1999 GMC 1500

1999 GMC 1500 4WD Shifting

Dec 2, 2006 Vincent Ciulla

A 1999 GMC 1500 4WD that won't shift in and out of 4WD.

Question: 1999 GMC 1500

I recently purchased a 1999 GMC 1500 truck. Truck is spotless, believe me. I used the 4x4 last week while towing a small trailer out of the grass and then pressed the 4x4 button back to 2 wheel drive. It would not flash and go out of 4x4. Eventually it went out a hour or so later, when I started the truck up again. I don't know what I did to make it go out. Tried it later and it worked fine. Today I tried it going down the road (to see if it would work) and it would not go out again. The lights in the switch did not blink, like when it does work. Only blinked when I put it in 4x4.

Now, when I got home, I shut of the truck off and turned it back on for the first seconds (when all the dummy lights go on in the dash). All the lights in the 4x4 switch lit. I quickly hit the 2 WD dr button and it flashed a couple times and went into 2 wheel drive. My question is, what would make it not go in and out while going down the road? Would it be the dash switch, actuator in the front axle or something electric in the trans? Please give me your input, if you would.

Thanks,

Fred Avila

Answer: The first thing we need to do is checked for stored trouble codes. To do this:

1. The diagnostic trouble codes are displayed on the three transfer case shift select buttons. The shift select buttons are located on the instrument panel when the connector pin 13 on the data link cable is grounded, and the ignition switch has been OFF for at least five seconds prior to positioning the ignition switch to run the shift select buttons will blink various times together in order to indicate a diagnostic trouble code from 1 to 4.

a. Position the ignition switch to OFF. Ensure the ignition switch is positioned to OFF for at least 6 seconds.

b. Connect pin 13 on the data link connector to a vehicle ground source. The data link connector is located in the cab under the instrument panel on the drivers side.

c. Position the ignition switch to RUN.

d. Note the shift select buttons for blinking codes.

2. If the shift select buttons all blink one time and stop, and do not continue to blink, no fault codes are stored in the TCCM.

3. The transfer case shift select buttons will blink in order to identify any stored DTC. If only one code is stored in the TCCM memory, that code will blink repeatedly with a three second delay between blinking sequences. If more than one code is stored, the first code will blink once, then after a three seconds delay, the next code will blink. This sequence will continue until pin 13 is no longer grounded.

4. When reading the diagnostic trouble codes, the number of shift select buttons blinks will indicate the code number.

This procedure will only work with the NVG 233/243-NP1 Two-Speed Selectable New Venture Gear transfer case. If you have the NVG 236/246-NP8 Two-Speed Automatic New Venture Gear you will have to use a scan tool to access the DTC's.

Once we know what the codes are we can then figure out what we need to do to repair the problem.

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