How Does The Auto Tcase Work On A '07 Silverado

andyems · April 5, 2010

Hopefully someone can explain how this Auto 4wd system works.

I have a GMT 900 Silverado 1500 with the electric Auto transfer case. I have noticed that sometimes after I have been in and out of Auto or 4wd at highway speeds, when I turn off the ignition I hear the transfer case cycling/clicking.

For example, tonight I had to use Auto for a short distance at highway speeds and it seemed to go in fine, heard and felt the "clunk" as it went in, and it seemed to come back out into 2wd fine, didn't notice any issues, sounds when turning into the driveway when I got home like it was still in Auto. Lights on 4wd dial show correctly for 2wd, auto, 4wd, etc. When I turned off the ignition, with the knob already back in 2wd, I heard the transfer case clicking like it was trying to come out of auto again. Don't know if this is normal for these transfer cases or not. Thought once you go in and out at highway speed it has to do that normally to maybe reset or something?

If I'm setting still in the driveway, not moving, and I turn the knob I can hear the transfer case working in and out of Auto, 4wd, etc. and I turn off the igniton, then it doesn't do it.

Seems to only do it when I am actually driving and I go out and back in to 2wd. And then it seems to come out fine, I never notice any noise or tire hop etc on the road, but when I turn off the ignition I can hear the transfer case cycling or clicking. Tonight I used Auto about 45 minutes before I got home, and when I turned off the truck, I opened the door and heard the transfer case clicking. I figured that with someone knowing the wiring or how the system works, electric, vacuum, etc of the 4wd switch and system someone might know what is going on or if this is normal or not.

Just hoping that someone could explain how the Tcase works and could offer some insight on if this is normal or not.

Thanks in advance.

Nytemare · April 5, 2010

Hopefully someone can explain how this Auto 4wd system works.
I have a GMT 900 Silverado 1500 with the electric Auto transfer case. I have noticed that sometimes after I have been in and out of Auto or 4wd at highway speeds, when I turn off the ignition I hear the transfer case cycling/clicking.

For example, tonight I had to use Auto for a short distance at highway speeds and it seemed to go in fine, heard and felt the "clunk" as it went in, and it seemed to come back out into 2wd fine, didn't notice any issues, sounds when turning into the driveway when I got home like it was still in Auto. Lights on 4wd dial show correctly for 2wd, auto, 4wd, etc. When I turned off the ignition, with the knob already back in 2wd, I heard the transfer case clicking like it was trying to come out of auto again. Don't know if this is normal for these transfer cases or not. Thought once you go in and out at highway speed it has to do that normally to maybe reset or something?

If I'm setting still in the driveway, not moving, and I turn the knob I can hear the transfer case working in and out of Auto, 4wd, etc. and I turn off the igniton, then it doesn't do it.

Seems to only do it when I am actually driving and I go out and back in to 2wd. And then it seems to come out fine, I never notice any noise or tire hop etc on the road, but when I turn off the ignition I can hear the transfer case cycling or clicking. Tonight I used Auto about 45 minutes before I got home, and when I turned off the truck, I opened the door and heard the transfer case clicking. I figured that with someone knowing the wiring or how the system works, electric, vacuum, etc of the 4wd switch and system someone might know what is going on or if this is normal or not.

Just hoping that someone could explain how the Tcase works and could offer some insight on if this is normal or not.

Thanks in advance.

What makes you think it was the transfer case that was clicking?

It has an electric actuator that engages 4WD, Depending on the case you have, a 246 or a 249, the Auto feature uses a Viscous coupling or you use a clutch pack for the auto feature, and the 4HI/LO uses mechanical drive.

249 is the Viscous coupling.

brims · April 5, 2010

Does the Tcase even do anything when you go into Auto? I would think it would only do anything when it detected you needed 4wd.

gmtech4 · April 5, 2010

i wrote a novel about it on another post resently,go look,

Nytemare · April 5, 2010

Does the Tcase even do anything when you go into Auto? I would think it would only do anything when it detected you needed 4wd.

It sure does...it engages the clutch pack, when slippage is "detected" the T/C is cluched in and puts power to the front. When it goes into 4HI/LO that clutching action is taken out of the equation, and the front axle gets power no matter if there is slippage or not.

i wrote a novel about it on another post resently,go look,

This is very true...

andyems · April 6, 2010

I'm not sure which Tcase I have, but will look tomorrow. Its the new body '07 with the dial switch on the dash between the steering wheel and DC outlets. It has 2hi, Auto, 4hi and 4lo settings, but again not sure which actual Tcase it is.

