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SOLVED! 4WD Glitch

TJBaker57

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rotate to an invalid position
I would expect that if it rotated to an invalid position the encoder sensor would report the out of range value to the TCCM and a code or the like would result. Assuming the encoder sensor is functioning properly.
 

Mooseman

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Since you've had previous issues with uncommanded 4x4 engagements, I would suspect the TCCM and replace it as well as the switch just for good measure. I've never seen or heard of this happening before. I doubt it's the TC as the TCCM is commanding the shift. If it was the TC moving around on its own, the TCCM should see this and throw an error sensing the unwanted position.
 

Mektek

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I guess I have to try the parts changing routine. I'll start with a tccm from a 05+ - that will have revised firmware. It appears that the tccm has two parts of the firmware - the OS and calibration data. I would even speculate that the tccm may have been paired with the TC at the factory, but the company is out of business so there's no way to find out.

After installing the relubed encoder motor I now get a c0327 so I'll have to remove it again and check what went wrong. Also, this seems to have caused the tccm to shut down completely and I have no lights on the mode switch ( except the test on startup).

I just can't get a break :badday:
 

budwich

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The transfer case has a "notch" / valley in the "control plate". It runs about "30 degrees" wide with "plateaus" equal distance from the end positions of the rotation of the plate.... so perhaps about 30 degrees on the plateau on each side.

Interesting "new info" about the uncommanded shift to 4WD over the past "few years".... I assume these happened without any service 4wd lights? certainly not a good situation. Based the "happenings", I would suspect a faulty switch / wiring.
 
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TJBaker57

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Take a DMM and read the voltage on the brown/white wire going into the TCCM from the encoder motor.
 

Mektek

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I pulled the tccm and checked the impedance at the encoder wires - it was open :no:
Checked at the encoder motor connector - also open. So I removed the encoder motor and then cut apart the encoder. The ceramic disk that the conductive ring was painted on was cracked. I can't imagine how that can happen. So I have to find another encoder.

I drove with the ATC fuse removed and there was no issue with that. Something new came up - the radio did not turn on. It appears that something happened to the Scosche radio interface box and it's now non functional. I can turn on the radio with the manual switch I installed but the chimes and automatic turn on are dead. So I have a new speculation - could a Scosche box send spurious network traffic and cause the tccm to shift? Perhaps the mysterious U1000 rear seat audio and HVAC codes which the hardware doesn't exist in my regular length TB somehow come from the Scosche box. I'll disconnect it and see today.
 
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TJBaker57

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can't imagine how that can happen. So I have to find another encoder.

I know how mine got cracked. I had a tough time getting into it! The initial trouble with my encoder was cracked solder joints where the terminal strip is soldered to the ceramic board.

IMG_20190703_101102.jpg

I have installed this one in 3 different trucks. Working so far.
 
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TJBaker57

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could a Scosche box send spurious network traffic and cause the tccm to shift?

I do not believe it could initiate a shift. But if it disrupted the network enough for the TCCM to not get the memo that the truck is not in neutral, then a poor connection in the mode switch circuit could appear as a shift request to the TCCM and there you go. Seems like a longshot though, as the TCCM would have to miss all the announcements of normal transmission upshifts/downshifts.

I have been studying the message traffic and indeed the PCM announces all transmission shifts, either by the driver moving the shifter as well as the normal upshift and downshifts to both the TCCM and the EBCM. Normally the TCCM and the EBCM acknowledge these messages. I cannot say if the PCM would retransmit the message if it failed to get an acknowledgement from the TCCM or EBCM.
 

Mektek

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TJBaker57

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very different from the original

I don't think they make the ceramic types anymore. This one looked like cheap chinese stuff when I got it but it is the AC Delco replacement. The new ones seem to use an open design unlike the original.

There are some observations I made when I replaced mine, also an 02 here....
 

Mektek

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OK, it's good to know that I wasn't the only one who cut one of those encoders apart :laugh:
Of course the usual disappointment is that genuine AC DELCO is made in China:hopeless:
There is an NGK/NTK TM0003 which is a cross reference to the acdelco.
I hoped I could fix the old one but it had too many cracks to save. You've also confirmed my theory that the encoder motor only rotates approximately 110 degrees.

