Service 4wd light

Well how about that 80k miles of no hassle and suddenly and very infrequently my service 4wd light has started to flash and all the lights on the selector as well. In between mass flashes I tried it and it will shift on the fly, shift parked, shift in nuetral. So are we all thinking its the switching unit? Anyway its a 2000 4.3l for all those who ask and never look at profiles

Now its randomely locking in and out of 4wd drive meanwhile all of the lights are flashing on the selector.

Come on, no one has ever had this happen, let see we have the control module under the dash and the motor on the transfer case, and some vacuum hoses and some wiring and a couple buttons. So if you had to guess which would you say. Also how can I disable it until I can repair it, I dont like going into 4 low in rush hour.

Man this is sad, still no thoughts, anyway. Today I decided to see about resetting the tccm so first I disconnected the battery while I went to town cleaning up the battery tray from a leaky battery I had. Well an hour later after checking all of the connections from the inside buttons to the tccm to the vacuum hoses to the selector cable as well as all of the connections on the transfer case, I hooked the battery back up, turned the key and... still the same thing, service 4wd light on and all the mode lights (auto 4wd, 2wd, nuetral, 4lo, 4hi) are on. I pulled the 4wd fuse and the other one ctsy lp or something like that. and my transfer case is the np 236 (new process my arse, what ever happened to the good old cast iron manual transfer cases). I am thinking it is the tccm or the encoder motor, The actuator under the battery seemed to be fine and the few times it has just acted normal i was able to get it to switch. I am thinking I will call a few junkyards for prices on the tccm and the encoder motor, god knows it will cost less for both parts than for just one of them new.

If anyone has any ideas on how to disengage the 4wd until I get it fixed I would love to know, unfortunately if you just pull the fuse for 4wd it also disengages the fuel gauge, not fun. I was thinking of unplugging the dash switch, think that would work, I need an idea here people.

Hope this isn't too late. The easiest way to disable the transfer case I've found was to go to the fuse panel under the hood. Pull the 20a fuse marked ATC. This should disable it. The reason I know is I'm having the same problem. Let me know if you get yours solved. I've tried the encoder motor and the TCCM with no luck.Man this is sad, still no thoughts, anyway. Today I decided to see about resetting the tccm so first I disconnected the battery while I went to town cleaning up the battery tray from a leaky battery I had. Well an hour later after checking all of the connections from the inside buttons to the tccm to the vacuum hoses to the selector cable as well as all of the connections on the transfer case, I hooked the battery back up, turned the key and... still the same thing, service 4wd light on and all the mode lights (auto 4wd, 2wd, nuetral, 4lo, 4hi) are on. I pulled the 4wd fuse and the other one ctsy lp or something like that. and my transfer case is the np 236 (new process my arse, what ever happened to the good old cast iron manual transfer cases). I am thinking it is the tccm or the encoder motor, The actuator under the battery seemed to be fine and the few times it has just acted normal i was able to get it to switch. I am thinking I will call a few junkyards for prices on the tccm and the encoder motor, god knows it will cost less for both parts than for just one of them new.

If anyone has any ideas on how to disengage the 4wd until I get it fixed I would love to know, unfortunately if you just pull the fuse for 4wd it also disengages the fuel gauge, not fun. I was thinking of unplugging the dash switch, think that would work, I need an idea here people.

I forgot to mention, I also tried a new switch. Still nothing.

my service light came on, this is what the factory gmc mechanics found

error codes co305 and co374, found open circuit in front speed sensor circuit (broken wire) repaired the broken wire at the speed sensorconnector and cleared the codes, road tested everything aok!

thats what the reciept says. cant help ya anymore than that but maybe a place to look good luck

I got it to stop jumping around and when I can I will be working on fixing it, a wire running to the speed sensors... fixing that would cost alot less than a new tccm, encoder motor... To stop it I just waited until it went back into two wheel and acted normal then I pulled the plugs from the switch and it still flicks on the service 4wd light but it doesnt change on its own. So where would I locate the possible split in the connection to the speed sensor, do you think it would be closer to the actual wheel maybe a short due to excessive wheel travel?

Thats right we are reviving a dead thread... well not totally it is my thread, I am getting ready to see about fixing this in the next couple weeks (taxes you all understand how that is). So i was wondering if any one had an idea as to what specifically it is, or how i can narrow it down to one item? I would much rather buy just the encoder motor or just the TCCM to fix it than to buy all new all around. Also does anyone have a reliable source for these parts or happen to have good ones lying around?

Anyone.... anyone at all.

Man, with all of these problems I just might sell my Jimmy. I wish I could help you guys but I just bought mine 3 1/2 weeks ago and I'm already haveing problems with mine.

I'm begining to wonder if these vehicles are junk.

Follow these instructions to determine if it's the TCCM or the encoder motor

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.

4th Pull your vac switch on the transfer and press the ball valve. If it's sticky, replace.

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.

I did all of that and it still went nuts when the power came back so i guess it would be the encoder motor, I just didnt think the encoder motor would change all on its own, I thought it would just not work.

Would a bad encoder motor cause it to randomely change the mode it is in, without ever touching the button? Or would this be more of a problem with the TCCM going bad?

If I can get a confirmed answer to this I will be in heaven, and then just need to find somewhere to get a replacement part.