Scantool reports Auto while switch says 4Hi

[TJBaker57]

So an owner on Facebook was testing his vehicle to see that the 4WD worked Ok. He spun his wheels a bit on gravel and felt like the fronts didn't engage.

So he get his scantool out and sees that with every shift to 4Hi the scantool shows the shift motor initially goes to 4Hi but then drops back and reports the encoder gear position at Auto 4WD even as the switch reports 4Hi. No warning lamps, no codes.

This led me to look into a few things. I have previously tested encoder return voltage readings against PID reported gearing positions and made the assumption that these encoder return voltage values don't change.

That assumption looks to be incorrect.

I have previously read that these TCCM 'learn' a range of shift motor position for the A4WD mode. I thought this applied to only the starting position for A4WD. I didn't realize that the higher end of clutch apply for A4WD might drift higher as clutches wear.

Tests today lead me to believe that the starting point for the A4WD mode of the shift motor is indeed a fixed point right about 2.86 volts (hex $92), but the point at which the range reported as A4WD vs 4Hi is a learned value.

This seems to have induced a condition in his vehicle where the 'system' by all outward dashboard displays is in 4Hi but the PID that reports the encoder range reports Auto 4WD.

I tested 6 different TCCM and no two of them agree at an encoder return voltage signal level to signify 4Hi vs A4WD.

For the high end of A4WD and the beginning of 4Hi I show anywhere from a low of 3.43 volts encoder return (hex $AF) to a high of 3.77 (hex $C0).

His graphs show even higher voltages being judged as "Auto 4WD".





And I can think of no reason his TCCM would initially go as high as 4.33 volts when switched to 4Hi but then right away drop back down to the neighborhood of 3.88 volts.

He switched the TCCM to his other TrailBlazer and saw the same results.

 

[Mooseman]

So he get his scantool out and sees that with every shift to 4Hi the scantool shows the shift motor initially goes to 4Hi but then drops back and reports the encoder gear position at Auto 4WD even as the switch reports 4Hi.

On this alone, I would be thinking that the brake in the encoder motor has failed. It's supposed to hold the set position and prevent it from drifting to another position. During shifts, the brake is unapplied by powering it, the shift is done, and then the power to the brake is removed to re-apply it.

 

[TJBaker57]

On this alone, I would be thinking that the brake in the encoder motor has failed. It's supposed to hold the set position and prevent it from drifting to another position. During shifts, the brake is unapplied by powering it, the shift is done, and then the power to the brake is removed to re-apply it.

I initially thought the same thing,,,,

However, when he swapped a TCCM from his other TrailBlazer it worked fine. The issue disappeared. And the issue followed the suspect TCCM to the other TrailBlazer.

So that should eliminate the motor brake I would think.

 

[Mooseman]

Unless the TCCM is defective in controlling it, keeping it energized continuously except when the ignition is off. Further testing at the encoder motor should prove or dispel this. Otherwise it's just a borked TCCM, especially if it's an original 02 that were defective.

 

[TJBaker57]

Unless the TCCM is defective in controlling it, keeping it energized continuously except when the ignition is off. Further testing at the encoder motor should prove or dispel this. Otherwise it's just a borked TCCM, especially if it's an original 02 that were defective.

He has ordered a replacement TCCM and says he will update me when he installs that.

I still wonder if it's possible he actually is in 4Hi while the encoder mode PID reports Auto.

I may ask him to send me the suspect TCCM after he gets the replacement in and it works. I'd be interested in experimenting with it.