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Tuning guru question.....

This is a discussion on Tuning guru question..... within the Computer & Tuning forums, part of the LSx Technical Help Section category; I finished a 2004 6.0 install in my 72 blazer. It's controlled by a 2005 yukon computer. I'm going through ...

  1. #1
    Moderator Firebirdjones's Avatar
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    Tuning guru question.....

    I finished a 2004 6.0 install in my 72 blazer. It's controlled by a 2005 yukon computer. I'm going through and tweaking things. There is one thing really bugging me on the drivability.
    Every time I let off the gas in 4th gear with the converter locked up,,,,it unlocks the converter. I touch the gas and the rpm raises and drops when the converter locks up again. Real annoying when the guy in front of you can't maintain speed.

    I'm used to the converter staying locked all the time until you get below a certain speed or you hit the brakes (brakes aren't an option here).

    The table I found for unlocking the converter vs TPS has "100" accross the board. I'm not clear on the description at the bottom of the table either. What can I change exactly to keep the converter locked on decel (0% TPS) but still unlock when I reach a certain mph???

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    TunedbyFrost.com Tuner Frost's Avatar
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    This can be more than one thing... If you have a 4l80, it's a way of life unless you do some wiring mods and use a 4l60 segment instead. If you are running a 60 trans, the brake switch feedback circuit is missing a state. Which trans is it?

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    Moderator Firebirdjones's Avatar
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    Thanks for the reply Frost. I'm running a 4L60 behind it.

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    TunedbyFrost.com Tuner Frost's Avatar
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    Cool, then this is just a wiring fix.

    Now, I may have the states BACKWARD (been a while) but I have had to track this down on more than one or two swaps over the years.... When the brake pedal is NOT depressed, it either sends +12 or GND back to the PCM. When it IS pressed, the opposite state is sent. This is done so that the PCM can detect switch failure and protect the car (which is what it thinks it's doing now by unlocking the TCC if you are off-throttle). They make the assumption that the brake and throttle aren't used simultaneously, so the PCM allows sustained throttle lockup at cruise.

    If you don't have a multi-pole switch on the brake pedal, you can use a DPDT relay to feed the both states back.

    Tell me if any of this does not make sense.

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    Moderator Firebirdjones's Avatar
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    I understand, makes sense. I had to do this on an LT1/4L60 swap in a jeep years ago. I wasn't sure if it was still necessary on this LS swap or not being a drive by wire and all. The tables in HPtuner look quite a bit different on this 6.0 than they do on older LS1 stuff. So I had to ask.

    My biggest hangup will be pinning the computer to run the brake switch wire to. My harness from Howell didn't come with a brake switch wire in the setup.

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    TunedbyFrost.com Tuner Frost's Avatar
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    For an auto trans, it definitely should have... It's one-wire back to the PCM that toggles +12/gnd though. What do you need, a PCM terminal?

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    Moderator Firebirdjones's Avatar
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    I thought it should have too. All others I've dealt with had it. This setup doesn't have it though. Somehow it's using the parameters in the tune to deal with it.
    There are tables in the DBW throttle settings for the TCC, somehow it's using those to lock and unlock the converter using just the throttle position. I was hoping to make the converter stay locked until a certain designated MPH regardless if I let off the gas. Like say below 40 mph it unlocks? Or unlocks over a certain throttle position percentage? Does that make any sense?
    I just don't understand how the DBW throttle tables work in relation to the TCC, the explanation in HPtuner is vague. I was thinking it's just a simple setting in the tune that I'm not grasping.

    If it comes down to it then yes, a PCM terminal should do the trick, and run that to the brake light switch.

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    TunedbyFrost.com Tuner Frost's Avatar
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    I didn't go into details, but it's absolutely nothing in the tune. It's absolutely the feedback from the brake pedal. Believe me, the first swap that had this missing took me a bit of time to track down. The PCM makes the assumption if the throttle is steady that you are not braking. When you ARE braking, the TCC always instantly unlocks. If the PCM can't tell where the brake pedal is, it assumes you aren't pushing it if you are accelerating so it allows lockup ONLY on throttle. All you need to do is get this circuit in place and it will work as you want it to. You can't use the brake light switch because because it's single state. +12 VDC (or either gnd) and the other state is just open. The PCM needs a state for no-brake and then for any-brake amount.

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    Moderator Firebirdjones's Avatar
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    Okay sounds good. I beleive all I need then is a pin-out location on the computer to make sure I hook it up properly. I guess Howell didn't feel the need to supply that wire, so I'll have to run a new one from the computer. Would you happen to know what that pin out is? Thanks.

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    TunedbyFrost.com Tuner Frost's Avatar
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    Pin 33 on the C1 connector (has the BLUE internal shield). The OEM wire in that spot was purple, if it matters. That's the input that is monitoring the brake pedal. You may tell the harness builder about it and ask for a terminated lead to add the circuit. If they don't, let me know and I can make you up one as well. For 4l60 applications, it's definitely necessary.

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    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by Frost View Post
    Pin 33 on the C1 connector (has the BLUE internal shield). The OEM wire in that spot was purple, if it matters. That's the input that is monitoring the brake pedal. You may tell the harness builder about it and ask for a terminated lead to add the circuit. If they don't, let me know and I can make you up one as well. For 4l60 applications, it's definitely necessary.
    Thanks Steve, I'll call Howell today and see. If not I'll get in touch with you and have you do it if that's not a problem? Thanks.

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