I've verified that the mode switch is working properly - all the positions show the correct impedance. Whatever is causing the random shifting it is not the mode switch. So I'll go to the PYP and get a tccm and an encoder ring - it will be worth the trip to get them both.
 

TJBaker57

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to the PYP and get a tccm and an encoder ring

Might be best to just look for a recently replaced or newish looking encoder motor. Very few people even know you can get the encoder ring separately and many, if not all, original ones will already have the wear that develops at the 2hi position.
 

Mektek

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I had the battery disconnected while connected to a charger to run a repair cycle. When I reconnected it the Scosche box was dead - or so I thought. All I had to do was disconnect and reconnect it and now it works fine. So I don't think this had anything to do with the original issue.
So I'm now down to two potential causes.
I still can't rule out some kind of strange mechanical malfunction, although the transfer case normally shifts modes and drives OK.
The second is the TCCM. Through some highly unlikely combination of firmware and electronic glitches, a mode shift occurs.

So the plan is to get a replacement TCCM from a 05+ truck. And install a power switch so that the TCCM can be powered down when not in use. That will eliminate any possibility of electronically shifting the TC mode and leave only mechanical issues.

I just have to wait a few days so the mosquito infested swamp that the local PYP turns into after several days of rain has a chance to dry out:badday:
 

Mektek

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Here's a section from the service manual:4loshift.jpg
First problem is that it says the engine must be "funning" :laugh: How do I make my engine do that???
I was in drive at around 30mph, so according to the second two conditions what happened to me should be impossible!!!!

But it's happened to others, yet I haven't found a concrete fix or conclusive diagnosis......

There is also another consideration when replacing the TCCM. According to the manual it is programmed with the gear ratio and tire size. I don't know how critical it is, but I'll try to get one with at least the same gt4 as I have.
 
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TJBaker57

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According to the manual it is programmed with the gear ratio and tire size.

This would be in the calibration?? That's an arena I know next to nothing about. Except how to read my TCCM calibration part number in Torque! (Or I suppose OBD Fusion though I haven't tried that yet)

I believe my 2002 has the original TCCM, Operating System, and calibration from the factory. Mine has a build date of 08/01 according to the door sticker and my vin is just outside the breakpoint(s) of those that had defective transfer cases that required replacement.

Screenshot_20200613-200243.png
 

Mektek

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HAH! Twins :laugh: Mine has the same build date as yours! What are the odds of that?

How do you determine if you have the defective range shift collar? The history report just says that some of these vehicles had the defect. Although I imagine if it really was defective it would not have lasted 18 years.....
Neat trick with getting the TCCM numbers. I'll have to try that when I get my system reconnected.
 

TJBaker57

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HAH! Twins :laugh: Mine has the same build date as yours! What are the odds of that?

How do you determine if you have the defective range shift collar? The history report just says that some of these vehicles had the defect. Although I imagine if it really was defective it would not have lasted 18 years.....
Neat trick with getting the TCCM numbers. I'll have to try that when I get my system reconnected.

Being a serious safety issue I think GM was pretty thorough in getting those defective transfer cases changed out. Surely would not have lasted 18 years! According to the listing I found here...


...mine is 6746 vehicles later.

I'm real curious what your TCCM numbers are. I would expect there to be differences which might explain the memory address thing not working for you as it works in mine. Or maybe these memory address things are not as static as I had expected.
 

Mooseman

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There might have been a change in TCCM part numbers, especially from '02 to '03+. The failures were just the TCCM would crap out from bad solder joints IIRC. I experienced it on my '02 EXT whenever it got below -15c right at start up. Some programming changes could have been done to take into account traction control and Stabilitrak or possibly other improvements over the years.

If you go here, you can plunk in your VIN and see if there has been updates to modules, including the TCCM (or ATC module). Also usually explains the changes. You don't need an account for this.

Tis2Web calibration info

In '02, there were 4 versions.
 

