I was driving up a canyon road. One lead to battery positive, the other lead to the pin on the relay that the ECU is supposed to ground. With the engine hot and on 10% of right-hand turns, the voltage would shoot around between 13 and 8 volts. I thought to myself: "Finally! It must be a loose pin at the ECU!". However, one thought lingered: I should see the voltage go all the way to 0 and not 8 volts... right?

While cruising on the drive home, it acted up again, but this time the whole truck shut off completely with no power to anything whatsoever. I excitedly pop the hood to diagnose the issue. The negative wire was scalding hot, so I knew something major had failed. It only took one quick peek under the truck to see burned copper and melted insulation on the starter wire. I turns out that when the engine is hot, the insulation around the starter wire became more flexible, and each right turn I made would cause the starter wire to come into contact with the crank pulley.

One quick phone call to the wife brought her out on yet another recovery mission (this happens a lot, but she knew I was a car guy who loves to DIY before she married me) with the necessary tools to remove the starter wire, which had firmly melted itself to the crankshaft pulley. An unorthodox use of some jumper cables got the truck started and back to the garage. Problem solved! Even though I didn't need it, this is exactly why I carry a fire extinguisher with me on any of my own builds.

[ns158sl]

Now that is awesome.. and Congrats on the finding the culprit!