SOLVED! 4WD Glitch

[Mektek]

While driving yesterday at 30mph the 4LO suddenly engaged. I thought the tccm was supposed to prevent this unless you were stopped in neutral. The light changed from 2hi to 4lo, but the knob was not touched. After a while as I slowed to a stop it switched back to 2hi.
Aside from the annoyance, this would probably be very bad if it happened at 75mph.....
Of course the "service 4wd" light came on.
The next day I scanned it and the 4wd light was off. Other codes came up p0171,p0340,b2575,b2615 and oddly enought a u1000 for the rear AC - and I don't have rear ac! The SES light just turned on that day.
The switch itself is in good condition - all contacts inside and out are in like new condition.

Is this a sign that my TCCM is on it;s way out? 18 years - I guess it's possible......

 

[TJBaker57]

Does your scanner read the TCCM codes??? These will be C0xxx or similar. Car gauge pro and a Bluetooth OBD reader will get them.

I have read in several posts about a supposed interlock but I can state that my TCCM will shift to 4Lo without the engine running and that's not supposed to happen either. Makes me doubt this reported interlock is for real.

In 2Hi the mode switch should return about 4.06 volts on the wire (light blue on an 02) from the mode switch and the TCCM. For 4Lo the voltage is about 1.6. A4WD and 4Hi are between those two in voltage returned. The voltage can be read at the TCCM by backprobing the correct wire. In the second image here I am reading the return voltage from the encoder, a brown/white wire. The switch return wire is directly below the encoder return wire. I must say, getting the backprobe in place is not an easy task through the access at left side of the dash, near the driver door hinge.

 

[TJBaker57]

Then again.....if we can figure out why your Torque won't allow more than 6 characters we can have you reading that 4wd switch output right in Torque!!

 

[TJBaker57]

So....what kind of RPMs did you get at 30 mph in lo range!!!

 

[Mooseman]

Must have been a bit violent to shift into 4lo at 30mph. Certainly shouldn't be doing anything on its own.

What year is your truck? (you don't have the year in your profile) 02 and some 03 had defective TCCM's but it would just stop working, nothing like this. First I've heard of it shifting on its own.

 

[TJBaker57]

My 02 once dropped into 4Lo while I was turning into a parking lot. Felt like all 4 tires went flat at once since I was hard over on dry pavement. Was only going walking speed or thereabouts.

 

[Mektek]

It was quite a "thunk and shudder" and I thought the transmission crapped out at that instant
Took me a moment to figure out what happened and why the engine was revving to 2500.
I tested the modes and was very relieved to find that I didn't blow the transfer case.
I'll try the new tccm PIDs tomorrow (I fixed the problem with the long PIDs). When I scanned the next day the service 4wd light was out and Car gauge pro did not find any TC codes.
I have had it shift itself before, but only into 4hi (which is not so shocking) and maybe 2 or 3 times a year so I wasn't worried about it. I do worry that a high speed shift into 4hi could do damage.

It's an 02, but this is not the defect the old tccms are known for. The indicator light works correctly so the range sensor is probably OK. What could cause possibly an instantaneous switch to 4hi?

I may pick up another tccm from a newer truck at the PYP. What else can I try?

I think I'll drive with the tccm fuse removed so it doesn't do any undesired shifting until I replace the tccm.

 

[Mooseman]

I'd also replace the switch. Since you've had past issues with unwanted shifting, I would do both.

TCCM codes won't show in CGP unless it's currently active with the light on. I think the Tech 2 is able to pull historical codes.

The current version of Torque Pro is 1.10.114, Nov. 2019. I don't think it automatically updates on version changes so you might have to uninstall and reinstall.

 

[budwich]

I don't know but I think you can't disregard the B2615 ... base on limited reads.

 

[TJBaker57]

I agree on the update to the Torque App!! That's an old version you have there.

Car gauge pro WILL pull up old, history, and intermittent codes. You need to request these specifically during the process though. However, I think it's not likely to have set a MIL code for a single incident. It very well might show an intermittent code though. Mine shows a ridiculous number of intermittent codes mostly because of all the testing I do..