Sparky

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FWIW when my TCCM in my old 02 died, right before that my metra interface forbly aftermarket radio would randomly shut down and I wouldn't have any audio at all until I ignition cycled the truck. Once I replaced the TCCM after it died, I never had that issue again over the next several years. My only theory is that the TCCM before it quit started babbling random garbage on the bus and the aftermarket radio interface would just lose its mind and shut off or something.

I too got a newer used TCCM and never had a problem with it. I think it was from an 05 Envoy but it has been years and I don't remember for sure.
 

Mektek

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Picked up a TCCM and encoder ring from an 05 TB XL and a spare encoder ring from an 02. The 05 had the new style ring and the 02 had the old type. Installed both parts from the 05 today and the 4wd is switching normally again :2thumbsup:
Both the old and newer TCCM have the same part number, but the 02 had a serial number of 03550 and the 05 was 35540. It has a motorola microcontroller with ram, flash and EEPROM.

When removing the encoder motor from the 05 the connector was on the side, while my 02 has it on the top of the transfer case and is much more difficult to remove. So I made up an extension cable to put the connector on the side for easier disconnection.EM extension.jpg

Unfortunately I can't say if I fixed the original problem. It was an occasional random event so I won't be able consider the issue solved until a year passes by without incident.

I still have the various OBD codes to deal with. The ECM gives a p0340 with both the old and new cam position sensor after one drive cycle. A u1000 comes up with torque, but not with car gauge pro. I'm puzzled by this because the engine is running well - possibly it's idling smoother with the new CPS.

B2575 and b2615 are still coming up even though the headlights are working normally. Maybe it has something to do with the DRL killer? Although that's been installed years ago and never gave a problem.
The new version of Torque has an occasional problem connecting to the VGATE obd adapter. The older versions never did. Maybe that has something to do with the mysterious u1000 that appears for rear seat audio and HVAC which my regular TB doesn't have.

And the cruise control doesn't work - probably related to the p0340.
Still more problems to solve......
 

Mooseman

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Since the original problem in this thread is apparently fixed, I would recommend that you start new threads for the other codes and issues.
 

TJBaker57

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A u1000 comes up with torque, but not with car gauge pro. I'm puzzled by this because the engine is running well

So as my last post here on this 'solved' issue I will say that as far as Torque vs Car Gauge Pro returning different results that there are at least 8 different code status that can be requested together or separately. Torque requests a grouping of 4 of them and doesn't necessarily display all that it sees in the responses from the various vehicle modules. If you select 'App Default' in Car Gauge Pro it also requests 4 of the 8 status but they are not the same 4. 3 are the same but one status is in the request is different.
 

Mektek

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I don't know if it's coincidental that the various error codes appeared the same day that the 4wd shifting problem occurred.
I would say the problem is "potentially" solved - only time will tell.
 

Mektek

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Here's what the restoration guide says about encoder motor brake function:
tccm info2.jpg
So the engineers have concluded that it is possible for the TC to "pop" out of position.
I have checked and the brake is strong enough to stop the motor from rotating when 12v is applied to the motor. But it is not a solid lock - I can still turn the gear with my fingers. Once it goes through the reduction it takes much more torque - I hope that is enough to resist whatever force the TC can apply to change the mode.

Here's a pic of the brake assembly:
encoder motor 2.jpg
It's at the end of the motor. To remove it I think the gear at the end has to be pulled off. Next time I go to the PYP I'll try this on a junked unit.
 

Mektek

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A few months have gone by without the problem recurring, so I'll say that the issue is almost certainly solved.
My theory on the solution is that the encoder itself was "flaky" and the single, actual cause. The TCCM is known to fail completely, not for uncommanded shifting. But if the tccm sees a value from the position encoder that does not coincide with the currently selected mode, it will attempt to activate the encoder motor in an effort to make the two match. My encoder failed completely after reinstallation which leads me to believe there was a flaky internal connection. I initially had an issue with 4hi shifting, which later changed to 4lo - a more serious problem. I hope that the newer encoder design is improved over the old ceramic one and not simply a cheaper redesign......

How many of these vehicles end up in the scrap yard with strange problems like this? The professionals probably give huge estimates of the cost to repair and people are shocked. When in fact the cost to repair was small.......
Hooray for the amateurs!:Lager Louts:
 
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