'94 Sonoma SLS 4WD Problems

1994 4.3 Vortec, Automatic, Push Button 4WD, 233 Model Transfer Case.

My 4WD will only go into 4 Lo. It goes in and out of 4 Lo easily but will not go into 4 Hi. I have replaced the Transfer Case Motor, the Transfer Case Shift Module and after taking it to a GMC dealer and getting a diagnostic run I have changed the 4WD switch in the dash.

And still I can not get it to shift into 4 Hi. When I shift into 4 Lo the 4 Hi and 4 Lo lights flash a couple of times. Sometimes they flash 3 times sometimes they flash 5 times.

When all of this started I wasn't sure wether it was the motor or the module. Since the module was the easiest of the two to change I changed it first. It worked fine and would go right into and out of 4 Hi with no problems. Also, the 4 Hi and 4 Lo lights did not flash, the 4 Hi light just came on. Then about 3 days after being fixed it stopped working, and was only going into and out of 4 Lo.

I then changed the transfer case motor. I assumed it was an intermitent problem with the motor because the dealer said they had been selling alot of the motors and that it was a common problem. Changing the motor out solved nothing.

At this point I took my truck into the dealer and had them run a diagnostic and they told me it was the switch in the dash. I just replaced it and it still does not work.

Has anyone run into this problem before?

And does anyone know if the flashing 4 Hi and 4 Lo lights is a code?

Any information would be great!

Thanks, Kirk

1st Check vac actuator under battery tray for tears, or leaks, and that the vac hose is connected. Replace if found

2nd Check vac hose leading from vac actuator to vac switch on transfer case for cracks and dry rot. Replace if found.

3rd Check remaining 2 hoses from vac switch on transfer case for dryrot or cracks Replace if found.

If all of the above are fine, disconnect your negative battery cable for 1/2 hour then reconnect it. If your 4x4 works after that you probably have a failing TCCM (Transfer case control module)

If that doesn't solve it, it's most likely the encoder motor. (Wouldn't be surprised if the new one you put in was bad, though unlikely)

1st Check vac actuator under battery tray for tears, or leaks, and that the vac hose is connected. Replace if found

2nd Check vac hose leading from vac actuator to vac switch on transfer case for cracks and dry rot. Replace if found.

3rd Check remaining 2 hoses from vac switch on transfer case for dryrot or cracks Replace if found.

If all of the above are fine, disconnect your negative battery cable for 1/2 hour then reconnect it. If your 4x4 works after that you probably have a failing TCCM (Transfer case control module)

If that doesn't solve it, it's most likely the encoder motor. (Wouldn't be surprised if the new one you put in was bad, though unlikely)

Thanks for the reply but my 4x4 is not controlled by vac, it's electric.

Thanks for the reply but my 4x4 is not controlled by vac, it's electric.

No, actually it's vac controlled with the exception of the transfercase motor itself. When you hit the switch on your dash it activates the transfer case motor and the vac switch on the transfer case pulls a vaccume actuator under your battery which engages the front differential so that it connectes to the front prop shaft on the transfercase thereby creating your 4wd... Again, follow the intructions above and you will find your problem.

No, actually it's vac controlled with the exception of the transfercase motor itself. When you hit the switch on your dash it activates the transfer case motor and the vac switch on the transfer case pulls a vaccume actuator under your battery which engages the front differential so that it connectes to the front prop shaft on the transfercase thereby creating your 4wd... Again, follow the intructions above and you will find your problem.

Ok, I followed your instructions and I think it is the shift control module.

All the vac lines were in great shape, in fact I think they are new. And the vac actuator was in good shape too. When I squeezed the actuator I could here air comming back out of the vent so I figured it all the lines were in good shape.

After getting the battery back in I was able to shift into and out of 4 Hi.

So, like you said I am assuming it's the shift control module.

My only question is, I recently replaced this module. Granted I did get it from a junk yard, but should I have disconnected the battery while replacing it?

I just unplugged the old one and installed the new one without disconnecting the battery. So, I'm worndering if I just needed to reset it or if it is actually failing.

Thanks again for all your help!

Be sure to check the battery and cables. I had a similar problem when my battery was bad and thought to replace the battery first. The second time it happened the OEM battery was almost new, I took it to the dealership and it was diagnosed as a bad control module, when it turned out to again be the battery. I recommend a battery of 650 cca, preferably higher. The OEM batteries are not designed for off road use and do not tolerate desert heat.