Automatic 4-Wheel Drive not engaging/disengaging

hello guys…

I have an automatic 2000 GMC Jimmy. It has automatic 4-Wheel Drive. After a recent snow storm, in which I was stuck, I could not get it into 4-wheel drive by selecting the automatic button, the 4-hi button, or the 4-low button. The buttons would just flash and the 4-wheel would not engage. Finally a friend pulled me out and then “bumped” me into a straigt line and the 4-low button - that we had last selected - finally engaged. Now it will not go out of 4-wheel drive. Using the same method that got me into 4-wheel drive, i have selected every button (on a dry, flat straight away) and it will not disengage.

1. Why?

2. Is there something manually I can do on the exterior to take it out of 4-wheel drive?

3. Outside of really using a lot of gas, what negative mechanical impacts will driving in 4-wheel drive on a dry flat road might this have on my vehicle?

4. anything i haven’t asked that you might advise?

thanks! bareezy

You need to take the truck to a dealer, or a good transmission shop (not AAMCO) for an evaluation. Your 4WD is controlled by electrical switches and solenoids, so this could be a mechanical or an electrical problem. Would need more info to give a more specific response.

Take this vehicle to a mechanic, ASAP, and have the 4WD engagement system repaired.

  1. Because something electrical or electro/mechanical is faulty.

  2. No

  3. Driving in 4WD on dry roads can cause all sorts of problems, some of which are very expensive to repair. I’ll bet the vehicle’s owner’s manual cautions against using 4WD on dry roads. Check it and see for yourself.

  4. Read the first sentence of this post.

Question: If your Jimmy has “automatic 4WD,” why do you need to turn it on and off manually?"

Another question: What does “bumped me into a straight line” mean?

Are all four tires on your vehicle the EXACT same size??

I’m assuming you have Auto-Trac with Auto 4wd, 4Hi, and 4Lo. Only Auto 4wd can be used on dry pavement. Using 4Hi and 4Lo on pavement will cause driveline binding and damage will result.

Earlier this year I was unable to engage 4wd in any setting due to a bad vacuum switch (mounted in the center of the firewall). The switch controls an actuator under the battery that engages the front axle. The default mode is the axles disengaged (2wd). Also check the condition of the vacuum lines, the lines on my 2000 Blazer are in bad shape and need to be replaced.

According a GM mechanic the Transfer Case Control Module (TCCM) will lock up if not used on a regular basis. To reset the TCCM, disconnect the wiring harness at the TCCM (inside the pass. side kick panel) or disconnect the battery (easier). This helped when I was having trouble shifting into 4wd. Also to shift out of 4Lo, it helps to be moving 2-3 mph while in neutral. Check the Owners Manual for this.

Ed B.