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Thread: 2014 Suburban 1500 6L80 First Gear Coasting/Engine Braking Issue

  1. #1
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    Angry 2014 Suburban 1500 6L80 First Gear Coasting/Engine Braking Issue

    Sorry for the longggg wall of text but I've been dealing with this for a couple years now, searched and searched could never find an answer and just gave up after a lot of failed tuning attempts. Now I've got a bigger trailer which has made my issues even worse so I'm revisiting the issue again to hopefully find a solution or atleast an answer if what I want is even possible. I'm usually pretty good at figuring things out but this has me beat. Other than this the trans works fine and doesn't give me any issues at all.

    I've got a 2014 1500 Suburban with a 6l80 & stock 3.42 axle gears that no matter what I do I can't get first gear to work like I want it to in Tow Haul mode without the gear selector being in M1 (Range Select 1st gear). I tuned tow haul mode to hold gears 1-3 until around 3300rpm & 2500 for going into 4th, then downshift automatically 1mph under the upshift speed for better hold back and power when towing or hold back when empty also. I've tried changing the TCC apply settings and I'm happy with how that worked out overall but I still have this issue. I live in a very rural & hilly area and this thing weighs over 7500lbs without a trailer hooked to it, I spend the majority of my time in first and second when towing and have to heavily ride my brakes in town even empty unless I put it into M1 because of this issue.

    This tune works great except for the 2-1 shift, it will downshift into first according to the logs but then coasts until you give it gas then the rpms will catch up for a short time then start coasting again almost instantly. It's very similar to how Rzr's with a CVT will do going down hill, but much more consistent. The only way to make it do what I want is to put the gear lever into M and select 1st gear then it acts as I want but then I'm constantly having to fidget with the gear lever. I want to get 1st to work in T/H like it does in M1 if possible, the other gears work just fine and act as I want them to. It did this on both the stock tune and every tune I've made since and I can't figure out why, not sure if its some hidden setting or just torque management stopping me but with it working in M1 I've got no idea.

    Any help would be appreciated, I'm really starting to think I just need to throw a set of 4.88 or 5.13 gears at this thing so it's out of first as soon as possible and I don't have to deal with this anymore lol. A copy of my tune and a screenshot from the logs showing what I'm talking about are included below.
    (UPLOAD) Suburban Tow Tune SHIFT POINT & TCC LOCK CHANGES.hpt First Gear RPM Slip In Tune.jpg

  2. #2
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    I have a feeling that 1 of pcs need to be on to get engine braking in 1st. On my phone so cant see your log. Does the log have the pcs channels included and do you have 2 very clear spots for each case so we can see?

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    Thanks for the response, what I posted above was my tune, I just had a screenshot of 2 points in the log not the whole thing. I'll put it below.

    Not sure what you mean by PCS needs to be on, the navigate function doesn't show a PCS anywhere in VCM Editor, and I'm not sure what that acronym stands for. I only found PCS 1,2,3,4,5 CMD Pressures in VCM scanner but unfortunately I wasn't tracking those at the time. I've added them and I'll do another log either Sunday or Monday when I have a chance to get out.

    The best log I have has 3 times showing it doing what I'm talking about then followed up with a couple seconds of the vehicle in M1 doing what I want right before it ends. The first example happens at around 13 seconds. Ignore trans temp it's wrong for some reason, it was at around 160*F when I was doing this.

    22-05-20 14-34-39.hpl

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    Just having a bit of a read of the service manual. So its the low / reverse clutch and shift solenoid 1 you need on to get engine braking. My manual says that should be on for both TUTD and normal drive mode. At some point in drive mode the shift solenoid and low / reverse clutch release. I can't see any where in the tune to adjust this.

    For the log add all the PCS 1 -5 channels and shift solenoids 1 & 2 if the scanner shows them.

  5. #5
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    Ok I had a chance to get out and get a couple, I have 2 logs for you to look at. 1st has the same stuff tracked as before with the CMD Pressures and something I found called Oncoming Clutch. The 2nd log is literally every single thing I could select to track in VCM Scanner minus GPS added in the order I found it below what I had before.

    Unless its called something different I didn't see anything for any shift solenoids sadly. From what you said above and what I've seen I'm starting to think the only way to truly defeat this is to just add another TUTD shifter in a better location and just leave it in range select anytime I'm driving.

    22-05-23 16-41-11.hpl
    Starts around 5 seconds, 3 times showing only T/H, then into M1 somewhere between 15-17 seconds.

    22-05-23 16-38-27.hpl
    Starts around 20 seconds, 4 times showing only T/H, then into M1 @ around 33 seconds. I noticed when I changed into that it shows PRNDL Status going from D6 to D5.

  6. #6
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    Yeah, it's definitely the low reverse clutch that needs to be on to get the engine braking working. I had a look at some of my logs (6L90) and the behaviour is the same.

    Not sure how you can get around it?

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    Ok thanks for the help, I really appreciate you taking the time. I was worried since I couldn't figure it out by now that it was probably impossible and this seems to confirm it.

    I'll just have to leave it in M and relocate the TUTD shifter. Found a couple ways of doing it just need to decide which route I want to go.

  8. #8
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    Bit of pain. I reached out on the 6L80e forum which I subscribe to and yeah, it's just how the OS works unfortunately.

  9. #9
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    You can change which shifter position the TUTD works in but it sounded a bit silly because if you move the shifter to the original TUTD position it will no longer work. Seemed like a silly idea in my opinion to recommend moving it to you.

  10. #10
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    Ok yeah that settles it then, appreciate you double checking for me.

    I think you may misunderstand what I mean, I was talking about adding a physical lever to control TUTD not changing what shift position it works in, something like this.
    https://www.kilduffmachine.com/uploa..._i5_w2560.jpeg

    I really don't want the PRNDM part of that so I'm still looking and debating my options. Love the overall design though, I hate push buttons and especially paddles. This or something very similar is gonna be the route I go then I'll do whatever necessary to make it work.