Weird 4wd issue. Stuck in 4hi!!!!

[Absmithing]

Hi all. New here. Another one drifting over from trailvoy.

I've got an odd 4wd issue. I'm stuck in 4hi right now, but if I pull the atc fuse the actuator retracts. Then I can pull the 4wd drive fuse, replace the atc fuse and I'm back in 2wd. (Possible temporary solution to someone stuck in 4wd) The selector switch keeps wanting to select 4hi no matter what I do. With 4wd fuse pulled the 2hi light is solid, 4hi keeps blinking. If I replace 4wd fuse it shifts back to 4hi and won't shift anywhere else. A4wd blinks and blinks then eventually solid, but is obviously still in 4hi. 2hi blinks for about 10 seconds then goes solid on 4hi. No service lights.


So I obviously have an electrical issue, since I can trick it into 2hi. I can hear the normal noises from the encoder motor, so I'm left with the tccm and selector switch as far as I can tell. Any body have an idea which one? Or if there is something else going on here.

Any idea why the actuator would retract when the atc fuse is pulled?

Thanks for any help.

 

[Hypnotoad]

I just want to say welcome!

Sounds to me like the selector switch is bad, but I'll leave that to someone with more expertise than I have.

 

[Juicy K]

I would like to welcome you as well, glad you found this site.

I as well would suspect the 4wd selector switch (seems to be a common piece to go bad), I have AWD so just give it a little bit, I know somebody with first hand experience will chime in.

 

[The_Roadie]

I agree it's 90% the switch or the wiring back to the TCCM, 10% chance it's the TCCM. Everybody should buddy up with a local GMT360-owning friend to swap parts like this with for troubleshooting.

The front axle actuator is powered by REAR fuse #48 (4WD). I never noticed before, but iif the TCCM and encoder motor (front fuse #8) is pulled, that would essentially ground the control wire to the front axle actuator, which would command retraction. Since the actuator still has power, that explains it.

You could always freeze the system into 4HI more but without the front axle connected through the splined disconnect, by also pulling rear fuse #48 at the proper time.

 

[Absmithing]

Hi guys. Thanks for the info. I replaced the switch today (had to buy from GM since between all the parts stores they only had ONE in all of Canada on the other side of the country). Didn't fix it so I traced the wiring from the actuator to tccm, and what I could see from the switch to the tccm. Didn't see anything out of place or damaged. Opened the tccm, didn't see any damage inside from what I could see. Guessing my next step is to go to the junk yard for a tccm.

Is it possible something is off with the encoder motor?
Also had a thought that this occurred not long after putting in my aftermarket HU and subs. If the gmos harness has a fault could that affect it?

Thanks for the breakdown of why the fuse pulling got me out of 4hi Roadie. Might be a possible addition to your sticky on 4wd diagnosing( thanks for providing that, it's a tremendous help)

Thanks again.

 

[Absmithing]

Got the tccm today for $25, but no luck. Still doing the same thing. Any other ideas?
I'm wondering if something happened when I got the alignment done the day before this started. Drove home in 2hi flipped to 4hi to get in the garage( skating rink in the alley) flipped to 2hi and shut it off. Drove to work the next day and when I got there I noticed the selector was in 2 hi but light was on 4hi.
There is a whirring noise from the right front tire that changes with speed not rpms, could that be involved.

I really need some more help here.

 

[The_Roadie]

The alignment tech could have messed with wiring, that's a possibility. Whirring is never good - but maybe he just bent your brake dust shield plate and it's touching the rotor. Or he screwed with the actuator harness.

To troubleshoot this properly will require a Tech II scan tool to command the actuator and encoder motor to specific positions and see if they succeed or fail. Or you could take the actuator off the splined disconnect and the encoder motor off the transfer case and then see if the TCCM will succeed or fail in moving them in mid-air, not restricted by resistance from the vehicle. The actuator may be frozen or full of congealed grease? A hair dryer might help.

You changed the switch, but haven't put a meter onto the TCCM to see if the switch is sending the right signal to the TCCM I expect. Have you downloaded the schematics, and do you have a meter and some electrical troubleshooting experience?