 

[TJBaker57]

If not the TCCM itself the voltage returned by the switch is the key. ANY poor connection in that circuit that drops that voltage will be seen by the TCCM as a request to shift.

 

[TJBaker57]

No new information here but seen in this video I monitor the return voltages from both the mode switch and the encoder motor with 2 DMMs...

 

[budwich]

where was the light at the first full switch over?

 

[TJBaker57]

'N', for Neutral! Hard to see that dim red N so I got a close up! As far as I know the only way to get into neutral is from 2Hi, with the transmission in Neutral and hold the mode switch for at least ten seconds in the spring loaded full clockwise position.

Edit: And make sure the wheels are chocked or at the least on flat ground where the thing won't roll away on you! No 'Park lock' with transfer case in neutral!.

 

[Mektek]

Knob was at 2hi - I wasn't looking at the indicator light while driving - I just felt the BAM! and shift to 4lo and noticed the solid 4lo indicator light.
I've pulled the tccm and found the power pins to have heavy oxidation. And the fuse had 18 years of oxidation on it. Cleaned all the pins - maybe that will make a difference......
I'll check the switch again. Early last year it was in good shape. And I'll check if the impedance changes correctly with switching.

 

[TJBaker57]

The original switches can be opened up for internal cleaning of the contact areas...



There are just a few tabs locking the two halves together. One seen here where the printed number is..

 

[Mektek]

I relocated the TCCM for easier access. No need to remove the lower panels and squeeze into the footwell upside down to access it. I've looked into the issue some more and found many people with same issue. Unfortunately no one has a concrete cause and solution. The worst outcome is this one:

The first major problem I had with this Trailblazer I was driving down the road at 65 mph when for some reason the 4wd decided to shift into 4 low on its own. Well, this ripped out the cv joint half shaft on one side, destroyed the transfer case internals and broke off the axle housing from the aluminum oil pan on the engine.

There is an interlock to prevent shifting in/out of 4lo unless in neutral. I thought there was a problem when I couldn't shift out of 4lo while moving even slowly, but then I read about having to be in neutral and found that to be the case.

This is my nightmare. At 30mph it didn't seem to do any damage, but on the highway it can be catastrophic.

 

[budwich]

perhaps investigate the "neutral interlock paths" to see if anything looks "soft" (ie. possible wiring, components). As mentioned, you are potentially dealing with electrical so the "Bxxxx" that you have / had should be investigated for possible contributing factors. The other question is history... is the truck stock and what work has been done in the area?

 

[budwich]

A "quick look" at the control system seems to suggest a "significant electrical" issue... maybe ...

First, the TCCM is unaware of any "neutral" status of the tranny directly. Further, whether the selector is moved or not, the TCCM can only "know" electrical current flows. IF it detects less current flow (for any reason... 2hi is the lowest resistance and the 4w LO is the highest), it is going to take the "action suggested". It would appear from the observations presented in the original post, that the TCCM was "seeing" a change because, in addition to the electrical adjustment to the encoder motor to switch the transfer case, it also lit the appropriate light on the selector switch. It appears it had some "brains".

As stated earlier, the TCCM seems to have to kind of get "permission" to select some of the modes.... in particular, dealing with 4w LO. It would appear that it sets a 4wLO signal towards the PCM. Although I haven't seen anything written, it would appear at this point that the PCM would then signal (data wise) to the TCCM to allow the selection IF the PCM knows that neutral has been selected in the tranny.... maybe. Again, since the TCCM has no other knowledge of neutral.... IF one can believe the operator's manual that the condition is required. (As far as the "engine running" condition, I think that is meant to mean "key in ON" as I know I can select LO with the engine off but the tranny selected to N).

One question that I have for the OP... do you have the timestamp for the existing codes? And do you have any way of perhaps looking at history to see if you can get back any of the potential other codes maybe related to the service 4wd indicator or other SES codes? Rather strange that the things happened kind of at a similar time period.... not weeks or months apart.
Is your PCM stock?

further looking would suggest a partial loss of the 5v reference supply internal to the TCCM.... although that doesn't explain the "neutral tranny condition"... but perhaps the "5v" is in common use within the tccm for logic control also. The service indicator and tests thereof might still be worthwhile... some concern about "b" type dtc's. It is worth noting that although the service indicator is cleared, the potential codes are likely still there in history.

