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Jerry, Master Mechanic
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Experience:  ASE Master tech, 30 years experience GM certified
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2001 gmc sonoma: if I put trans..neutral..transfer case vacuum switch

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my 2001 gmc sonoma 4wd crew cab will not switch into 4wd. if I put trans in neutral and hit the 4wd hi button and shift back into drive it will switch into 4wd low. I changed the transfer case vacuum switch but no change. any suggestions?

Hello and welcome to Just, Super mechanic here. About your vehicle.


You will want to check the transfer case control module for any trouble codes.



Diagnostic Trouble Code (DTC) Displaying

Diagnostic Trouble Code (DTC) Displaying
The transfer case shift control module is equipped with the ability to store diagnostic information, which is useful to a technician in the event of a module, component or wiring failure. This information can be retrieved from the transfer case shift control module by way of flash codes, which are displayed on the three transfer case shift control switch buttons. These buttons are located on the instrument panel to the right of the steering wheel. The following steps provide an easy way of retrieving the flash codes properly in the event that a failure has occurred.

  1. Turn OFF the ignition for 10 seconds.
  2. Connect PIN 13 on the Data Link Cable (DLC) to a ground. The DLC is located in the cab under the instrument panel on the driver's side.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the transfer case shift control switch select buttons for flashing codes. Important: If the transfer case shift control switch buttons all blink only once and then stop, no fault codes are stored in the transfer case shift control module.
    • When one DTC is stored into memory, that code will blink (the number of times they blink indicate the code number) followed by a three second pause and then repeat.
    • If more than one code is stored, the first code will blink, followed by a three second pause and then the second code will blink.
    • This sequence will continue until PIN 13 on the (DLC) is no longer grounded.





Jerry and 3 other GM Specialists are ready to help you
Customer: replied 7 years ago.
I grounded the 13 position on the data link. 2wd and 4wd low lights flashed once and went off. I tried many times, positions and grounds but with the same results. I took the truck to the dealer who told me (for 50 bucks) that I needed a transfer case motor and u-joints and gave me an estimate for over 1000 dollars! I got the parts at a local parts store for 200 bucks and put them in myself. I still seem to be stuck in 4wd high. When I push the buttons they flash and return to 4wd high or not at all. I also hear a clicking (electrical) under the glove box when buttons are pushed. As I previously said I also replaced the vacuum sending unit. What's next? Thanks
Let me try to find something on that. In the meantime, try disconnecting the battery cable for about 30 minutes, to allow it to clear and see if that will help.
Customer: replied 7 years ago.
I had the battery disconnected the whole time I was changing the transfer case motor. Same resultsFrown

Does it still have the code #1?



There may be a problem within the control module.






DTC 1-RAM Standby Power Loss

Circuit Description
The transfer case shift-control module normally has battery voltage supplied through the battery positive voltage circuit (CKT 40) at all times. When the ignition is ON, the transfer case shift control module conducts a self-test. This self-test determines if the current memory is the same memory that was retained when the ignition was last turned OFF. This self-test indicates whether the RAM standby power-supply to the transfer case shift control module pin C6 was interrupted. If the power is interrupted, the DTCs would be cleared from memory. If the self-test fails, the transfer case shift-control module sets DTC 1.

Conditions for Clearing the DTC
This DTC will clear when the voltage supply failure has been corrected and the ignition switch has been cycled 5 times

Test Description
The numbers below refer to the step numbers on the diagnostic table.

  1. This step inspects the CTSY LP fuse for an open.
  2. This step test the battery positive voltage circuit (CKT 40) for an open or high resistance.
  3. This step rechecks the CTSY LP fuse for an open.
  4. This step tests the battery positive voltage circuit (CKT 40) for a short to ground.


Customer: replied 7 years ago.

OK I replaced the switch in the dash (control module?). The lights seem much brighter.??? I disconnected the battery for over half an hour. Now when turn on ignition the 4wd light stays on. When I press either of the other two buttons the light flashes and return to 4wd. Now when I ground position 13 data link all the lights flash once about every three seconds as long as I keep grounded. I think we are getting somewhere if only by process of elimination.

PS the CTSY LP fuse was OK


Thanks again


Yeah...that sounds like a code 1, (TCCM) however, the control module is actually behind the right kick panel.





Customer: replied 7 years ago.
Thanks for all your help. I'll get one tomorrow and let you know how it turns out. Can you accept tips?Wink
Your welcome and yes, bonuses are always welcomed. As far as I know, you can just add it. Never been on that end though, (lol) and thank you.
Jerry and 3 other GM Specialists are ready to help you
Customer: replied 7 years ago.

Thanks for your help. The control module ($425.00 list but my little sister got it for me for $306.00) seemed to be the problem. It switches fine from 4wd high and 2wd. It doesn't however seem to want to switch into 4wd low. Any ideas. So far I've changed to switch in the dash, transfer case motor, vacuum sending unit and the control module. I hope that there aren't to many more electrical parts because they are breaking the bank. I can't complain though because the dealer quoted me $1067.00 and I only have a little less than $600.00 in it so far. It seems to drive fine in 2wd and 4wd high and I never have used 4wd low before so it won't break my heart if it doesn't work. Thanks again.



That seems like it might be in the transfer case, (a mechanical problem) hard to say though with all the other problems that it had.