Unusually 4wd behavior in 1999 GMC Jimmy
4-button switch seemed to work fine at first but after getting stuck noticed the front wheels would only lock in when switched to “Auto 4wd.” Transfer case would shift to low range and back out, it would go into “neutral.” All of the lights were working but maybe the TCCM was not locking the front wheels in “4hi” and/ or “4low.” I think the actuator under the battery is working because the “4wd Auto” would lock the front wheels.
Now, at start up the truck mysteriously selects “4wd Auto” or “2hi” and pushing the other buttons has no affect on it. Those two work properly with respect to 2wd or 4wd but I cannot switch it. I had the GM dealer in town run their scanner on the Jimmy and they concluded a bad switch in the dash. I ordered the switch from AMAutopart, on eBay, but it did not solve the issue, I even tried another switch but still nothing. I have unplugged the TCCM while the motor is running and I hear clicks from the TCCM.
Has anyone heard of getting a codes to replace the switch when it really was a bad TCCM. I wish I could find one to try before potentially blowing 300 bucks.
Any suggestions would be greatly appreciated, Thanks Again.

 

yeah mine is pretty much doing the same thing, though when mine first started it would select 4-lo on start up. if you werent paying attention it would shift into lo range when you pulled gearshift through nuetral. not having the time or money to deal with it i just unpluged the tccm and encoder motor. now that ive decided to deal with it, it is selecting auto 4 on start up, but never actually shifts. it basically times out and goes back to 2hi then within a few mins the service 4x4 light comes on. had the codes pulled and the active code was b2725 i believe which is the code for a bad switch in the dash. i did the same thing replaced the switch and no dice, same prob. so at this point i guess im gonnna wait till the spring (after it stops snowing) and try to buy a new tccm. should be less competition for them on ebay. fortunately i live in florida so i dont NEED 4wd. if the tccm isnt the prob then what else can it be? a bad encoder motor? that doesnt make since to me. i mean i cant imagine the encoder motor being able to trick the tccm into thinking the button on the dash has been pushed. i can imagine the tccm tricking itself into thinking that. so if you replace the tccm and that fixes the prob would you let me know? or whatever you figure out. Thanks

 

I installed a new TCCM today. I replaced the old one with Part#12577412. Not the same part# as the TCCM I removed, but I was told at the GMC dealer this was the replacement part, so I hope they were right. The transfer-case is shifting properly now, but I only have 4wd in the auto setting. It seems like the vacuum solenoid on the the firewall is not getting the signal to open I guess. Could this be a problem with the encode motor now? Not sure were to go from here... Maybe pull the encoder and check that out, I'll follow up as to where this goes.

 

If the transfer case is 'shifting properly', then the encoder motor should be doing its job. The TCCM is responsible for energizing the vacuum solenoid on the firewall. The solenoid should be energized in Auto4wd, 4HI, & 4LO.

 

I do have power at the switch, 11V+/- when the key is on, when grounded at the battery and testing one of the terminals. i.e. one terminal is hot. Now, from terminal to terminal I get 11V+/- in the 3 4wd setting and 6V+/- in the 2wd setting. Is this normal? Is there a vacuum switch on the transfer case I have not found yet? Why would the 4wdAuto engage the front tires and not the 4hi or 4Low, is the front axle engaged differently in 4wdAuto? Do I need the truck running to fully check this out?

Thanks Again for all your help!!!

 

You need to measure the resistance. Search for "NV236+resistance" with "swartlkk" as the author.

As I said, the front axle is engaged in Auto4wd, 4HI, & 4LO. It engages the same way for all modes. In Auto4wd, the transfer case controls transfer of torque to the front axle as needed, but it is engaged all of the time. In 4HI & 4LO, the transfer case locks the front and rear outputs together.

 

After my last post, I decided to go through the vacuum system more diligently, including the vacuum solenoid. What I found was a burnt vacuum line. It was hard to see and because I was getting what I thought was 4wd in 4wdAUTO I never went through the vacuum lines real close. After closer inspection, I found it, burnt through the protective loam and vacuum line. So I redirected the line to not be close to anything too hot. I put the battery back in and ran through the button settings and like magic it engaged the front end in the settings you said it should. Now, I wonder what was giving me more traction in 4wdAUTO if the front wheels went locked in. Now that I have this kink worked out, what would anyone say to the 4wdAUTO button not wanting to change with one push? It will switch but I have to push it repeatedly and then by chance it might change. Sounds like a bad switch, I have 2 others I need to find and see if one of them might work.

Thanks Again,
Nick

 

I have it all put back together and I put in the other button and all the settings work, I now can 4wdAUTO with one push. What has me confused is when the motor is running I jacked up the driver side wheel on the front. I engaged the 4HI and the actuator engages the front wheels but I can still spin the passenger side front, it is not moving real freely but I can see that the front drive line is turning when the front passenger wheel is turned. As the front drive line turns the rear drive line remains unchanged. I thought that in 4HI or 4LOW front and rear drive lines were locked together. Any idea what is going on here?

I have way more traction, can physically see front wheel spinning on ice, in the AUTO setting. And there seems to be no differance between 2HI, 4HI, or the 4LOW settings where only the rear wheels appear to be spinning...

Is my TC not shifting all the way or what, is there a clutch inside the TC that is slipping?????