1996 K3500 4WD issue

green 788 · December 30, 2015

I could use some help. Long time member... I don't post unless I can't find the info in the archives.

My 1996 GMC K3500 had its 4wd stop working the other day.

Thought the thermal actuator was the issue, so changed it to a new one, but still have no 4wd.

There is power on the line going to the actuator. I have not checked to see if the new actuator is working or not (haven't pulled it out to see if it extends when 12v is applied).

My transfer case has the floor shift, and I'm *assuming* that it's shifting is all manual, with no electronics involved (right or wrong?)...

Transfer case doesn't make any grinding noises... and no grinding or clicking from front end at all.

Just replaced the drivers side wheel bearing/hub assembly before noticing 4wd wasn't working. Could be coincidence, or could the mechanic (my friend) who put the wheel bearing/hub assembly in have done something to foul up the 4wd? I understand if you pull the right axle out too far, a thrust washer can be dropped and mess up 4wd operation--but this was the left (driver's side) axle that got the new hub assembly.

I'm hoping that the fact that I've not heard any grinding or popping is an indicator that the mechanics of the system are all okay. I guess that's not a 100 percent indicator, but still, all sounds well when driving down the road.

Is there some electronics on the transfer case that could be causing the problem? Or is the only electronic part of this system that thermal actuator (which I've already replaced)?

4wd 25 amp fuse is good, by the way, and there is power going to the actuator on the wiring harness side. It's a cheap Autozone actuator (Duralast, I believe) so I guess it's possible that it's a defective part, new from the box. :/

Any help appreciated.

Dan

grampadirt · December 31, 2015

Sorry for this dumb question but how do you know the front diff isn't engaging?

green 788 · December 31, 2015

grampadirt, that's not a dumb question. Truthfully, I don't know that it's not engaging, but I'm going to test that today.

I'm hoping that my transfer case hasn't thrown in the towel... but if the front diff is engaging, and since my transfer case shifts manually (with the floor shifter that you have to fight with sometimes) I'm guessing my only hope for an electronic fix to this problem is out the window, and I've got a bad transfer case or bad front diff.

I'm not hearing any grinding of any kind, though, so I'm hoping that's a possible indication that the gears and related hardware are good. Maybe kidding myself... but hoping.

Will post back later today. Thanks for the question, it is a good one.

Dan

grampadirt · December 31, 2015

Good luck with it. Put it in 4hi or 4lo and check to see if the front drive shaft is not turning,it should not turn when in 4x4 mode as long as the vehicle isn't moving.If it's turning you have problems in the transfer case.

I have to turn the engine off when putting it in 4lo or it grinds like crazy,but it always goes in.Going into 4hi can be done at any speed(I just go slow when doing that though).

grampadirt · December 31, 2015

One question,does the little panel on the 4x4 shifter light up when in 4x4 mode?

green 788 · December 31, 2015

Grampadirt, thank you so much. Your instruction about how to check the condition of the transfer case allowed me a lot of piece of mind. The front shaft locked up well in 4WD, and was loose to spin freely in 2WD. So knowing the transfer case was good was a blessing.

Then of course this meant the problem had to be somewhere else.

I removed the new actuator which I'd gotten 2 days ago at Auto Zone, and pressed a screwdriver inside the hole, and noted that the spring lever that locks the differential in was good, everything was fine with it.

Then I turned the switch key on, and checked the voltage at the plug for the actuator. Voltage was good (present in 4HI, no voltage on it in 2WD). I confirmed that the ground wire to the actuator was good also (with an ohm meter).

Plugging the new actuator back in with the voltage on the wire (truck in 4HI) showed that it was not working. I even re-checked the old actuator too, not working.

I wondered if there was a low current issue with the wiring, which would allow it to light the test light, but not provide enough current to operate the thermal actuator. So I bench tested the new actuator on a battery... and it did not work.

Back to the auto parts store for a replacement, and the new one works.

I'm good to go for now. Next time this happens I'll probably opt for the posi-lock upgrade.

Again, thank you so much for your help.

Dan

grampadirt · January 1, 2016

You're very welcome Dan,glad I could help.In the future consider what is an upgrade to that old thermal actuator..an electric one.The actuator is activated electrically,not thermally.It means there is no delay when going into 4hi or 4lo,and it will not disengage if it gets cold(like a thermal one will at times).There are kits available to do this,I did it and glad I did.

green 788 · January 1, 2016

I will definitely upgrade in the future.

And by the way, I guess the bulbs are out in the floor shift for the transfer case, they don't light. But my 4WD is working great now, I took it to our farm which is a muddy mess in places and all is well now.

thanks again.