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Thread: Still struggling with cruise control

  1. #1
    Tuner in Training
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    Still struggling with cruise control

    Hi Apologies if this isn't the right area.

    I still cannot for the life of me get cruise to work on my vehicle. It's a Landrover with a 2007 LQ9 and P59 PCM in it. It's as if there's a hard fault that's preventing CC from switching on but it isn't registering anything in scanner and I can't see anything in VCM editor that I've not enabled. All of the triggers - CCon/+/- brake/tcc etc are apparently working according to Scanner.

    I don't have access to GM's Tech2 with the the LQ9/ Silverado keys here in the UK to check to see if there's a fault that Scanner can't read.

    Does anyone have any suggestions as I've been struggling with this for a year now.

  2. #2
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    common problem with the gm vehicles if there is a gear change and a decent tire size change. A quick search and you will find a lot of us have run into this problem and no one has solved it. Submit a ticket to them and if there is enough tickets HPT will then look into it and get the part of the ecu/tcm opened that needs to be adjusted.

    I know that there are other tuner OS that have this sorted but not HPT

  3. #3
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    Thank you for the reply. I'm obviously not using the right search terms as I've found *nothing* where anyone has had the same problem as me. Could you provide links please?

    I presume from that then that if I could use some stock gear/tyre parameters then it should work, albeit the "speedo" wouldn't be collaborated. ( I presume there would then be knock on issues if it were to be outside the minimum/maximum speeds allowable for CC operation)

    can anybody then give me some stock figures for a '07 Silverado please?

  4. #4
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    I have found that the majority of the time, Cruise Control issues in a Gen 3 LS conversion vehicle, are related to the brake inputs.

    Cruise Control Switches ...
    - On / Off switch to enable the Cruise Control system (constant 12 volts to TAC Module when "On")
    - Set / Coast switch is a momentary 12 volt input to the TAC Module
    - Resume / Accel switch is a momentary 12 volt input to the TAC Module

    Brake Inputs ...
    - to PCM - a constant 12 volts when the brakes are NOT being applied (interrupted when the brakes ARE being applied)
    - to TAC Module - a constant 12 volts when the brakes ARE being applied (interrupted when the brakes are NOT being applied)

    *** brake inputs use a reverse logic system between the PCM and the TAC Module to confirm the status of the brake pedal position ***

    LED tail lights and / or brakes lights are NOT recommended. LED lights on the brake lamp or tail lamp circuit often inhibit Cruise Control function.

  5. #5
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    Jon,

    Ive checked, double checked and triple checked all of the inputs to the TAC. I'm not running LED brake lights and the PCM/TAC inputs are relay controlled so they switch simultaneously and absolutely -this is confirmed by my diag scanner.

    Unless there's something seriously wrong with my checking methods, mine is one of the few cases where it isn't brake light related.

  6. #6
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    Sounds like you have done a good job with the wiring. There are a couple of more things that you can try. This LS platform does not like "floating " voltage with respect to the brake inputs. The brake input to the TAC Module is the more susceptible of the two brake inputs.

    Try this ... use a relay for the brake input going to the TAC Module.

    The relay must be a 5 pin type, where the circuit between pin 30 and pin 87A is closed (conducts current) when the relay is NOT energized. When the relay IS energized, it the changes the active circuit to being between pins 30 and 87 (pin 87A is now open - no longer connected to pin 30).

    Wire the relay pins as follows:
    85 & 87A - connect to ground
    30 - connect to TAC Module connector C1 pin 6
    87 - connect to a 12 volt source
    86 - connect to brake switch (such that it receives 12 volts only when the brake pedal is depressed)

    This relay setup provides the 12 volt input to the TAC Module when the brake pedal is depressed (just like you have now), but also grounds this input when the brake pedal is not depressed (i.e. eliminates any floating voltage).

    Hopefully this gets your Cruise Control working. If not, you may have to use another relay for the brake input to the PCM - however this relay would need to be wired differently.

  7. #7
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    I'm being a bit stupid here. I thought the whole point of the problem with LED lights was that the SLSV pin had to see a resistance when the brake pedal is not pressed - the point being it's checking resistance through the stoplight bulbs to determine that the circuit is working? If I relay it and pull to ground when the brake isnt pressed, doesnt that defeat this idea?

  8. #8
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    No such thing as a stupid question when trying to sort out an issue ... and I do not hold myself out to be an electronics engineer of any sort. I have however, done a lot of work with Cruise Control systems in standalone Gen 3 and Gen 4 LS conversions.

    Depending on what scanner you use, and what PIDs you monitor, you may observe that one of brake inputs cycles between active and inactive (on or off), while the other brake input does not. This is often the cause of the Cruise Control system being "inhibited".

    The necessary signals seem to be 12 volts versus zero volts (ground), that consistently seem to bring about the desired results with respect to the appropriate Cruise Control related brake inputs. You may only need this arrangement on one of the two brake signals. You may need to experiment with your particular system to sort out what works.

    As long as there are no LED brakes in use, you should be able to get it sorted using a separate relay for each brake input. I had one vehicle that would inhibit Cruise Control if the running lights were turned on, as it used LED lights. The Cruise Control inhibit function can be highly frustrating until you get it sorted out.
    Last edited by Jon S.; 01-20-2020 at 02:42 PM. Reason: typo

  9. #9
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    Quote Originally Posted by fifty View Post
    common problem with the gm vehicles if there is a gear change and a decent tire size change. A quick search and you will find a lot of us have run into this problem and no one has solved it. Submit a ticket to them and if there is enough tickets HPT will then look into it and get the part of the ecu/tcm opened that needs to be adjusted.

    I know that there are other tuner OS that have this sorted but not HPT
    What tuner OS have this sorted out?

    I'm have the same issue, all the inputs are wired and working correctly. Simply won't engage.
    '05 5.3 DBW swapped into car with 25" tires and 3.5 axle ratio. I've got the speedo and and gear changes working fine.

    Can't get the cruise control to engage.