transfer case low range switch

Discussion in 'GM Powertrain' started by grumpy old man, May 12, 2010.

  1. My 99 GMC Suburban is quick shifting and it is my understanding that there is a switch somewhere that signals the VCM when the transfer case is in low range to alter the shift points to compensate for the lower gear ratio. Is there someone out there that can tell me how to disable the switch to see if that is the reason that my transmission is quick shifting 1-2/2-3. I have looked and can find nothing on the transfer case that looks like it could be the switch in question and I have found nothing in alldata. Any help would be greatly appreciated. If I can get this thing straightened out by Thurs. afternoon it will make the trip to the mountains a lot easier. Thanks in advance for any help.
  2. rnr

    rnr New Member

    Sounds like you have a faulty transfer case control module. There is a wire that runs from TCCM to the PCM (gray and black wire in terminal F7 of the TCCM) that senses the 4wd low signal. You can try backing out that wire to see if that changes anything. You might get a service 4wd lamp on after the wire is disconnected.

    If you do have a faulty TCCM, can I purchase your old one from you? I am doing research on failure modes of TCCM's.
  3. triple-4x4

    triple-4x4 New Member

    I also have a 99 Suburban K1500. Mine had a NP246 Transfer case until I just changed it over to a NP241 manual shift because I really do not like the complication of the electric shift system. When the road conditions get really bad, I don't want to worry if it will shift into our out of 4wd.

    If you have the electric type of transfer case, you should be able to remove the 4x4 fuse from the fuse panel just inside of the drivers side door. Do this while you are in 2wd. Now the computer for the Transfer case will be powered off. Also, the Transfer case motor and actuator for the front axle will no longer have any power.

    It is true that there is a signal going to the Engine Computer for the low range selector. This signal comes from the Transfer case computer. Therefore, if the transfer case computer is making a bad choice to tell the Engine computer to go to low range, removing the 4x4 fuse will eliminate all but a shorted output of the TC computer or a pinched wire in the harness.

    I have been driving around for a couple of months with the 4x4 fuse removed and mine don't have a check engine light issue. Once I get a chance to modify the wiring for the manual shifter and disconnect the transfer case computer, I will be putting the 4x4 fuse back in place.
  4. murdog94

    murdog94 Epic Member 5+ Years 5000 Posts

    And if this is the fault let us all know.. More knowledge, since it seems that there are so many problems with the auto 4wd etc..
  5. rnr,
    There would be no purpose in purchasing my old module as I can tell you what happened to it. I was suffering from a severe case of brain fade and was cranking the engine with the coil wire out. It took out the ignition module and back fed a high voltage signal through the pink wire, which feeds the ignition module and the trans shift solenoids, and burned out the normal shift circuit in the VCM. I got it running by replacing the ign module but then it was quick shifting, third gear in about a vehicle length. I replaced the VCM which cured the shift problem for a few miles. After replacing the VCM I plugged it into a tech 2 do dial it in. During the procedure to match the crank position sensor to the VCM it is normal for it to set several codes which subsequently get canceled. I noticed during the procedure that it set a code that read transfer case low switch or something like that but did not think anything about it. After I got everything dialed in and left for home it started quick shifting again but not nearly as bad as it was before so my assumption is that when I barbecued everything else it probably took out the transfer case module also. Would you happen to know which terminal in which plug on the VCM would be the gray/white wire in question? Thanks for your reply.
  6. The final chapter in the transmission quick shift issue. For those of you who are interested I finally have the transmission working correctly again. The gray black wire in the F7 position of the transfer case control module IS the wire which signals the VCM when you are in low range. If any of you out there are relying on Alldatadiy it is in reality some data not all correct. They show that wire as being in position 23 of plug C4 in the VCM for the 4.3L engine, position 16 C2 for all other gas and 28 C2 for diesel. I can not speak for any truck other than my own but after spending considerable time taking the module out from under the dash (the general still feels compelled to hide everything as completely as possible) and tracing the wire to the VCM, wonder of wonders I found it in position 23 C4. I will not guarantee much in this life but I will guarantee that the truck is not a 4.3 but is a 5.7. As everything else seems to be working as designed I am not going to spring for a new module, I have bought quite enough G.M. electronics for this thing. My 4WDL usage is practically non existent so I just backed the wire out of the VCM plug and will use it as is. I have not fully checked it out but am quite confident that by picking up a signal for low range at the dash switch it would be a simple matter to wire in a cheap relay (or switch) to signal the VCM if at a later date it proves to be necessary. If I have any advice it would be think about what you are doing. I have spent enough time working on G.M.'s to know better than to crank one with the coil wire out but was engrossed in something else and spent close to $400.00 and about 6 weeks of now and then time finding all the problems because of it.

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