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  1. #1

    Default 4 wheel drive problem

    1990 Silverado 3500 7.4L - we have recently had over 20 inches of snow and my driveway is solid ice but the main roads are in good shape at the moment. Calling for 2 to 4 inches of sleet tonight. So this morning I put the truck in 4 wheel drive to back out of my driveway and when I stopped the light on the floor shifter went back off, thought maybe it was just a bulb, put it out of 4 wheel drive and went to work. When I got home this evening I found that the 4 wheel drive is not engaging at all. Several years ago I had the (transfer switch?) replaced. The mechanic had bought the newer design which was supposed to engage faster but found that it would not fit on a 3500 and had to replace it with an original OEM part. Recently it has been taking longer for the 4 wheel drive to disengage or at least it has been taking a few miles for the light to go off. My first thought is a vacuum line somewhere. Does anyone have a diagram for the vacuum lines? Looking for any suggestions on where to start to diagnose the problem and hoping for a quick fix.


  2. #2


    You could be right about a vacuum line, some GM trucks use a vacuum "motor" to operate the disconnect in the front passenger side axle.

    '09 Avalanche LTZ - Black
    '05 Envoy XL (sold)

  3. #3


    Quote Originally Posted by RayVoy View Post
    You could be right about a vacuum line, some GM trucks use a vacuum "motor" to operate the disconnect in the front passenger side axle.
    My parents have an 83 S10 Blazer that had the same problem a few years ago which was just a vacuum line. On it a canister and everything was on the passenger wheel well which was easy to find. Popped the hood on my truck when I got home just as it was getting dark and didn't see any obvious problems. Too much snow to crawl under the truck this late in the evening.

  4. #4


    you might have to unscrew the actuator out of the front axle. Then put in 4x4 and see if the piston extends

    1995 Silverado 4x4
    6" BDS Suspension Lift-3" Body Lift-Add A Leaf in rear -Trailmaster SSV Shocks-Duel Steering Stabilizer Kit -AirAid Cold air intake-
    4.56 Gears with Detroit Auburn Locker-Pro-Comp Traction Bars with duel shocks-Aluminum Skid Plate Kit-38.5" x 16.5" Mickey Thompson Baja Claws-Constant Dropping fuel gauge

    2005 Yukon XL Jet Power Programmer, Bilstein Shocks, Bilstein rear springs, Helwig Anti-sway bars, EGR Window Visors, EGR Hood Shield, Denali Headlights, Headlight harness upgrade, GE NightHawk Bulbs, White Night Rear lighting system, Russell Braided SS brake lines, PowerStop Brake pads, PowerStop cross drilled and Slotted Rotors,
    2002 Silverado ext cab 2wd (Sold)
    2003 Yukon XL (Totaled)

  5. Thanks 08silverado3500hd thanked for this post
  6. #5


    Turns out it was the actuator. Replaced it last Friday. Now the old truck is ready for the next snow storm. Last Wednesday morning it had just started raining on the way to work and I hit a patch of black ice on a turn. Only doing 30 mph had it sideways to the left then the right but couldn't recover and ended up head-on into the ditch and road bank. Lost my fog lights under the bumper and bent up the driver side running board. Figured I was going to be stuck in the ditch since the 4 wheel drive wasn't working then but a little rocking and it came out on its own.

  7. #6


    Had the universal joints replaced in the rear drive shaft last week. Been hearing a small clank the last couple of months. Could not feel any play/wobble in the drive shaft but at the garage they saw some rust in the caps. Turns out the 2 caps with rust were missing a lot of needles. The guy at the garage was surprised that he couldn't feel any play either. Forget what brand universal joints they put in but now I have grease fittings on them. While on the lift found that one of the U-bolts on the springs-rear axle had broken in the accident. Noticed the bolts had puffed up a little with rust a while back just above where they go down through the plate under the axle. Had 4 new bolts made up and replaced them all. All the bolts where almost half rusted in two right where they go through the bottom plate. Truck is a 1990 and has been on the salty roads of WV all its life. Next time you are under your vehicle take a quick look at you U-bolts and clean off any rust for a good inspection of the bolts. Always better safe than sorry.

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