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2002 olds bravada
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Discussion Starter · #1 · (Edited)
Bravada NVG 126 Transfer Case felt like it was always engaging all 4 wheels, binding in low-speed turns, etc.

Long story short, see related thread http://forums.trailvoy.com/showthread.php?t=52279, it was a bad encoder motor sensor.

Wanted to show pictures, but couldn't upload for some reason.

Here is the text version of the resistance readings as it is rotated. The first picture was supposed to show an open (greater than 30KOhm) reading across the middle and right pins with the notch at roughly the 7 o'clock orientation using the pins as a 12 o'clock reference:

Spinning the notch counter-clockwise and stopping every 90 degrees, the readings across the middle and right pins are:
Notch Reading
7 o'clock open
9 o'clock 1.26K
12 1.70K
3 2.41K
6 2.43K

The same sequence, but across the middle and left pin:
6 o'clock 1.25K
7 o'clock open
9 2.47K
12 2.07K
3 1.26K

If someone could test a brand new unit using the same sequence, we now have an easy way to test an encoder motor sensor.

[If anyone wants to share the trick with me on how to upload pictures, I've got 'em, but I couldn't get one 3MB image to upload - and it was within the size and dimension limits.]
 

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2002 olds bravada
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Discussion Starter · #4 ·
Pics uploaded

Resized pictures to 800x600 per Roadie's advice. Success!

When I used the word 'notch', I was describing the round protrusion into the center of the sensor.

Note the numbers on the bottom: "02287". I suspect it is a manufacturing date code, corresponding to the 287th day of 2002.

If anyone else has one of these sensors, I'd be interested to compare the numbers on the bottom.
 

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2002 chevy trailblazer_ltz
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So I geuss this goes on ,or in the en. motor?... Is this what fails, requiring whole motor replacement?:hissy: (different for diff. 4wl systems:undecided)..... & thanks wagnewvt for addressing this issue.:thumbsup:
 

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2004 gmc
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It stacks in line with the encoder motor - #29 in this drawing. Often, the encoder motor goes bad mechanically or in its gear train. Not clear whether the cheaper sensor is the root cause of more failures than the motor or not. At least it's available separately, but an entire encoder motor replacement is often cheaper if you get it off Ebay.

 

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2003 olds bravada
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I really don't mean to get :eek:fftopic: or anything but...

and this might be a dumb question, but would the encoder motor (if bad) also cause the awd to not engage or not engage as "quickly" as it should?..this past winter i've gotten stuck trying to back out of my parking space at work (snow drift that was just below the rear bumper and on completely level ground)

and on cold days i would hear a rapid tick that seemed like it was coming from under the cabin (maybe x-fer case)
 

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2004 gmc
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...would the encoder motor (if bad) also cause the awd to not engage or not engage as "quickly" as it should?...
Certainly would. The encoder motor, like most actuators, has a gear train to reduce the RPM and amplify the torque coming from a small motor. When cold, the grease on the gears will congeal and slow down the engagement. Not much of an issue for selectable 4WD models, since we hardly ever choose A4WD mode if conditions are such that 4HI is useful. But for you, it might make all the difference.

Have you changed the transfer case fluid every 50K as the Owner's Manual recommends?
 

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What exploded veiw do you have of encoder?

It stacks in line with the encoder motor - #29 in this drawing. Often, the encoder motor goes bad mechanically or in its gear train. Not clear whether the cheaper sensor is the root cause of more failures than the motor or not. At least it's available separately, but an entire encoder motor replacement is often cheaper if you get it off Ebay.

The exploded view you show here is different than the one I got off the DYI site. The one I have just shows the controller motor going into the back of the case without this sensor in between. My 2002 Brav is having 4wd all the time issues. I changed transfer case fluid today, and my buddy took the motor off the actuator housing to check it. It runs great both directions. Looking for the sensor now, but before I take the whole thing off I need to find it. Was there two different models of sensor? Or did I get some bad paperwork?
 

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2002 olds bravada
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encoder motor sensor

My 02 Bravada has the same problem.
I have the encoder motor off and measured the resistance across the pins of the sensor as described above and got the same results. Open at the narrow 7:00 position.
I'm a bit confused though....are these the readings of a bad sensor or a good one ???
This motor is a new one (less than a year old). According to the part number on the motor, it's a ReTech motor.

Thanks
 

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The exploded view you show here is different than the one I got off the DYI site. The one I have just shows the controller motor going into the back of the case without this sensor in between. My 2002 Brav is having 4wd all the time issues. I changed transfer case fluid today, and my buddy took the motor off the actuator housing to check it. It runs great both directions. Looking for the sensor now, but before I take the whole thing off I need to find it. Was there two different models of sensor? Or did I get some bad paperwork?
Gently pry the bottom cover off of the bottom of the motor and you'll find the sensor.
 