 

[Mektek]

The TCCM is smart - looks to have a motorola microcontroller with flash memory. Appears that there there were 3 firmware updates, but none for an issue like this. I don't know if mine has any updates.
I've studied the schematic and fiddled with the system and found that in fact neutral is required for 4lo. The TCCM goes on the data bus and requests a "transmission range" from the PCM. So shifting into 4hi can be done at any speed, (as the manual claims) but 4lo will cause the light to flash until the transmission is in neutral - then it will complete the shift.
So here's my new theory on the problem. The encoder motor has a solenoid activated brake. I can only suppose that the engineers put it there to prevent unintended mode shifting - like is happening here. So the solenoid is not holding the brake (due to age/wear) or some other mechanical problem.
As a temporary measure I'll figure out some way to put a locking pin to keep the selector from rotating util I replace/fix the encoder motor or brake.
What prevented major damage was the low speed when the torque converter wasn't locked - that absorbed some of the shock.
Impossible to be a TCCM problem unless it was hacked by the North Koreans or Chinese

 

[TJBaker57]

I believe the brake works the other way around. Powered to release, normally on.

 

[Mektek]

Yep - makes sense. Going to pull the motor out today and check it.

 

[TJBaker57]

Have a look at this issue of gears magazine, around pages 25, 26. EDIT: maybe pages 24, 25

Good info there that many have asked about regarding the proper encoder motor orientation for installation.

http://www.gearsmagazine.com/resources/issues/130543208322118000.pdf

 

[TJBaker57]

Another thing that is hard to figure out from the theory of operation document over at Offroadtb is the order of positions of the moter/shaft. I have studied those diagrams for hours and have come to the conclusion the order is this:. TC shift shaft fully ccw is 4Lo, going clockwise from there is Neutral, 2Hi, A4WD, then fully clockwise is 4Hi.

 

[TJBaker57]

Make certain to chock your wheels, it's only a little ccw from 2Hi to Neutral!! Park Lock will not apply when TC is in Neutral

 

[budwich]

can you do any history recovery of codes?

as for the encoder motor positions, 2wd position is at the first notch away from full clockwise while 4wLO is all the way to full counter clockwise.

I find it somewhat "funny" that the TCCM appears to not show any "gear indication" (ie. not neutral / neutral) in its data according to the service manual but do expect some form of data transfer from the PCM to complete things.

The theory of the motor "running" has some potentially but then again, it also need the brake solenoid to be energized. IF such a condition happened, it would be possible that the motor ran fully counter clockwise to its physical stop / end point which happens to be 4wLO.

 

[Mektek]

The "service 4wd" light clears when the condition that caused it is gone after cycling the ignition. So when I got to it the next day - no light and no code. After slamming into 4lo it eventually returned to 4lo when I slowed to a near stop, but still in gear and rolling. If only I had a scanner then.......
I ticked the history option in car gauge pro and nothing came up.

I pulled the encoder motor today - what a PITA! It requires you to give the TC a bear hug - the connector is on top of the TC and takes two hands to disconnect.
Either the reduction ratio is very high or it's a worm drive because I couldn't turn the gearbox shaft with a pair of pliers. The brake solenoid sounds like it's working normally, and even with the brake solenoid active I still could not turn the shaft manually. So this leads me to believe that the brake function is only two stop the inertia of the gears and motor at a precise point determined by the encoder - not to stop the shaft from being moved by the TC.

So I'm back to where I started - having no idea why it shifted itself.

An electrical fault would require two conditions - both the motor and solenoid to be powered. Those two are highly unlikely to occur simultaneously by accident. And even more unlikely to correct themselves and later operate normally.

Could the transfer case shift itself without rotating the shaft? Unlikely......
So far I've heard of others with the same problem but none with a conclusive cause and repair.

