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.

 

[Mooseman]

If he ordered a new or rebuilt, he may need to get it programmed. That's why I always recommend used.

 

[TJBaker57]

If he ordered a new or rebuilt, he may need to get it programmed. That's why I always recommend used.

Pretty certainly used. From Ebay. I could have sent him one myself, I have like 6 or 7.

 

[budwich]

Qestion: are the posted graphs the same for both units / vehicles that he tested.... as you indicated that the "other tccm", the "mismatch indicator / position" disappeared for one versus the other unit (ie. tccm)?

 

[TJBaker57]

Qestion: are the posted graphs the same for both units / vehicles that he tested.... as you indicated that the "other tccm", the "mismatch indicator / position" disappeared for one versus the other unit (ie. tccm)?

The posted graphs image is from the vehicle/TCCM he encountered the issue with first.

Later on while looking into the issue he swapped the suspect TCCM to a different TrailBlazer and using the scantool he witnessed the same 'fault' there in the second TrailBlazer. This would seemingly prove the trouble is with the suspect TCCM.

He then swapped the non-suspect TCCM into the first vehicle where he had first discovered the 'fault'.

With the non-suspect TCCM in the vehicle where the fault was first witenessed there was then no 'fault' observed with the scantool.



Just for informations' sake:

In the TCCM there are 4 PIDs pertinent to this issue.

Two Pids report the voltages returned from the 4WD mode switch, and the range position sensor (encoder) in the shift motor. These two voltages should be in the range of 0.5 VDC to 4.5 VDC.

Two other PIDs report what the TCCM has determined are the current 4WD mode switch 'Mode' position, and what is the current operational 'Mode' position of the shift motor. These values are based on the reported voltages from the respective switch/sensors. These are displayed as text in a scantool.

We would expect that if these two 'Modes' (switch vs shift motor) do not agree then a fault would be present and a code would be set.

But that is not happening here. That is what I set out to investigate when I discovered that the encoder return voltage threshold for the decision between A4WD and 4Hi appears not to be a static value.

 

[TJBaker57]

So the fellow got back to me yesterday with a status report.

He got two used TCCM from Ebay or wherever. He says both of them seemed to run through all 4wd operations normally. But as he backed out the driveway he said it "rolled like" it was in 2Hi. He did not say if he observed scantool data.

So based on observations only he decided to swap encoder/shift motors between the two vehicles.

The trouble moved with the encoder/shift motor from the problem vehicle.

As soon as the encoder/shift motor from the troubled vehicle was tested in the other vehicle (that previously was OK) it displayed the trouble where Auto 4wd was displayed when 4Hi was selected.

And when the "known good" encoder/shift motor was placed in the troubled vehicle it "worked great".


So now he is evaluating getting a replacement encoder/shift motor.


I find this all pretty baffling.

There is only the one encoder return voltage coming back from the encoder/shift motor to the TCCM. There is no other signal output from an encoder/shift motor to say it is in Auto position when the voltage return says it is in 4Hi.

From literature I see these TCCM are "learning" devices that accomodate clutch wear etc. To me that offers a reasonable explanation for the variance in encoder return voltage as to the Auto/4Hi threshold decision performed in the TCCM.

In the Tech 2 data there are data points for "Current Slip Adapts" and "Slip Adapt PWM". I figured a TCCM 'learns' using these values during drive cycles where the A4WD is engaged and there is rear wheel slip events.

And based on that the TCCM may alter the default encoder return voltage value for the changeover from A4WD (Auto) to 4Hi.

But then a freshly swapped encoder/shift motor should not immediately display the sort of Auto/4Hi trouble this owner is experiencing.

As I said, baffling. The owner said he "has no problem" sending me the old parts once he gets everything straightened out. I hope this happens as I really want to get my hands on the suspect devices for testing and explorations.

 

[budwich]

The posted graphs image is from the vehicle/TCCM he encountered the issue with first.

Later on while looking into the issue he swapped the suspect TCCM to a different TrailBlazer and using the scantool he witnessed the same 'fault' there in the second TrailBlazer. This would seemingly prove the trouble is with the suspect TCCM.

He then swapped the non-suspect TCCM into the first vehicle where he had first discovered the 'fault'.

With the non-suspect TCCM in the vehicle where the fault was first witenessed there was then no 'fault' observed with the scantool.



Just for informations' sake:

In the TCCM there are 4 PIDs pertinent to this issue.

Two Pids report the voltages returned from the 4WD mode switch, and the range position sensor (encoder) in the shift motor. These two voltages should be in the range of 0.5 VDC to 4.5 VDC.

Two other PIDs report what the TCCM has determined are the current 4WD mode switch 'Mode' position, and what is the current operational 'Mode' position of the shift motor. These values are based on the reported voltages from the respective switch/sensors. These are displayed as text in a scantool.

We would expect that if these two 'Modes' (switch vs shift motor) do not agree then a fault would be present and a code would be set.

But that is not happening here. That is what I set out to investigate when I discovered that the encoder return voltage threshold for the decision between A4WD and 4Hi appears not to be a static value.

Not sure if you covered what I tried to ask.... you indicate that the trouble moved with the encoder (even with his subsequent other "purchases")... BUT I wanted to know about the "response graphs". You seem to indicate the "observable event"(things worked or didn't work) but not necessarily all the information that a "picture / graph" provides for analysis... such as spikes and other components.

 

[TJBaker57]

Qestion: are the posted graphs the same for both units / vehicles that he tested....

I have no way to know. The original post contains one video clip taken on the troubled vehicle.

There is no other graph data available from the working vehicle.

As I understand it, if he individually swaps either the TCCM or the encoder/shift motor from the 'troubled' vehicle to his 'working' vehicle the troubles are then evident in the 'working' vehicle.


When he swapped the TCCM from the 'working' vehicle to the 'troubled' vehicle he observed no troubles, but he didn't test drive it.

When he swaps the encoder/shift motor from the 'working' vehicle to the 'troubled' vehicle he reports it "seems to work great".