4wd wont dis-engage

RowdyRuss · November 12, 2019

I'm new to the forum so forgive me if this has been discussed or answered already. I searched but could find nothing.

I have a 2003 silverado 2500 hd LT gas 8.1L crew cab 4x4 allison transmission with 208k miles.

I live in Michigan but I bought the truck from arizona and it had approx 175k miles on it when purchased. It has been through one winter here in Mi. It had no rust or corrosion anywhere including the frame. Very clean truck, well maintained and taken care of.

Recently the service 4wd message came up. I promptly had it scanned for error codes but none came up so I asked the service tech what he thought. He said it was more than likely nothing more than a reminder to service the fluid in the transfer case and differentials, so I figured no immediate emergency, I'll get it done in a month. Shortly thereafter I started having probs with 4wd engaging. I would push 4wd H or 4wd L, the light wld flash for a bit and revert back to 2wd position w/o ever engaging 4wd. I could hear the encoder motor cycling but it wld not engage. I thought ok it's prob the actuator. However, if I shut the truck off, restarted it and pressed the 4wd switch in the first few seconds it wld engage into 4wd H or L. I then began to think transfer case control module or perhaps a faulty 4wd switch. In quick succession the 4wd wld not engage at all, the encoder motor stopped cycling and i lost the lights on the 4wd switch. It was time to remedy. I started with the body ground beneath the driver side door. Ground down to bare metal, new terminals and a fresh coat of primer/paint to cover. Checked for and had continuity and had a nearly perfect ground at .01 ohm. No help. On to the 4wd switch (which btw is a 3 position switch w/o auto 4wd) replaced it and still had no lights on the switch. On to the control module, replaced it with remanufactured from doorman and holy wah, I have lights on the switch. Then tried to engage 4wd H and it went into 4wd low and there it stayed. Wld not go into 4wd H or 2 wd and the encoder motor wld no longer cycle. Getting frustrated at this point so I replaced actuator and encoder motor, in that order. Had to place trans in neutral and manually shift tc spline into 2wd (shifted easily w/o resistance other than detent) in order to install new encoder motor. Started the truck, pressed 4wd H switch and SOB if it didn't shift right back into 4wd low again. I AM able to shift between 4wd H and 4wd L but ONLY when restarting it in neutral. Will not work when in park. The lights on the switch will not light when pressed until I restart. Yes I tried it both while rolling under power (on the fly) and sitting still. It will NOT shift into 2wd under any circumstances and the service 4wd message is still present. It should be mentioned there was a "shift range inhibited" message that flashed briefly then went away, right after installing encoder motor and attempting to shift into 4wd. The good news is the switch, control module, actuator and encoder motor are all working, the bad news is they are not working as they shld and I'm no closer to solving the issue other than the process of elimination.

In summary I have verified good ground, replaced 4wd switch, control module, actuator and encoder motor in that order. Am I missing something??? It leaves me with more questions than answers. Intermittent electrical short? Pcm? linkage? safety switch? Faulty remanufactured control module? Trans? Tc? Ect. Ect. I don't have the extra time or money to throw at it and I'm losing money and business every day it's down. As of today I lost a plow contract with a local business that I had in the bag as this is my personal plow truck and we were just hit with our 2nd plowable snowfall of the season. The next step today will be draining and replacing the fluid from the tc and inspecting old fluid for metal shavings. I hope someone has the answer and I know if there is an answer I'll find it here.

Edited November 12, 2019 by RowdyRuss
typo in title

Formulabruce · November 12, 2019

Is the switch new? and your truck has the auto 4wd, right?

This sounds like a bad rebuilt encoder motor.

Check your T-case for any leaks in the case, they are known to wear through the case, and there is a fix for it, but you have to take apart the T case.

Good luck !

RowdyRuss · November 15, 2019

Yes the switch is new and no it does not have auto 4wd. I stated both of these things in my question. 3 position switch means no auto 4wd. The three positions are 2wd, 4wd H, 4wd L.

A 4 position switch has those plus auto 4wd.

RowdyRuss · December 9, 2021

UPDATE. Sorry its been almost 2 yrs since The issue was fixed and I forgot to post it. Hope this helps anyone with the same issues.

I ended up replacing the entire 4wd system. Encoder motor, 4wd switch, front axle actuator, TCCM, etc. initially I used aftermarket parts (doorman), then returned them and went with OEM in case the aftermarket parts were junk, none of which helped the issue.

Finally I brought the truck in to have all the control modules checked 1 by 1 since there were no dtc codes. It turns out the ground for the TCCM had a high resistance. All this headache and $2k spent in OEM parts and diagnostics for a ground with high resistance. At least the issue was identified and repaired. Hope this helps someone.