Offroad Trailblazers and Envoys

Diagnosing a TCCM or wiring problem

Something not working right?

by Bigemergency » Thu Apr 19, 2012 6:22 pm

I have been having an issue with my 4wd selector switch flashing and going into different gears at "wake Up" causing 4wd service light to come on and truck to lock in 4 low. I took it to a mechanic. He ran a test that came up with code 550. Reccomended changing out encoder motor. After doing this I still had the same problem. I called the dealer and they reccomended the newest software update to correct the issue. I just had that done. Same problem. The dealer put it back on the computer and I have a new error code c0327. It says it has a bad encoder. The options given to me at this time are: swap out encoder with a new one since I did not have this error code prior to installation, second if that does not work consider wiring problems and possibly the TCCM module. At this point my wallet is tired of getting beaten.

Your suggestions ?
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by The Roadie » Fri Apr 20, 2012 8:34 am

Uncommanded mode changes that flash lights on the switch without moving it are often the flaky pre-2005 switches. Did anybody try that?
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by Bigemergency » Fri Apr 20, 2012 12:00 pm

Good to hear from you Roadie. I have a new switch coming today. I'll let you know if that clears it up. Gm ordered a replacement encoder to ensure I didnt get a bad part, as well.
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by Bigemergency » Fri Apr 20, 2012 4:46 pm

Update: So far so good. The brand new switch seems to have solved the problem. I will verify over the weekend. It was interesting to watch the diagnosis computer while the tech was checking it. When it was in 4hi it actually said the gear it was in was 2lo(wich doesn't exist of course), but it said it. After the tech powered down the truck and removed the #8 fuse to reset the TCCM it said it was in neutral at 4lo when he replaced the fuse and restarted the truck.

Lesson learned: Start with a brand new switch with this type of fault. The upside is I have a new switch and encoder with the latest software upgrade for my TCCM. Down side: the $219 for the encoder motor, $90 diagnosis fee and the additional $90 for the programming I paid for when the Encoder motor didn't solve the problem. I wish I would have thought about the switch still being the issue in the first place, but I had already replaced it before.
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by Bigemergency » Sat Apr 21, 2012 12:08 am

Back to square one. The new switch did not reolve the issue. Light came back on this evening and the 2wd and 4lo are on at the same time again. Maybe the new encoder is bad as the code suggests?
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by Trail X » Sat Apr 21, 2012 8:27 am

The switch was suspect because it works off of resistance ranges. It's a really strange design, because any corrosion in the switch can affect the resistance and therefore throw off the TCCM.

Now, seeing that you're saying two lights are on... It'd really help if you could explain everything that's going on. Is the 2WD light solid, while the 4LO is blinking? Where is the switch pointing? Were you switching into a certain range from something else?

When you do attempt to switch modes, what happens? Do you hear both motors turning? I'm wondering if there's an issue with your disconnect actuator motor. The dealer could be reading your issue wrong and interpreting it as the encoder motor, when it's the actuator motor.

Need more details... maybe a quick video of you trying to change modes? The pattern of the lights and sounds from under the vehicle can sometimes offer some clues.
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by The Roadie » Sat Apr 21, 2012 9:49 am

The encoder CANNOT feedback bad light commands to the switch. The resistance of the switch tells the TCCM what mode you want it to be in. If the switch says 2HI, and you have 4LO blinking, then that's an unsuccessful attempt to go INTO 4LO mode, probably thwarted by the Neutral interlock. Nothing outside of the switch->TCMM path needs to be examined, and I would have them quit using the Tech II tool and put a meter right at the switch connection at the TCCM and see if the resistance reads properly. None of this crap about what the TCCM interprets the switch commanding and then telling the Tech II along the data line. Any abrasion and intermittent open on the switch->TCCM wiring and you will get uncommanded mode changes. GM was crazy when they designed it to interpret the LOWEST resistance as 2HI, and the highest resistance (open) to be transfer case Neutral - beyond 4LO.

A problem with the encoder motor would show up if you have the switch a legitimate turn and asked for a mode change, and the TCCM failed to get the expected feedback from the encoder motor (or the front axle actuator) that it did what it was told to do. Then you get a blinking light on the switch over the mode you wanted it to go to. But you could never get an uncommanded mode change initiated by the encoder motor.