None of the TSBs and firmware updates that I can find cover this condition. So my "fix" for now will be to install a switch on the encoder motor or TCCM power to disable it when not needed - which is 99% of the time. So no electrical/firmware glitch can possibly affect it. Then I'll know for sure if it happens again IT IS the transfer case shifting itself

Someday I may take a spare TCCM and do a firmware dump to really figure this out ...............
An electrical fault would either short the output and burn out components or go open circuit and not work at all. I can shift modes all day long and it works fine so no such issue here.

 

[budwich]

check the timestamp of the codes that you posted... are they all the same? Not sure how history works on some units. I know on the torque one, it seems to be somewhat hit or miss depending on the vehicle.

the code for the service 4wd should have stayed in the history maybe as yes the key sequence clears the indicator but the code should have stayed with history. Has your PCM been "modded"?

What are the chances that one of the B codes applies to the selector switch (ie. wrong code).
Related to this, what are the chances that the TCCM went to sleep and then woke up with a "hang over" and thought it seen a "selector switch request".

 

[budwich]

OK... here is the theory. You have a bus problem caused by an issue with BCM or another component. The "babble" on the bus causes an issue with TCCM waking up. In essence, it remains asleep but since you are in the "selected drive" and require no change, things appear OK (of course, you would hope that the PCM would flag an important module not responding... maybe).
Anyways, somewhere along the way, the TCCM awakes and does some form of initialization checks (there are some that are done as part of a key on) which cause some form of selector switch test and encoder motor test. Potentially, if it was a "wake up" sequence, the program would not check for the "neutral request" as it would expect to be in a "vehicle startup" type mode since that is when the "normal wake up" would happen.

Of course, I know nothing of this, just general reading and my truck (2008) doesn't have a TCCM seperate from the PCM. It would be nice if you had access to a tech2 or someone who has one.

 

[TJBaker57]

The brake solenoid sounds like it's working normally, and even with the brake solenoid active I still could not turn the shaft manually.

This sentence confuses me. The brake is part of the encoder motor assembly. So if you have the encoder motor removed I see no way for it to have any effect on the turning of the shaft in the Transfer Case.

Am I missing something here??

 

[Mektek]

Not turning the TC shaft, just the encoder motor gearbox.
It appears to me that the function of the brake is to stop the motor at a precise point - exactly where the encoder indicates the shift point is. When I grab the gearbox output shaft on the encoder motor with a pair of pliers - it can't be rotated - with or without the brake solenoid being powered.
Along that same line, if I drove without the encoder motor installed, it would not cause any problems other than the inability to change transfer case modes and a service 4wd light.
While I have the encoder motor out I'll take it apart and relube all the moving parts. Being a early production TB the grease is probably hardened and the motor bearings could use some oil.

Until I get a conclusive cause of this problem I'll pull the ATC fuse and that will stop any uncommanded shifting.

About there being a bus problem, well, it could be. The TCCM is certainly awake - the mode switch lights are on. I've never noticed any active encoder motor test, although there is a passive test that measures the return voltage. To do a shift to 4lo requires a specific routine to executed by the TCCM, and part of that routine is a check for neutral and a signal is sent to the pcm that the TC is in 4lo. So it should not happen by some kind of random fault unless there's a bug in the firmware somewhere - what are the chances of random faults triggering buggy firmware in GM products????

I'll clear the codes and drive it with the ATC fuse out for a while and see if any codes come up.
That should identify any issues without risking grenading my driveline

 

[budwich]

I think the "asleep" state is more to do with the processor and not light states with the switch which are more likely "latched hardware states". At the key start cycle of the journey of the "event", did you notice what lights lit on the selector? All, or just 2Hi?

quote: "Along that same line, if I drove without the encoder motor installed, it would not cause any problems other than the inability to change transfer case modes and a service 4wd light."

that is also a good test of the coding system. disconnect your tccm, cause the warning, check for codes. Put the tccm back on line, do another key cycle, check status of light, check status of any codes. This will tell you what a "normal failure path" might look like in the overall operation of the system. I would still expect codes to remain in history for a bit.

