I am a current owner of a 2008 trailblazer with an inline 6. Current miles is about 60k. I bought it used about 18 months ago. It has been a great vehicle accept one thing....the 4 wheel drive turns on and off intermittently. This has happened off and on over the course of the last 12 months. This will happen during highway driving as well as slow driving such as in a parking lot. While this is happening the 2 to 4 wheel light will light up both the 2 and 4 wheel lights on the switch but it is in 4 wheel drive. The actual switch is still in the 2 wheel drive position. It also flashes at times as well while this is going on. The service light comes on that that looks like there are 4 wheels with a wrench as well. When I shut the vehicle off everything resets and it seems as if it is back to normal. I have taken it to the dealer but they say there are no stored codes as to the problem. Any ideas would be helpful.
Phantom switches from 2 wheel drive to 4 wheel drive
- Thread starter Fpatka
- Start date
The dealer is an incompetent goof if they believe all problems need to store a code before they can be properly troubleshot. 99% probability it's the switch, the harness from the switch to the TCCM, or the TCCM. You might get it to work better if you rotated the switch CW and CCW 100 times or so to clean up the contacts. The switch used to be less reliable but they improved the design around 2005 and you have one of the good ones. If there's any warranty, get them to change the switch and inspect the harness. The redesigned switch is also cheaper, and you can get them on Ebay for under $20 usually, and even at the dealer for $35 or so. Whatever you do, don't let it go into 4HI mode while you're driving because it will chew up the transfer case clutches in short order.
Thanks for the tips...I have checked the wiring all is still in like new condition. I will exercise the switch several times to see if it was a contact issue. As far as "letting"it go into 4h while driving at hi speeds...that seems to happen by itself with out my interaction with the switch. It has not gone into 4L. Just seems to go into 4H by itself (switch is still in 2H position) but the indicator lights lit at the time are both 2H and 4H until I turn off the vehicle and restart it. Then it normals up. I have also, when it is in this this state, slowed down and tried to manually move the switch to 4H and then back to 2H with no changes to the indicator lights. It has on occasion taken itself out of this condition and normaled itself up on its own while driving.
The GM designers goofed when they made the switch control circuit tell the TCCM to go into 4HI mode when the contacts get oxidized or the wiring harness goes open. A more safe position would be to force it to 2HI under those conditions. However, they DID get it right when then installed a double interlock to make sure it NEVER goes into 4LO unless you are rolling less than 3-5 MPH AND the gearshift is in neutral. That way it will never destroy itself and cause a certain crash by going into the 2.7-to-1 low range on the fly.
No. The TCCM diagnostic should fail and lock out all mode transitions, but a broken wire / open switch will try to send it to neutral mode and it will stop at 4HI.
Pulling the fuse is the definitive way to safe the system in 2HI mode.
Pulling the fuse is the definitive way to safe the system in 2HI mode.