All I know it after I used Auto the last several times, I get out at home and hear the Tcase cycling/clicking under the truck. It's the same sound as when I have been sitting in the driveway with just the key on, no motor running, and cycle through the settings and can hear the case clicking from 2hi to auto to 4hi, and back, etc.

Last night I went from 2wd to auto for about 1/4 mile and then went back to 2hi. 45 minutes later when I got home, I turned off the truck and heard the Tcase clicking like it's just then going out of Auto and back into 2hi. This is like 5 or so seconds after the truck has been turned off and they key removed, long enough for me to have the door open and be getting out.

I only hear it after I have been on the highway using it. If I set in my driveway and turn the dial, it cycles/clicks fine then, into and out of 2wd, auto etc. But when I use it on the highway and turn it back to 2wd, later I shut off the motor and hear the same clicking sound. I get alot of clicking from the heat shields, and it's a totally different sound, so I know it's not just that.

I was just thinking it might have some type of vacuum system in it, and once it used once while driving it doesn't completely release vacuum until the motor is totally shut off even though it's actually releasing back into 2wd?

I wish I could explain better what is happening. I'm not very mechanically inclined and know what I want to explain, but have a hard time getting it into mechanical terms for this.

gmtech4 · April 6, 2010

you have warranty left i hope,5 yr powertrain

andyems · April 6, 2010

Yes, I bought it used, 1 owner with 6000 miles on it last May. It has the remainder of the 3yr/36,000 factory warranty until this November, and I bought the extended warranty with it, too. Just want to take it in now if it's messing up, but don't want to waste a day if that's a normal to happen. The dealer is over a hour away and hate to take time and gas to get there for them to tell me it's normal.

nickvrebel · April 6, 2010

Yes, I bought it used, 1 owner with 6000 miles on it last May. It has the remainder of the 3yr/36,000 factory warranty until this November, and I bought the extended warranty with it, too. Just want to take it in now if it's messing up, but don't want to waste a day if that's a normal to happen. The dealer is over a hour away and hate to take time and gas to get there for them to tell me it's normal.

keep in mind that the manufacturer put a 3 yr/ 36,000 mile bumper to bumper AND a 5 yr/ 100,000 mile powertrain warranty on that truck, take it in and have it looked at.

gmtech4 · April 7, 2010

+1,get the description and operation of it printed off for you while you are there,

BrockwayMT · April 8, 2010

The transfer case is just relearning when you shut it off after being in auto. Don't worry about it.

The NQH doesn't necessarily need to detect slip to apply torque to the front. It is preemptive in certain situations. For example if you floor it on dry pavement in 4auto you'll get torque to the front axle before slip occurs.

andyems · April 9, 2010

Thanks to all for the ideas and help. I will schedule an appointment with service next time I get an oil change and at least ask them about it. My main concern was that the AUTO was not disengaging after turning the dial back to 2wd. I used it the other night, and 45 minutes later I get home and turn off the engine and hear the T-case servos cycling. My first thoughts were that I had drove the last 45 minutes in AUTO because it was messed up and hadn't switched back into 2wd. I know it does not hurt to drive in AUTO but I want it to work right and not waste gas.

I have messed around with it on my drive home today. Every time I'd go to auto I could feel the clunk of it engaging. I'd turn it back to 2wd, a mile or so up the road try auto again, and feel it engage, so it always seemed to be in 2wd as the dial read and engage as normal when I'd turn it to auto. I drove up the neighbors gravel driveway today and again it seemed to be working fine in 2wd and auto.

Aside from the servo cycling/clicking noise when I turn off the engine after being in and out of auto, I can find no fault or problem when I've been trying to diagnose the problem. It's not throwing up and codes or anything in the DIC. I never heard it this winter, but it could have been there the whole time. I just heard the noise after using auto and then once I started listening for it, kept hearing it all the time.

I also learned how the AUTO feature works. I was under the impression that in auto the front wheels had power applied to them, but in the auto setting the center differential was just unlocked so they could turn independently of the rear and by using actual 4hi the center differential was actually locked. I had no idea that it was an on-demand type system. Thanks again to everyone for the help and input on this.