4x4 Won't Engage -- Opinions and help needed

Discussion in 'Chevy C/K Truck Forum' started by Strino78, Apr 17, 2013.

  1. Strino78

    Strino78 Member

    About a month ago my 4x4 quit working. I checked the 25A fuse and it was not blown. I am able to put it into 4-LOW, and the transfer case engages -- as my rear wheels are running in low gear. I suspected the 4WD actuator to be the culprit, it is the 2-pin thermal style. I changed that out today, and my 4WD still doesn't engage. I'm seeking advice and suggestions as where to look next.
  2. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    One of the more common failures is the passenger axle disconnect.

    It is disconnected in 2wd and operated in all of the 4x4 positions.

    It's purpose is to lock the outer and inner axle halves together. Without it being locked, the inner axle shaft spins and the truck acts like it is in 2wd.
  3. zippstripp

    zippstripp Member 1 Year

    Has it blown the fuse anymore? What year and model are we workin on?
  4. Strino78

    Strino78 Member

    Thanks for the suggestions - this is a 1994 GMC K1500 - and I'm happy to report I have resolved the issue.... but I'm somewhat
    baffled... as what the fix was... the bulb in the floor mounted 4WD shifter display.....

    My friend and I used a test-light to test the electrical. We were getting power at the 25A fuse (which has never blown), power at the 4WD actuator, and power at the transfer case switch. (.. if it wasn't for the 3" body lift, I wouldn't have been able to get to the top of the transfer case to test the electrical connector... talk about a tight spot...) We removed the NEW 4WD actuator from the front differential, left the electricalconnector attached and engaged the 4WD – the actuator did notextend. We tested the old actuator the same way... same result. Iapplied 12V direct power to both actuators, and both of themworked... the old one was slower than the new one, but it worked.

    While we were both under the truck..tracing electrical connectors .... my friend noticed that the wiringharness that runs to the floor shifter. My friend is a Ford guy...and I am an old-school Ford/Dodge man too. Neither one of us weresure what was under the floor shifter... sensors or what-have-you. Iremove the floor shifter cover.... just bulbs... after going throughthat effort.. I decided to check them and I discover that the 4WDbulb is burnt out. Okay... no problem.. I have spare bulbs on theshelf and I replace the 4WD indicator bulb. Just for giggles.. myfriend says “hey, you should see if the stupid thing works now...” I crawl back under the truck... connected the old actuator to theharness....holding it in my hand .. my friend pulled the lever backinto 4-HI... after 15 seconds.. the 4x4 light came on... and theactuator extended outward!?!?!

    It was a bit of a surreal moment – Ididn't believe what was happening. With the NEW actuator in thedifferential and the electrical connectors fastened – I put therear of the truck up on axle stands to give the front wheels a testengagement. I put the selector into 4-LOW... heard the heavyclunk.. 4x4 indicator light was on... put it into drive.. and allfour tires were turning... Same results for 4-HI. We were both inan immediate denial moment – dropped the truck on the ground,backed it out of the shop and tested the 4x4 in the driveway and aquick run down the road. Sure enough... it works.

    My curiosity got the better of me... Iremoved the 4x4 indicator bulb... and once again.. the 4x4 stoppedworking.... the old actuator would not extend. The only conclusionwe both came to is that there's some kind of ridiculous fail-safe not allowing the 4x4 to engage if the bulb is burnt out. Bothactuators work if tested with 12V power directly... but neitherworked if connected with the burned out bulb in the 4WD floorconsole.... even though the harness shows power with a test light.

    Strange... just strange... $65 part gets trumped by a $0.39 bulb.

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