 

[Absmithing]

I had my upper and lower ball joints done a few days before the alignment by a friend who is a mechanic and he checked the front end parts and couldn't find anything else that was worn. I'll check on the dust shield, that's what I was thinking it is.

First thing I replaced was the actuator since when doing the initial diagnoses it made no noise. Hooked the new one up and it didn't move either so tried the fuse reset of the tccm, but while on run, which is how I discovered that the actuator retracts when it's pulled. So it doesn't move with the switch but it does fully extend and retract with the fuse pull. Something is telling the system to be in 4hi no matter where the switch is. Even with the fuse pulled and the light solid on 2hi, the light above 4hi continuously blinks.

I'll check the schematics out and go through the testing to try and see what's going on.

Thanks for the help
Ill post my results.

 

[IllogicTC]

I myself smell faulty wiring somewhere.

The actuator would retract because it is powered "independently." Instead of being a system where power makes it go one way and removing power retracts it, it's considered always hot. The system is controlled by receiving a signal from the TCCM to either engage/disengage. The two states are set by a low signal and a high signal. The feedback loop will send a low or high return signal, depending on the current position of the probe. What this means is that, assuming the logic in the actuator isn't screwed, the problem lays in the command/feedback wiring. The actuator itself gets power off a different fuse (48, rear fuse block), as compared to the TCCM module.

The signal on the light-blue wire will go high to command extension, low to command retraction.
The signal on the feedback wire (black/white) will be high if the unit is retracted, low if it is extended.

Using a tester, you can see the different states of the wires. I'm feeling like something is affecting the light-blue wire to make it go high, it may be that a bare spot is contacting a bare spot on a hot wire or something. Follow both wires as much as you can and inspect the whole way.

 

[Tofer76]

I had a similar issue with my f250 last spring ( got up and it was in 4lo and wouldn't come out but the front wheels wernt locked in )?? everything I did wouldn't work . ended up being the transfer case relay not sure if ours has the same relay but ..... in my truck I went through a selector switch, a tc motor, a pcm, and numerous fuses before I found this out , cost me around 500 to find out it was a $40 relay

 

[Absmithing]

Thanks for all the info guys. Probably not a bad thing to have the extra parts that I have now. Plus I know a lot about the 4wd system now which is a big plus since this is my first 4wd.

I'm guessing that roadie and illogic are right about there being a wiring issue since the rest has been ruled out. Makes a lot of sense about the signal wire being the issue. Now I just need some time to get it checked out. And hope I can find it.

I'll let you guys know what I come up with when I get a chance.

 

[The_Roadie]

... f250 ... not sure if ours has the same relay but .

No relay. The controls and switching are all done in the TCCM.

 

[IllogicTC]

Good luck. Remember to check it all thoroughly - after all, if you can fix it yourself, fixing a wire literally costs pennies as compared to the dealer/shop jerk-around.

 

[Absmithing]

Update:

No luck finding any faulty wiring that I can access. Double checked tccm, axle actuator, fuses, etc. about ready to take it in to get checked. Been spending more of my free time lately sorting out and fixing my wife's caravans transmission issue. I must say I love limp mode I did manage to get a $600 job done for $32 though making the $200 scanner I bought worth it.

A thought I've been having. The encoder motor makes noise 2-3 times when I try to switch from 4hi. Is it normal for it to try 3 times? Could the encoder motor be failing to shift stopping the actuator, or are they completely separate?

 

[IllogicTC]

The two systems should be independent. If the actuator fails once, the TCCM "gives up" and throws a service 4x4 instruction to the instrument cluster.

 

[Absmithing]

I was figuring they were independent, just hoping for another option.
The actuator is what's got me stumped. It doesn't even try to move. Still no service 4wd light.
How does the fuse for the actuator tie in to the loop? As far as I can tell the wires from the actuator go from the actuator to the tccm.

 

[The_Roadie]

The front axle actuator runs off REAR fuse #48. If it fails, the clue is that the "N" light on the switch fails to light up during the lamp test time when the ignition goes on. It's in the schematics. Thought it would be obvious, but I can see how it might not be. That's why I always ask if ALL lights light up on the switch during the lamp test. The "N" light counts.

The TCCM and the transfer case encoder motor run off FRONT fuse #8.