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2001 Suburban stuck in 4WD 4H

Discussion in 'Chevy Trucks' started by rjpjnk, Jan 28, 2010.

  1. rjpjnk

    rjpjnk Junior Member
    from NJ
    Messages: 14

    Hi all,

    First post here. I'm hoping you can help me with a problem I'm having with my 2001 Suburban. It is stuck in 4H. This vehicle has the standard 4 button control switch. When I press any button (other than 4H) I first hear the TC encoder motor run, then I hear the front axle actuator motor run, then I hear the TC encoder motor run again, then the front actuator run again, then it gives up and settles back on 4H. All through this process the button I pressed is flashing (2H for example). This whole process takes about 7 seconds.

    I have tried this with all wheels off the ground. Initially all wheels are engaged, i.e., if I turn the front left, the front right turns the opposite way. If I turn the rear the front turns as well, etc. After pressing the 2H button, if I go out and manually rotate a front wheel while the shift motor sequence is running as described above, I can feel the front axle unlock momentarily into 2WD and then it locks back up again. So I believe the front actuator is working, but for some reason the TCCM is not satisfied. Is there a position sensor for the front?

    I have already fixed the switch a couple of years ago by resoldering all the terminal connections on the PC board. Also, since every button lights up and makes something happen when pressed I am assuming the dash switch is not the issue.

    I have removed the TC encoder motor and verified on the bench that it does turn over its full range in both directions as voltage is applied and reversed. I also got the schematic for the encoder motor sensor switch (located outside the encoder case) and verified that all 4 internal switches appear to be opening and closing at various positions of rotation. I opened the encoder case and it's gears are clean as well. I am able to manually shift the TC into 3 distinct positions. One of these is Neutral. I assume the others are Hi and Lo. The front shaft always turns, but I understand this is normal. My guess is that the TC, the enocder, and its sensor are therefore working.

    I am wondering if the front axle actuator might be the problem or the TCCM itself. Something is apparently telling the TCCM that the shift from 4H to 2H was unsuccessful. Is there any position feedback from the front axle actuator, or is it just a simple 2 wire device?

    Sorry for the long post, but I've been working on this for a while.

    Thanks for any help,

    Rich
     
    Last edited: Jan 28, 2010
  2. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    I admire your through diagnosis so far Rich. Many guys don't know how to at least check the basics such as you did before looking for answers. And by your descriptions so far I'd say it needs the switch assembly replaced as it's sending errored signals to the TCCM.

    The only other real cause would be a defective TCCM but without a good scanner to view data as it's being received there's no way to 100% positively say for sure that it's the switches, but I'd bet it is as they're the most troublesome component in the system by far for your year of truck.
     
  3. rjpjnk

    rjpjnk Junior Member
    from NJ
    Messages: 14

    Thanks B&B. You are probably right about the switch. I did fix the switch a few years ago when I re-soldered it. I was not working before then and re-soldering the pins fixed it instantly. I re-soldered it again yesterday just in case with no change. Do you have a schematic of this 4WD system? I thought the switch was just a simple resistor network. Maybe there's more to it.

    Is there any position sensor in the system other than the one on the TC encoder? I'm looking for any possible source of a feedback signal to the TCCM that could be invalid.

    Regarding the scanner. It does have the Service 4WD illuminated, but there are no stored P DTC codes. I only have a basic scanner. If I take it to the dealer and pay them 100.00 they say they will diagnose it. This might be worth the cost, but is not nearly as satisfying;-) When they pull the TCCM codes will they be able to tell me with some certainty which part is defective? In other words, should I pay the 100 bucks or buy a switch?

     
    Last edited: Jan 28, 2010
  4. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    I may have the schematics for the Auto 4WD 246's. I'll check and post them if I do.

    Yes there's also feedback on the white/black wire from the axle actuator to let the TCCM verify the actuators position and whether it's correct for the command function.

    Depending on the specific model you're using, many of the basic scanners can't retrieve 4WD specific codes as they're stored in the TCCM. So just because there isn't any showing with your basic scanner doesn't mean there aren't any in there. A good scanner would be able to better identify the issue not only from retrieving any codes but also watching the commanded functions as they are pressed, as well as using the bi-directional controls to perform the function manually.

