4wd Not Working, Where To Start?

tackepj · August 11, 2007

I have a 95 K1500 W/T, 142k, with Insta-trac and the lever on the floor. I tried 4WD-hi yesterday on a dirt road, going about 15 MPH, but it never engaged. The lever moves fine. I tried while stopped, too, and it never engaged. Tried forward and reverse. The light never went on, and I could tell it never engaged (still could spin the rear tires). It shifts fine into low range, but still not into 4WD. I can tell the hubs aren’t locking either. There’s no clunk or noise or anything, it just won’t engage. No binding around corners. I never had this problem before, and it worked fine in the snow all winter. Any ideas? The only thing I can think that’s changed is I had the original clutch replaced a few weeks ago. Not sure where to start. Thanks

Brian L · August 13, 2007

If it is like my 88, on the front differential there is a sensor that screws into the side that has 2 wires on it. I had the same problem once and that was it. You can get a new one at a parts store (Napa) but they arent cheap. I remember it cost about $80. If its original that is most likely the problem.

vortec22 · August 13, 2007

I am not sure how the front end is actuated on this truck, I had an S-10 that locked the front halfshafts with vacuum, there was a vacuum switch on the transfer case that ran to a diaphragm to pull a cable. If it is similar I'd looked for vacuum lines that are not hooked up or leaking. Same if it is electrical, something that didn't get reconnected after the clutch fix.

tackepj · August 15, 2007

Just an update. I know for sure that the hubs aren't locking and that the front driveshaft isn't turning. I put the truck in low range out in a field and let it creep in 1st gear, and I hopped out and the shaft is definitely not turning. Same thing in reverse. Kind of surreal to watch your truck drive along without you in it...

The only fuse (25 A) I could find is in the dash panel by the steering wheel, labelled 4WD or something, and it's fine. There is a space for one labelled "transfer case" but it's empty. Must be for the push-button T case or something. I called the shop that did the clutch and they're not sure, but asked me to bring it in.

Any idea how the system works? Do the hubs lock with a solenoid or something or do they need input power from the front axle?

98 Z71 SLT · August 15, 2007

One of 2 things.

Faulty actuator on top of the front diff is worn out.

Or

Bad vacuum switch on top of the transfer case.

Try the actuator first...85% of the time it is the fix.

If no 4x4 after the actuator, repl the vacuum switch

dgilfillen · September 2, 2007

One of 2 things.

Faulty actuator on top of the front diff is worn out.

Or

Bad vacuum switch on top of the transfer case.

Try the actuator first...85% of the time it is the fix.

If no 4x4 after the actuator, repl the vacuum switch

On the fullsize, the front actuator is electric, not vacuum..

When in 4wd mode, a switch on top of the transfer case conducts 12 volts out to the actuator. Wire #50 (brown) is the 12v feed, and it conducts to the light blue wire (#900) to the actuator.

When shifted out of 4wd, the switch opens up and no voltage goes to the actuator.

I would check voltage at the connector for the front actuator (at the front differential - it looks like a large bullet that threads into the front diff.) If it has 12 volts, the actuator is the problem. If it does not have 12 volts, check the transfer case switch and the power feed to the transfer case.

Let us know what you find.

dgilfillen

tackepj · September 15, 2007

dgilfillen,

Thanks for your help.

I checked voltage at the connector near the front diff, and it's getting 12 volts when in 4WD. I also went and re-checked the driveshaft, and I was wrong, it's definitely turning when in 4WD. The truck still isn't engaging into 4WD though. The hubs aren't locking, and I can drive in 4WD for a while and it never goes in. Looks like the T case is fine after all, so it's probably the actuator, right? Is that hard to replace? Any tips?

Thanks,

Phil

dgilfillen · September 15, 2007

dgilfillen,

Thanks for your help.

I checked voltage at the connector near the front diff, and it's getting 12 volts when in 4WD. I also went and re-checked the driveshaft, and I was wrong, it's definitely turning when in 4WD. The truck still isn't engaging into 4WD though. The hubs aren't locking, and I can drive in 4WD for a while and it never goes in. Looks like the T case is fine after all, so it's probably the actuator, right? Is that hard to replace? Any tips?

Thanks,

Phil

Phil,

No problem. I assume it is the actuator, if the plug to the actuator is getting 12 volts when in 4wd.

The way this actuator works, is it is a sealed chamber that gets heated by voltage. When it heats, the pin in the end swells and pushes a shift fork in the front differential. The shift fork engages a spline on the passenger side of the differential, and boom! you have 4WD.

GM offerred an upgrade to this actuator that is a motor instead of a heated device. However, it requires a small harness addition, a spacer, and a new actuator. It is about a $150 to $200 option.

If I were you, I would buy a new heated actuator. They are available at most auto parts stores, and are about $90. They are also on eBay for about $55 plus shipping.

http://cgi.ebay.com/ebaymotors/ACTUATOR-4X...sspagenameZWD1V

These are very easy to change - just thread out the old one, and thread in a new one.

Let us know how it goes.

Dan

wrathtag · September 27, 2007

That actuator is usually the problem 90% of time, ive replaced 2 of them on my '95 k-1500

tackepj · October 8, 2007

Update:

Thanks for the help! It was the actuator. I got the new part from NAPA, it was $96. Sure was an easy repair, and 4WD works fine now.

Interesting that GM would use a heated actuator. I just figured it was a little solenoid in there. Oh well--I know it's the original, and it lasted 12 years and 140k+, so that's pretty good life. Probably the only time I'll ever have to replace it.

Thanks again.

mikeleg · July 11, 2011

im having the same problem,but mine is popping fuses.... i replaced the actuator and still does it.. how can you test the switch on top of the transfercase if it keeps popping fuses time you throw the power on ? thanks for any info i have an 89 k1500

Update:

Thanks for the help! It was the actuator. I got the new part from NAPA, it was $96. Sure was an easy repair, and 4WD works fine now.

Interesting that GM would use a heated actuator. I just figured it was a little solenoid in there. Oh well--I know it's the original, and it lasted 12 years and 140k+, so that's pretty good life. Probably the only time I'll ever have to replace it.

Thanks again.

sk · July 12, 2011

Well I found this thread a little late to help some of you, but if possible replace the thermal actuator with the motored one. I did it a few years ago and it only cost me around $100 (found the actuator on Amazon for around $60 and the harness on ebay for around $40). Not only is the new one more reliable, but it engages INSTANTLY instead of having a 15-30 second delay like the original one does.

Here is an old thread where I supplied a link to a great tutorial by a previous member:

http://www.gm-trucks.com/forums/index.php?showtopic=90640

Check Post #6. Unfortunately most of the links have been changed since the post is over 2 1/2 years old! I didn't have any luck locating the tutorial but the Amazon link for the actuator is still valid.

justin35 · September 30, 2011

i have the same problem on my 1990 1500 i pulled the actuator out plugged it in turned the key on the i can see the actuator push out but it still wont engage the front diff. i dont know what else to do.

schepacarter · March 7, 2014

justin, sorry that no one answered you,but this is for everyone. I have been trying to find out why my 4x4 is not working for weeks. number one, replace the actuator. they go bad consistently and you should really look into getting the electronic motor one if at all possible. my truck does not have that option. if you are not getting power to the actuator and it is still not working after being new, then go to the switch. the transfer case switch is on the driver side middle of the transfer case. replace that, and if you are still not getting any power, then you have to check the fuses. i've been doing this for weeks and still can't figure out mine, but i hope i helped everyone else.