NEED HELP Went from 2 high to 4 high on its own on freeway.

[welches_fruit_snacks]

Going freeway speed the 4 wheel drive selector was on 2 hi. I look down and the 4 hi light started blinking and then steady on. I switched to auto and then back to to 2 hi.

When I got home I parked and switched through all the 4 wheel drive modes and they seem to be working as least the lights are working on the selector and I can hear something actuating at the front axles.

In other words WTF?

 

[jsheahawk]

Mine has done that before. I assumed it was the switch, so I replaced it. Hasn't happened again. Roadie suggested switching it through the modes about fifty while the truck is off to clean the contacts on the switch. Worth a shot.

 

[Sparky]

Yeah, that's a classic funky switch symptom.

 

[Thisorsomethingbetter]

Glad to find this discussion since this recently happened to me but with a bigger problem. Prior to the random switching at freeway speed, I had the issue that since first trying my 4WD this season, it has seemed to be stuck in 4 lo. Regardless of what the switch says, my 4WD only operates in low, so I have been driving in 2WD and don't use 4WD. When it randomly switched at that speed it all but locked up the front wheels. This happened a couple more times as I limped home at lower speeds. I did run the switch back and forth about 50-100 times and it hasn't done any random switching since. But I wonder why would it be able to switch into low at such a high speed? I thought there was some sort of safety lockout, which must have failed also? 4WD still seems to work even after that, but like before, still stuck in low. I would guess something would have gotten fried in the transfer case when that happened. Does anyone know, would a bad TCCM cause any of this?

 

[Mooseman]

I think it's a software lockout in the TCCM that prevents engagement of 4lo above a certain speed. Your 02's TCCM, if it has never been replaced, is a known bad part that fails. I would start by replacing that since it is the easiest. Anything from an '03+ will work.

 

[Thisorsomethingbetter]

I did replace it once, with a used ebay part, so not sure if what I put in was from an 02 or later. I have no problem replacing it again with something newer. That sure seems like the easiest place to start.

 

[AtlWrk]

Welcome
Was it just the binding/locked up feeling that makes you think you were in 4LO?
Given the design of our transfer case it would be nearly impossible for it to shift into 4LO on the fly/at speed. The shear mechanics simply wouldn't allow it. As mentioned, there is a speed (<2mph) and transmission neutral interlock to prevent people from trying or accidental attempts but even a software glitch wouldn't let you just slip into 4LO.

However, you can (and it is perfectly acceptable to) switch to 4HI (or A4WD/Auto) on the fly BUT to be in 4HI in our trucks requires truly slippery conditions: ice/snow covered roads or being in sand/gravel--just wet roads aren't enough. If you were testing or the transfer case was inadvertently shifting into 4HI in dry conditions due to a flakey switch then the drivetrain will bind up. On the highway or straight-ish roads it can gradually feel like the whole truck is dragging an 18-wheeler and steering gets brutish but usually isn't too dramatic. Once you start maneuvering (like in a parking lot or pulling out of a driveway in particular) the binding occurs much more rapidly and you get what we call "crow-hopping" as the tire with the least traction releases the bound up energy, hops, and the process repeats over and over.

It's just the way the system works as these aren't true "All-Wheel Drive" systems. The compromise was Auto/A4WD mode where the front wheels aren't locked to the rear (so no binding occurs) but the transfer case is ready to lock them quickly to send power forward if it detects the rear wheels slipping.

I suspect the "locked-up" feeling you experienced was actually just going into 4HI in conditions that don't let the wheels slip--but pardon me if I'm making some bad assumptions about your situation.

 

[Thisorsomethingbetter]

Hello - thanks for the reply. Yes, it was mainly the front wheels binding up but the switch also had flashed and locked onto A4WD then it somehow switched into 4 by itself.

I had already felt like the 4WD wasn't right after trying it out on snow and sleet a week or two earlier. I'm up here in the Mini-Apple and don't use 4 until it snows so its kind of anyone's guess how it will work each winter. I can be criticized for "out of sight, out of mind" mentality, I realize. When trying it this winter in 4HI, or A4WD - anytime it was clearly in 4 - it felt so bound up, with no speed and lungey (must be what you're calling crow-hopping) that I made an assumption it was stuck in low. I just drive in 2WD and decided to worry about it later. Then it went off and switched as mentioned above. I guess luckily (?) that happened on a slippery slushy night so it was like slamming on the front brakes on slush. If the rears had locked up I would have skidded across the road, but instead I was able to slow down and let it shift itself back into 2WD. It switched again a couple more times before I got it home.

I supposed it could be something else in the transfer case binding up. It's still stuck like that now any time I put it 4, so if its not 4LO then who knows?

 

[Mooseman]

Maybe it's just the encoder motor not working right. Maybe pick one up cheap at a pick-a-part and give it a try.

 

[Thisorsomethingbetter]

Hey Mooseman - i did pick up an encoder motor to try as you suggested. The old one came off no problem but i can't seem to get the keyway lined up to get the replacement on. I recall a thread regarding taking the housing off to rotate the motor in order to get it lined up, but i can't seem to find it now. You don't have a link to it at all do you?

 

[Mooseman]

If the Transfer Case shaft or the encoder motor are in other than 2WD, things won't line up. Try plugging the motor in, turn on the ignition and turn the switch to each mode. The motor should rotate at each mode. I'd try one mode after another and try to insert it at each one.

 

[Thisorsomethingbetter]

Hooked up the new (used) encoder motor and turned on the ignition but now have the "service 4wd" light on and no lights above the modes on my selector switch. I suppose I could have gotten a bad encoder motor from the yard. I am going to try to wake up the TCCM by pulling fuse 8 as others have suggested elsewhere.

 

[Chickenhawk]

Ah, classic failure of 02 TCCM. I would be willing to guess that you replaced the 02 TCCM with another 02. Go 03 or higher.