fsxbob

Hi, I know there are quite a few 4wd problem threads and I have read through many of them and still can't solve my issue.

So the truck was throwing a 4wd service code which i scanned with a SnapOn MODIS and it said it was likely the actuator motor on the side of the transfer case. replaced that unit and now it actuates when i press the 4wd hi button. Well for some reason after it engages 4wd the 2hi light starts blinking and it automatically disengages.
I also tried 4wd auto and it seems to work, it won't switch back in 2hi. But then in the rain the other day i was starting on a hill and wheels started to spin so it tried to engage the 4wd automatically and then threw a service 4wd code. The code went away next time i started the car.
So no idea whats going on, I have checked the following things:
1. new actuator, it does actually move when 4hi/4wd auto buttons are pressed. even when removed from case it automatically just goes back into 2hi
2. case seems to have fluid, opened the fill hole and stuck a zip tie in, came out wet
3. removed actuator and pressed the pin in with a screwdriver, seems to slide just fine
4. took selector switch out and re soldered all the pins to make sure nothing was loose

Any suggestions are appreciated

ukrkoz

Ahhmmm... The scanner "told you" it's bad actuator?
You realize those switches are known issue on those trucks? And the way it responds to moisture in the air, again tips towards it? I had mine replaced at 50 or so K miles. Flashing lights and 4WD code. And we have same trucks, one year apart. 'twas $60 or so 10 minute job. 143 K miles on it by now, still runs no problem.

ukrkoz

Oh, and to have my t-s crossed. You have to be a heck of soldering pro to solder tips as they should be. That you tried it, means nothing. Replace the switch.

fsxbob

Those SnapOn computers are a full diagnostic center. They will read the code and then show you the common solutions.

But I will try the switch anyways and update the thread.

ukrkoz

Yeah, they are known headache. I drove around with toothpick jammed between 2HI button and housing for about 2 months, before it finally gave up and I replaced it. Pity will be, if it were just a $60 or so switch and you did all the "recommended" work for nothing.

fsxbob

quick update. replacing the switch did not change anything. still engages and then just goes right back to 2Hi

ukrkoz

DArn. And I was so proud of myself and it worked for me. Sorry, brother.

ToooTall1

First check all your grounds especially the one under drivers door on outside frame rail looking forward. If the grounds are good and you have replaced the actuator and switch then the last one which is always the problem is the module located behind dash panel on the left side. Make sure that you get the correct one by part # listed on the module. I have 3 Silverados with 4x4 and my 2001 had the same problem as yours. Did the normal replacement as per everyone advise and finally took it upon myself to replace the module. Found a used one on E-bay for around $50 as new one from the stealer are beyond my pocketbook. Also sometime the dorman rebuilt actuators are garbage if they come from china so look for a USA rebuilt actuator.

fsxbob

So I finally got the 4wd working on the truck, all it took was reading the service manual to learn how the system works. My problem was I didnt realize there were two motors involved in making the 4wd work. You have your transfer case motor and then the front diff locking actuator. I had to replace both because I wasn't sure which was broken.

So common problems with 4wd as I've learned over the past 4 months, hopefully this helps people diagnose their problems a lot quicker than it took me. If i'm incorrect in any of this information, please tell me so i can edit this.

The Switch
The selector switch is known to go bad quite often on these vehicles. Bad solder joints is what i've heard. you can buy Doorman replacements for about $30 or less on Ebay. If you are too cheap for that however you can also sometimes repair the one that you have buy removing it, taking it apart and then reheating all of the solder joints in the back to make sure they are making proper contact. To remove just lower your steering wheel all the way down, put the vehicle in gear (to get the selector out of the way) and then just gently pull the entire dash off. The switch is easy to access at that point. 2/10 on difficulty scale.

You know you switch may be bad if 1. you don't see it illuminated 2. it doesn't respond in anyway to button pushes 3. your 4wd module throws a code, you can only read these codes with a SnapON MODIS or GM diagnostic tool.

The Front Axle lock actuator motor (aka 4wd actuator)
This motor is a linear actuator that locks the front axles in with the driveshaft. It replaces the old method of getting out of your truck to manually lock the axles in place. It is also very exposed under the truck and can easily be damaged and rusted by debris, salt etc. So this one is a common problem but also easy and quick to diagnose. Its a very loud motor so with the truck off you can usually hit the 4wd auto or hi button and hear this motor engage. But if your not sure, lets pull it out.

Go to the front diff, disconnect the power plug and unscrew the actuator. Once the actuator is out, plug it back in and then push the 4wd auto button. This should make just the actuator move. If it doesn't move, check all your electrical connections and check for harness damage. If it looks good, then chances are the motor is bad. They are about $70-90 depending on the source. This is a link to a video showing you how to remove the actuator, it is about a 1/10 on the difficulty scale.

The transfer case motor
This is an external motor that mounts up the the transfer case. It turns a shaft which engages the front drive shaft and can also put the truck in Hi or Low range. It is also a common problem on these GM 4wd systems.

This one is a bit more difficult to diagnose because it takes significantly more effort to get to the motor. It is located on the transfer case immediately next to the front drive shaft. As well you should not try and actuate this motor when its removed. However it does often throw a code in the 4wd module if its bad, so again SnapON MODIS or GM diagnostic computer will help.

The poor mans way of diagnosing is to just listen for a noise. So hopefully you know what the axle motor sounds like, because this one is must softer sound. So when you hit the 4wd Hi button, first thing the truck will do is lock the front axle. Then it will actuate the transfer case motor to put the truck in gear. So as soon as you hear the front axle motor do its thing, listen for a softer motor spinning, obviously listen near the transfer case.

To replace if needed, you need to drop the front drive shaft to get to the motor. Depending on the amount of rust and your tools, this can be between 1-4 hrs. It took me about 2 hrs laying on the ground with air tools, but my truck is old and fairly rusty. A new motor will set you back about $200, used on ebay typically $70-90. Difficulty scale 5/10 without lift or air tools.


Other common problems
Grounding or harness issues. People say there is a grounding wire to the chassis below the driver side, so check that and make sure its still there and making contact

Check all of your fuses before buying expensive replacement parts! There are fuses in the cabin and the engine bay that contribute to 4wd, check them all twice.

4wd control module. I didn't mess with this. it is deep in your dashboard and fairly well protected from the elements, but on occasion this will go bad. If nothing works and no codes are being thrown, this may be your problem. Its also an expensive part, so don't mess with it unless the evidence points towards this being broken. If you truck is throwing 4wd codes then its probably not this module.

Fletch130

This post was awesome. Can't tell you, it's been three years and 5hrs on the computer today to find this. Thanks, I should be able to find the problem soon. My 4wd engages and I drive 25ft, the "service 4wd" light comes on, and the switch goes into fail safe mode, cuts power, and won't work for a day or two. My guess is a possible ground issue, or the module behind the switch (although you said that wont throw a code). Are you sure about this? You can also add to your post that you can test the 4wd selector switch with a multimeter set to ohms on the two intermost plug pins. There are various readings you should get, this can be looked up on youtube.