Tow / Haul mod
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Adding the Tow / Haul function

I have no idea what the range of vehicles this will apply to. The BCM has many variations depending upon the RPO codes your truck came with. This page describes what was done to enable the Tow / Haul function on my 2001 Crew Cab.
My truck has the factory fog lights. These are controlled by the BCM and I did want to retain that. The donor vehicle in this case was a 2000 Blazer 4 door with the factory fog lights and Tow / Haul.
this is the glove box code list:


In this list
ANL denotes the rear deflector and fog lamp package.
AUO keyless entry.
AU3 power door locks.
A31 power windows.
C60 manual HVAC control.
DK2 heated outside mirrors.
DK7 roof console.
K34 cruise control.
M30 4L60E transmission.
NP8 push button two speed transfercase.
T61 DRL.
UN0 AM/FM , CD radio.
U16 tach.
U89 trailer wiring.
ZQ3 and Zq6 are sales convenience packages.

Overall a pretty good match for the functions controlled by the BCM.
I assume that the U89 code is the tow package and this may include the Tow/Haul mode for the BCM.

This mod does disable the SIR. At this time we do not know how to correct that condition!

Parts list:
  • Tow / Haul switch or suitable momentary switch (normally open)
  • Wire connector to switch , wire to dash connector and proper terminals
  • BCM with RPO code list to include the Tow/ Haul function and your included options.


    The vehicle must be off. Set the emergency brake and observe all precautions for working on the vehicles electrical system. Disconnect the battery and use precautions for working near the vehicles air bag (SIR).

    Tools:
  • 1/4" drive 7mm socket , universal and driver
  • Pick or thin scribe
  • 3/16" drill bit , drill , flat file and Coping saw (or Dremmel tool) These may be replaced with a drill bit suitable to mount an aftermarket switch if used.
  • Diagonal side cutters/wire cutters
  • Electrical tape

    This is the donor BCM on the left and the stock BCM on the right:


    This is located on the transmission tunnel, in front of the heater. The drivers side and center lower dash covers will need to be removed for access. The three BCM wiring connectors are removed by squeezing the retention clips. Once the three connections are removed the BCM has two plastic clips that hold it in the bracket. Top and bottom of the left (drivers) side. This is tight , but can be removed from the vehicles with the storage trays, without removal of the center console. When salvaging parts from the donor vehicle the center console will need to be removed.The Tow/Haul switch is mounted next to the floor shifter in the center console.



    Ignore the brown connector in the above picture as the blue wire needed is already present in our truck. You will need to seperate the three wires from the switch to the seven pin connector on the end. Cut away the extra black ground and gray dash dimmer light wires and release the terminals from the connector after pulling the red retainer.


    The terminals are released from the hole in the front side. Use the pick to gently compress the retention tang and it will slide out to the rear. There are two sets of holes on the front for each terminal. One will release the other is for the male part of the connection.

    Now that you have the three wires to the switch released and cut loose it should look like this:



    That is all the donor pieces ready to go. Now out to the truck.


    The vehicle must be off. Set the emergency brake and observe all precautions for working on the vehicles electrical system. Disconnect the battery and use precautions for working near the vehicles air bag (SIR).

    Start by removing the lower dash pieces from the driver side and center. Carefully disconnect wiring connectors and set them aside, or fight working around them as I did.
    Once those are removed follow the left (driver) side of the transmission tunnel to the firewall and located the wiring bundle tee pictured below.
    Behind it and the heater you should feel the seven wires and connector shoved behind the heater duct , against the firewall. Mine was pretty well stuck back there.
    This is what you will find:



    Release the connector with the green jumper wire.
    This is the connector you will add your three wires to. Pay attention to where the corresponding colors match up. You will add your black ground wire to one side, the blue BCM wire to the other and the gray dash light dimmer wire to the center. (if you are using an after market switch the black wire needs to momentarily ground the blue BCM wire)
    Once assembled it should look like this:



    Now use the electrical tape to wrap these three wire together in preparation for installation.
    Route the wires from where you will locate the switch and plug the connector back in.
    Next up is to install the switch. The wires allow for your choice of location. I installed mine in the dash. To do this the knee bolster and front dash cover must be removed.
    The switch is 1" square. I drilled the four corners with the 3/16" bit and use a coping saw to connect the dots. Some filing to fit and the switch snaps into place.



    Now you are ready to swap the BCM.
    The three BCM wiring connectors are removed by squeezing the retention clips. Once the three connections are removed the BCM has two plastic clips that hold it in the bracket.Replace with the donor BCM and attach the connectors.

    Reassemble the dash and all panels.
    Reconnect your battery.

    Now to program the components:
    The passlock system uses a lockout cycle in order to synchronize all of the passlock components when any passlock related part is changed. The passlock system requires 3 consecutive lockout cycles in order to complete the 30 minute learn procedure for the changed component.

    Changing the passlock components:
    The design of the passlock system is to prevent theft even if various theft deterrent parts are changed. The parts that can not be changed without going into the tamper mode are: The passlock sensor, BCM and VCM/PCM. If any of these parts are replaced the vehicle may start and stall for 10 minutes. This is the long tamper mode. If this occurs, the system must go through a long tamper mode cycle. During this time the security telltale will be flashing for the full ten minutes and the DTC B3031 will be set. The BCM and the PCM require a full ten minutes in order to complete a learn cycle. The ignition switch must remain in the RUN position until the security telltale stops flashing. You need to repeat the cycle if the ignition switch does not remain in the RUN position.

    Programming the theft deterrent system components:

    30 minute learn procedure:

  • 1. Turn ON the ignition, with the engine OFF
  • 2. Attempt to start the engine, then release the key to ON (vehicle will not start)
  • 3. Observe the SECURITY telltale, after approximately ten minutes the telltale will turn off.
  • 4. Turn off the ignition and wait 5 seconds.
  • 5. Repeat steps 1-4 two more times for a total of three consecutive cycles/30 minutes (the vehicle is now ready to relearn the passlock sensor data code and/or passwords on the next ignition switch transition from OFF to CRANK)

    Now the truck may or may not start. The light should no longer continue to flash. After a few tries mine fired up and ran. Push the button and the Tow/Haul lights up:



    Test your systems components
    This completes the install.

    Special thanks to "Steventruck" who paved the way!