    Spending the money on the switches or the diagnosis first is your call inevitably. But personally speaking I don't like to endorse throwing parts at something as a means to diagnose so I'd go for the diagnosis first since your lacking in proper tools prevent you from doing it yourself. The satisfaction of repairing it yourself isn't always cost effective, sometimes it's less expensive to simply pay the diagnosis time.
     
  5. rjpjnk

    rjpjnk Junior Member
    from NJ
    Messages: 14

    Thanks very much for looking for the schematic.

    Interesting that the front axle actuator has a position feedback path to the TCCM. If I had the schematic I might be able to determine what signal should appear on this line. Perhaps you already know? I would have guessed this sensor was the primary suspect since the actuator is dissengaging to 2WD and then re-engaging to 4WD every time. I am curious, why do you think it is more likely the dashboard switch?

     
  6. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Simply because it's doing just that, engaging, disengaging and then re-engaging again when it shouldn't (2WD), which it shouldn't do unless it being commanded by something, namely the TCCM, which I suspect is receiving errored data from the stitches thus it's not functioning the actuator correctly.
     
  7. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    If I recall correctly the black/white feedback wire supplies a 5V reference when the actuator is engaged and should show 0V when it's not. But I don't suspect an issue with the actuator anyway. It's only doing what it's told.

    Here's some diagrams if you'd like to chew on them a little. I may even have the TCCM pinout diagram too just for fun...

    Actuator.gif

    Encoder.gif

    Switch.gif
     
  8. rjpjnk

    rjpjnk Junior Member
    from NJ
    Messages: 14

    B&B,
    Thanks so much for the schematics. It appears, from the third one, that the TC control switch can easily be tested on the bench. The only lines that do anything appear to be H and A, which select one of 4 possible resistance values. There does not appear to be any active circuitry for selection. All the other lines appear to be for illumination only. Of course there could be hidden portions. The TC encoder motor logic (2nd schematic), for example, definitely requires the 8V supply on pin E, as the internal switches, though they look simple, are actually active magnetic 3-terminal devices. I discovered this during bench testing it. Same could apply to the dash switch. I'll find out this weekend.

    This is a great help. Now if only I knew the values of these resistors.

    Also, it should be fairly simple to build a substitute box to emulate the actual switch and use this to control the system as a test.
     
    Last edited: Jan 29, 2010
  9. rjpjnk

    rjpjnk Junior Member
    from NJ
    Messages: 14

    B&B,

    What is the normal sequence of events when shifting from 4H to 2H?

    As you know in my case I am seeing TC motor activity followed by front axle motor activity, and then it repeats. If it were working properly what should I observe?

    Should there in fact be any TC motor activity in this case? I don't see why there would be since there is no TC change required between 4H and 2H if I understand correctly. They are both High ratio.
     
  10. bocefus78

    bocefus78 Junior Member
    from Indiana
    Messages: 11

    stuck in 4hi

    prob. the front axle actuator motor. If its anything like mine, (05 avalanche) its plastic housing cracks & even though you can hear the motor work, its not lined up right anymore so therefore, no engage, or disengage in your case. Mine luckily stuck in 2wd. Hope that helps.
     
  11. rjpjnk

    rjpjnk Junior Member
    from NJ
    Messages: 14

    It seems like the front axle actuator is engaging and disengaging because I can spin the front wheels independently (up on jack stands) for a second during it's attempted shift. Then it re engages back to 4H and the axle locks up again. So I'm pretty sure the motor part is good. Was just wondering if the sensor part was not good, so that even though the actuator has done its job, the TCCM is not satisfied because the sensor signal is not what it is expecting. I have a theory that it may then try a second time, then give up and stay in 4H.

    Just wondering. If I simply unplug the actuator will the vehicle be in 2WD then, or will it be in 4WD? If I could get it into 2WD at least I could drive it.

     
    Last edited: Jan 29, 2010
  12. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    It will only attempt to complete the shift once and for a period of approx 15 seconds. If its then not satisfied with the results it returns to the previous range.

    Just to verify Rich, this is a 1500 Sub correct? And it's stuck in 4hi, not Auto 4WD correct?