If they think THEIR new encoder is bad don't they owe you a free fix under warranty for the attempted repair?
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by Bigemergency » Thu May 10, 2012 11:28 pm

James and Roadie,

Sorry it took so long to return more information on this, Im at annual training for the military. Current status:
Replaced encoder. I also replaced the TCCM and updated to the newest software update since last post. It seemed to prevent the problem for three days. It is now doing the same thing but fewer times than it had been. It will show solid orange light for 4Lo and Flashing 2wd at ignition while the switch is located in 2HI. If I shift into N it allows it to shift into 2wd and have no further problem. Technician said that I need to test the wiring between TCCM and Encoder motor to see if there is a bad wire or corrosion at the connections. The plug under the vehicle seems free of corrosion and has dielectric grease on it.

This only happens at ignition. When i turn the vehicle off its in 2hi, upon ignition 4lo(sometimes). I hear the motor move when it switches position.

Yesterday when I was trying to test all positions it would not go into 4hi the first time, it quit attempting and moved to awd. I restarted the vehicle and it made the switch. I was able to repeat the failure one more time.

What do I need to do moving forward?
Do you have a proceedure for testing with a meter? What is the porper resistance? Am I off in left field and need to start over?

Thanks
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by skobie » Thu Dec 29, 2016 2:05 am

Dear All:

New to this Board, but was recommended to try my luck here from the GMT Nation Board as I've heard you guys seem to have a good handle on the chevy trailblazer drivetrain, specifically the transfer case and attached encoder motor for a 2003 chevy trailblazer ext LT I6.

I am getting C0327 code, which seems to be indicating a bad or open ground from the encoder motor to?. The 4-Hi and 4-Lo will work when I first start the car up, but never the A-4WD (like it's not even there). After driving awhile, the "Service 4-WD" light comes on and cannot shift into or out of any of these modes at all. So even if I've cleared the C0327 code, it will then come back up at this point. If I shut the car off awhile, system resets and can go thru the above again. Also getting a Loss of Class-II Communication with ATC (code U1026) and No State of Health from Module (U1000) codes.

Replied to this thread because all other info on this subject does no seem to apply to my situation of course (encoder motor works as a brand new one does the same thing, proper wheel(s) spin, fuses are all good, proper actuator motors are making their noises, TCCM and Switch have been replaced already, etc.). This is a new problem within the last week as I've used the 4WD several times already this season and I thought I had it licked because there was a broken wire near where the encoder motor plugs in to the wiring harness (4-WD would not engage at all and only got buzzing at the encoder motor, though selector switch said I was in A-4WD no matter where the knob was, but I was only in 2-Hi in actuality) that I fixed by bypassing the plug and just tying the wires together (orange wire I believe) and the system worked completely as designed for 2 days.

Question is:
1.) Can I just run a new ground from the plug at the encoder motor (bypassing the plug, but will be spliced in to the encoder motor wiring of course)?
2.) If so, and assuming it's the solid black wire coming out of the encoder motor, where do I ground it too (chassis, direct to battery, etc.) to see if that will work?
3.) If not either of the above, what do I test with my multimeter to tell me which wire or wires are bad from the encoder motor as my scantool will give me P and C codes, but won't get more specific than that for diagnosis?
4.) Should I just replace the whole wiring harness if even possible and not ridiculously complicated?
5.) Am I way off on all this and I need to do something else?

Sorry for all the questions at once, but I can't seem to find anything on the internet that pertains to my exact problem and I figure one of you guys will know what's wrong if explained to you properly. Feel free to ask follow-up questions, but that about sums it up for now.

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by Anthony Hernandez » Thu Dec 29, 2016 12:08 pm

I don't know about everything else but I never run in auto 4wd! That is worthless!
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by skobie » Thu Dec 29, 2016 5:18 pm

I don't run it in Auto 4WD either, but it seems to be part of the problem and I don't want to leave anything out. I've already cleaned up and re-attached my chassis grounds to take them out of the equation, but no change. Again, any input would be greatly appreciated at this point (there may be a pun in there somewhere!).

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by Anthony Hernandez » Thu Dec 29, 2016 10:11 pm

It may be the actual switch bro
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by skobie » Fri Dec 30, 2016 2:28 pm

That's already been replaced, Tony. And so has the TCCM. And I even tried a new encoder motor with the same result, so that's not it either. Very frustrating, but seems to be related to the wiring and a bad ground (and I cleaned up all the chassis grounds too, so it might just be a bad wire, period). Just looking for advice on that.

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by Anthony Hernandez » Fri Dec 30, 2016 7:26 pm

Ask tbyoda, he's an electrical engineer
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by KingBird » Sun Jan 01, 2017 6:27 pm

Honestly, I'd start ohming out the wires to ensure they are good. I'd ohm them while disconnected from pin to pin just to make sure you don't have a problem in the encoder motor connector down low (which is exactly where I think the problem is).
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by rScherzer » Sat Feb 04, 2017 3:25 pm

loaded circuit test! :shoot:
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