 

[TJBaker57]

Ahhh... I looked at pictures of when I had my encoder motor out and didn't think there was even enough of a nub to grab with pliers!! I had mine out to replace the actual encoder sensor but never went so far as.to inspect the reduction geartrain. The encoder sensor does wear at the 2Hi location so if your is original I would expect some degradation of the sensor. You can check the sensor with an ohmmeter while the unit is disassembled. Look for fluctuations while slowly turning the sensor.

DTC C0327 Resolved

So after many years of occasional issues with the transfer case shift motor encoder I decide to fix the damn thing. Hooked up the Tech 2 to display the encoder return voltage and take it under the truck with me. Wiggle this, push pull that, tap here and there all the while watching that return...

gmtnation.com

I had the equivalent part out of my Yukon last year as well and it has a large gear reduction



The article I linked to Gears magazine details using a 9 volt battery for releasing the brake as well as another 9 volt battery for energizing the drive.

I have seen the class 2 broadcast from the TCCM after the shift to 4Lo or back. Just the other day I monitored and recorded ALL class 2 data bus traffic while shifting in and out of neutral and also when shifting 2hi to 4lo. An announcement from the TCCM to functional address 3B is seen after the shift is felt but the TCCM sends no traffic out on the bus before the shift. Responses are seen at functional address 3A from node $29 (EBCM) and node $10 (PCM). This makes me believe the TCCM monitors the broadcasts made from other modules when a shift to/from neutral is made and perhaps retains this information in memory.

 

[budwich]

so you are saying the tccm keeps track of "neutral / not neutral" state of the drive train. I don't see any of that data that the tech2 can find in the tccm as labelled points in the service manual.

More interesting that no "request" or otherwise is sent to "announce" a shift is going to happen.
Related to that, it is funny that the TCCM sets a "hard line" indicator towards the pcm for "4w LO activated".... is that all there is?... for which PCM responds.

 

[TJBaker57]

so you are saying the tccm keeps track of "neutral / not neutral" state of the drive train. I don't see any of that data that the tech2 can find in the tccm as labelled points in the service manual.

More interesting that no "request" or otherwise is sent to "announce" a shift is going to happen.
Related to that, it is funny that the TCCM sets a "hard line" indicator towards the pcm for "4w LO activated".... is that all there is?... for which PCM responds.

Regarding the neutral state, that is what I am seeing in the class 2 data, yes. I suspect there is a whole lot going on in memory and at a software level that GM does not divulge even to their own technicians so it doesn't surprise me to not see all with a tech 2.

This afternoon I looked over class 2 data during transmission shifts as well as 2hi to 4lo transfer case shifting. I isolated the messaging from the PCM that causes the instrument cluster display to reflect the new transmission position and secondly these '3B' messages that the transfer case replies to. I was able to identify the specific bytes to send and I even spoofed these messages and changed the cluster display as well as telling the transfer case we were in neutral when we were not. I then shifted the transfer case from 2hi to 4lo while in reverse!! Key On, Engine OFF of course,,,,I don't need to abuse my drivetrain at it's age.

I expected to maybe see periodic messages from the PCM 'reminding' the TCCM or anyone else what the current status of the transmission shift selector was but I saw no such messages over the course of maybe 3 or 4 minutes.

 

[budwich]

wow... great stuff. So my question back to you is: the difference in messaging between a "neutral" selection and others.... is it multiple bytes or bits? somewhat related and probably more difficult to ascertain but inquiring minds like info, is: you know the messaging that occurs as a result of gear shift "detection" (ie. the PN / transmission position switch) path.... but what about normal transmission shifting (ie. as the tranny goes thru in running gearing / rpm selection). The reason that I ask, of course, is that is there a possible "fake neutral" thrown out by the tranny at times during operation? not knowing anything about auto transmissions or transmissions in general beyond gear 1, 2, D, etc.

Is it possible that IF there was a "fake neutral" transition somewhere along the way and the TCCM "sucked it up", with a possible selector switch fault at any time, the two could meet up so to speak at a "bad time". Of course, there is more coincidence with also a "speed limit" but perhaps that is more to do with speed difference between from and back axles which running down the road would be 0.