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gmc envoy_slt
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Anyone have a theory on this workaround?

I have a GMC 2002 GMC Envoy V8 and have the exact problem described in TSB #1848894 (Subject: Inoperative 4WD/AWD Lamps, Inoperative 4WD/AWD System, DTC C0550 Set No Communication with TCCM (Reprogram Transfer Case Control Module) #02-04-21-006E - (07/20/2006) ) and in the posts above.

I took the encoder motor apart and checked the resistances and got similar numbers, except I can't recall getting any opens. (I feel stupid for not reading your post more closely and writing down all the numbers because I have what I believe is a good encoder motor sensor.) :eek:

I think I can confirm your part imprint theory, mine said something like 01023 or something that fit your year-dayofyear pattern.

I may get the opportunity to revisit it because I put it all back together and still no joy.

I was trying the "pull the ATCM or TREC fuse" method when I stumbled across a way to get the encoder motor to fire up and change into AWD and 4HI and back to 2WD. (I am too chicken to try 4LO).

I switched the console selector to AWD, turned off the car, turned it back on, turned it off and back on, and I heard the encoder motor run and change to AWD. Same results with AWD to 4HI and back to 2WD eventually. When you key up, the selected mode flashes just for a second, then all the lights flash and then go dark.

Anyone know if this narrows things down? At this point, the motor and sensor seem fine.

Many thanks. And thanks for those great photos. I have never touched 4WD before and this gave me the courage to have a go at it!

Regards,
Weezy
 

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2004 gmc
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Your workaround is one that, well, works.....

But the root cause, especially seeing that you have a 2002 with a known design defect in the TCCM, is probably the TCCM, not the encoder motor or sensor. Just locate a 2003+ TCCM from Ebay or a junkyard and it should be a plug and play fix. Search for earlier discussions of the TCCM to read up on how bad the 2002 versions were.

Some early TCCMs respond to a dealer reflash to solve a "won't wake up on a ignition run signal". But you can usually locate a junkyard one for less than the dealer charges for a reflash. So you don't have the uncertainty of a possible failed attempt at a repair that wastes your money. Buying a new TCCM from the dealer or a GM discount on-line parts place like parts4chevys.com will require a dealer flash to configure it for the first time (evil GM designers couldn't make it work right out of the box), so you might as well go for the used one first time out.
 

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D'OH! So that's a TCCM!

Thank you very much for the reply. Like I said, this is my first time looking at 4WD. I thought a TCCM was the entire back end of the tranny!

I looked them up on eBay and now I get it.

Thanks!
 

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2002 olds bravada
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Discussion Starter · #17 ·
Just getting back to this now...

The readings I showed originally were from a suspected bad position sensor.

Since I had the dealer do the install, I never had the opportunity to test a new/suspected good sensor.

Very curious that a new (suspected good) sensor would have showed an open - if I were the TCCM firmware programmer, I would consider an open to be a fault condition and not risk activating the motor and potentially doing expensive damage to the drivetrain.
 

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2002 chevy trailblazer_lt
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Measure your encoder motor position in vehicle

My TCCM set a service light and then a few weeks later my transfer case slipped into neutral. I'm going to make a TCCM bypass that will all me to set the transfer case and front axle motors manually but I need some help from the community (This can serve as a handy reference for yourselves too). Can you guys measure the resistance and/or voltage of the encoder signal @ the TCCM on a working system?

This thread describes how you get to the TCCM: http://forums.trailvoy.com/showthread.php?t=49200&page=4

If you look @ the back of the black 16 pin connector, there are numbers and letters. If you put your ohmmeter probes in the brown/white B6 and black/white A7 then you can read what the encoder is feeding back to the TCCM (in volts). You should also be able to read the resistance of the encoder too since the TCCM should only decrease the true reading by a negligible amount (test by reading the resistance, then unplugging the TCCM and taking another reading. They should be the same). Leaving the probes in and flipping the 4wd switch, you can watch the reading change and settle in each mode.

Anyhow, I managed to get out of N and into 4lo and 4hi manually, but have no idea where 2wd is and don't want to risk insufficient engagement of the high speed gear. For reference, with the encoder motor run until the stops, I get a reading in 4lo = 1.38kOhms and 4hi = 2.39kOhms
 

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2002 chevy trailblazer_ls
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Looks like a rotary encoder switch out of an old VCR! (Sends signals to the microprocessor corresponding to positions of the mechanism.)
 
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