I tested the 4WD while in the yard. When I was not moving and in neutral, I pushed the 4 High button, nothing happened and the light did not come on. I pushed the 4 Low button and I heard a sound below that I assume was the transfer case switching to 4 Low and the light came on. I pushed the Auto button and heard the same sounds (I assume from shifting back to high range) and that light came on. I pushed 4 High again and the light came on this time. I drove around some and I could tell it was in 4WD by the way it steered when I turned sharp. I switched in and out a few more times and everything seemed to work normally. So, maybe something was just sticking and freed up. The thing I'm not sure about is that the front wheel drive axles turn all the time, even when 4WD is not engaged. I'm referring to the shafts that have a boot on each end (which I assume is covering U-Joints) going to each front wheel. I know this because I had my wife drive real slow while in 2 High and I could see them turning. The front drive axles on my 1989 Suburban do not turn when it's in 2 High. It has a straight axle with automatic locking hubs.
It has a switch on the top that says "Auto 4WD". The manual doesn't call it Autotrack, it says it engages the front axle but doesn't apply power to the front axle unless it senses a loss in traction. I'm starting to wonder if mine is in "Auto 4WD" when I think I'm in 2 High, since my front drive axles are always turning. Sure wish I'd noticed that before I bought it!
You've got Autotrac. That's important to know because the diagnosis is different. I still think it's the switch but that part number I gave you before isn't the right one.
GM has had problems with the Autotrac system, mostly electronic. You may have to have the dealer diagnose it for you.
Autotrac is a set-it-and-forget-it 4wd system. You have it correct buy stating the axle is engaged but not activated unless a loss of traction is encountered. It's only available on the 1/2 ton models.
I'll do some more diggin', but I'd bet you'll need to visit a dealer. [img]/forums/images/icons/frown.gif[/img]
OK, I guess I'll calm down a little now. I called 2 dealerships and told them about the front drive axles turning all the time and they both said it's normal. There is no power to them unless it's in 4WD, but they are supposed to be turning all the time. They said it's that way for the Auto 4WD system.
HAD THE EXACT PROBLEM WITH MY 99 ALSO DEALER
REPLACED THE 4WD SELECTOR SWITCH IT SHORTED
OUT. COST AROUND $150. [img]/forums/images/icons/smile.gif[/img]
Let me calm you down even more. I don't want you to feel that the Autotrac system is junk. It's actually very good and works as advertised. Most of the problems that I'm aware of are electronic in nature; bad switch, actuator motor, etc. Granted, these aren't exactly cheap to replace, but I'd much rather pay $200 bucks for an electronic item than >$1000 for a transfer case. I think those Autotrac transfer cases are well over $1000.
My sister has a 2000 'burb w/ Autotrac and running around town with kids and groceries, she has told me more than once that she feels more secure on wet roads. Especially with something that big... [img]/forums/images/icons/shocked.gif[/img]
More people actually complain about the noise and vibration that you get with Autotrac than anything else.
Jim - Thanks for all the info! Since the light has not come on in over 300 miles now, I'm going to just wait and see what happens for a while. The 4WD is working and I'll be using it on my property in the dirt and grass every once in a while all summer and fall anyway, so I'll see how it goes. If the light comes on again, I'll take it in and get it fixed before winter. So, time will tell if/when I have to do that. Maybe I'll get lucky (for once). [img]/forums/images/icons/smile.gif[/img]
My buddy has a 99 2500 that has the same problem. The dealer said that it was the module and would be 400 bucks. He continues to drive it and use 4WD when needed. His started out every so often but now comes on after it warms up.