 

[TJBaker57]

I need to do more data acquisition and analysis. Yesterday I set my adapter to listen solely for the 3B messages. While driving to town and back I observed two more values not seen while just shifting at rest. They seem to be related to moderate/high acceleration and deceleration. I am wondering if maybe this value is bitmapped.

So how does this relate to the thread subject? We're getting away from it here though it is related.I think I will start a thread for these matters of some interest.


But for now...I am wondering what if the TCCM somehow "missed the memo' due to some comm issue and the last message it got was that we were in neutral? Then along comes a voltage drop due to a degraded connection in the switch circuit making the TCCM think a shift has been requested. But then it has also been said there is a speed prerequisite that is supposed to be met before a shift to 4Lo as well. Seems like we are grasping at straws trying to come up with a plausible explanation for the errant shift to 4Lo, which may or may not lead to a solution.

I agree with Mekteks idea of either removing the fuse (easiest) or installing a switch to disable the TCCM. But would this cause something undesirable like losing the cruise control?

 

[budwich]

I think your theory on "missed message" is probably pretty good. Even though you think the subject thread is not directly related to the discussion, I think it is as the event leading to "bad outcome" are unknown. The "root cause" is not known and even if the TCCM is replaced, may still result in the "bad outcome" at speed. Of course, it depends on how much time and investigation some one wants to do at any point along the "question cycle".

One other point based on relooking at the original post. I am not fully versed in fault codes and the differences across OBD implementations, but relooking at the ones that the OP listed, the U1000 is a bus communication error for most references that I see and nothing to do with rear AC... although I guess there maybe some module id associated with it. A BUS error falls into your "theory" somewhat. Of course, as requested, what are the time stamps and other data associated with the codes.

Just browsing thru stuff I see a PIT3832C about loss of battery to the rear fuse box causing some strange symptoms like U1000, 4WD light, HVAC, etc. Not exactly the same, but the weird areas is somewhat worrisome. Another is PIT3154.... in this one, the module setting the dtc may not even be in the tech2 monitor list... sounds similar. It is somewhat related to the B2615 which could be set as a "result of loss of TBC 3 battery in the rear fuse block for the BCM for more than one second".

 

[TJBaker57]

Just browsing thru stuff I see a PIT3832C about loss of battery to the rear fuse box causing some strange symptoms like U1000, 4WD light, HVAC, etc

Just time for a quick note about this one regarding the 4WD light(s). While the TCCM controls the lighting of the indicators it is indeed the BCM that supplies the power for the transfer case indicator lights. I think this is done to provide the dimming function and this is the BCMs arena. I helped a guy online somewhere, Facebook I think, troubleshoot his no 4wd lights issue. We eventually found the cause, looked like something got dripped onto the ribbon cable at the BCM.

 

[Mektek]

Here's something I found in the service manual:

When the clutch pressure plate bearing fails, it causes the clutch lever to over-travel, allowing the transfer case encoder motor (actuator) to rotate to an invalid position.

Could this have something to do with my problem?

I got to disassembling the encoder motor. It was completely dry - appears that way from the factory. I greased the last two reduction stages and oiled the first two. The reduction ratio is so high that the output shaft can't be turned with pliers. I also checked the brake - it stops the motor from turning when applied and when powered it releases and the motor spins freely. And I tested the motor operation - the output shaft rotates fully in both directions. Also tested the encoder itself - the resistance moves smoothly and linearly.
So all appears to work normally.


I noticed that there is a lot of play in the transfer case shift shaft. It rotates without any resistance approximately 30 degrees before it hits a firm stop. Is this normal? Or does it indicate a lot of wear in the mechanism and perhaps this wear is allowing the transfer case under certain conditions to shift itself?

Another observation - the uncommanded shift to 4wd happens at 30-45mph. Over the past few years it has only been doing the occasional shift to 4hi, and for the first time lately to 4lo.

I was thinking of contacting the manufacturer for suggestions, but unfortunately after 124 years in business, New Venture Gear closed in the week of August 20, 2012,
So we have an "orphan" transfer case

Probably time to get a AAA membership - in case of disaster they can drag me away......