4WD will not shift into 4Hi


Asked by May 11, 2016 at 03:24 PM about the 2002 GMC Sonoma SL Ext Cab 4WD

Question type: Maintenance & Repair

Will shift into 4Lo and then into 4Hi, but not directly to 4Hi.  Have disconnected battery cable for 5 min to reset TCCM - same symptoms.  Seems like if it were vacuum line issue, it would not shift to 4Lo???

1 Answer


Will attempt to clarify my issue and question: 2002 GMC Sonoma with 3-button shift on dash - 2Hi, 4Hi, 4Lo Was able to shift directly from 2Hi to 4Hi, but stopped working last year. It can still shift from 2Hi to 4Lo (while stopped and in neutral) Once in 4Lo it can then shift into 4Hi Also have to be stopped and in neutral to shift from 4Hi back to 2Lo, then from 2Lo to 2Hi. It has basically lost the ability to shift on the fly. My question - DOES THIS SOUND LIKE A KNOWN ISSUE WITH THE TCCM? Research shows the most common problems to be vacuum lines, vacuum actuator or TC vacuum switch (ball stuck). My logic is that if the problem was with the vacuum system - it would not shift to 4Lo (or anything) --- maybe this is not good logic? I hate to just start changing parts and hope I find it. I'm unemployed at the moment and can't afford to pour lots of money into this truck. Special thanks to ANYONE for helping me out with some advice or benefit of their experience.

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