    Also, if you need to drive it simply disconnect the connector at the axle actuator. Of course it needs to be in the disengaged position first or it isn't effective. You already know how to check whether it's engaged or not.
     
  13. rjpjnk

    rjpjnk Junior Member
    from NJ
    Messages: 14

    Yes, it is a 1500.
    Yes, stuck in 4hi not auto (at least according to the dash switch lights)

     
  14. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Still sounds like a command issue but a scanner would remove all assumptions by verify what commands are taking place and how the system is responding to said commands.

    Did you check the feedback signal from the axle actuator on the white/black wire for any out of position errors? If so and it checks out good, then you'll either need to shotgun it by replacing the switches or get a scanner on it to see whats really taking place.
     
  15. rjpjnk

    rjpjnk Junior Member
    from NJ
    Messages: 14

    update TC encoder, front actuator, 4WD dash switch

    Updates from today

    (1) I put the TC encoder back on after disassembling it and convincing myself it and its sensor were working. When I had the TC encoder off, I manually switched the TC to neutral with a wrench. I also manually energized the encoder motor to line up with this new TC shaft position using a battery. I then bolted the encoder back on the TC. After this The red neutral light was on on the dash. Good sign I guess since it is now in neutral, but nothing worked at all. No movement at all in either TC encoder or front axle actuator. I thought I broke the darn thing. Then I applied battery voltage to the encoder and the TC rotated to 4Lo and the 4Lo light lit up, so I reversed the battery and the TC rotated to 4Hi. Then I plugged the harness back in and after that was back to the same situation as yesterday with both motors working two times. Interesting...

    (2) I measured the resistance between pins A and H on the dash switch. I got 5 distinct values depending on which button I pressed.

    Here is what I measured:

    No buttons pressed = 9K ohms
    4WD Auto = 76 ohms
    2Hi = 1.5K ohms
    4Hi = 670 ohms
    4Lo = 2.3K ohms

    This makes me think the switch might be good. Of course I don't know the correct values for these resistances, but it at least changes and shows signs of functioning.

    (3) I tapped into the blk/white wire front the front axle actuator. This is the wire that sends position feedback to the TCCM. When the vehicle is in W4D the voltage on this wire measured about 11 volts, and when it is (briefly) in 2WD, during one of its unsuccessful change attempts, the voltage drops to zero. I then removed the actuator and observed its operation as I pressed the buttons on the dash. (Remember, I am stuck in 4Hi). The actuator is initially extended and the voltage on the sensor line is 11V. When I press 2Hi the actuator fully retracts (winds up) and the voltage on the sensor line drops to 0V. So the actuator appears to be working perfectly I think. Now because my system is defective, the actuator then starts to turn the opposite way and goes back to the extended position (4WD). This completes one cycle. After this the TC encoded runs again and the actuator does the same in/out dance a second time. Then system gives up and stays in 4Hi.

    Amazingly, I did get it to go into and stay in 2Hi one time! I don't know how, and I can't repeat it yet.

    So I looks like ther is no problem with the TC encoder or front actuator. The switch seems to do something, but is it the right thing?

    Suspicion at this point is that either the switch is bad, or the TCCM is bad.

    UNless something occurs to me tomorrow, more than likely I'll take it to the dealer, pay the 100 bucks and get the proper diagnosis now, but I wanted to give it my best try. I learned a lot about how this system works by this, and I hope this thread is useful for someone else someday.

    Rich



     
    Last edited: Jan 30, 2010
  16. M.S.P.M.

    M.S.P.M. Senior Member
    Messages: 107

    I had a similar problem andit ended up beingthe transfer case encoder motor.
     
  17. rjpjnk

    rjpjnk Junior Member
    from NJ
    Messages: 14

    B&B,

    When I looked on gmpartsdirect.com I see a second electrical part in the front axle. It lists front actuator (8) and engagement switch (7). Is this a different model? Looks like my actuator already contains a switch. Maybe the one pictured is the old thermal style?

    Also, how can I find the correct GM part numbers for the following components on my vehicle:
    TCCM
    4WD control switch
    TC encoder motor assembly.

    Thanks.

    [​IMG]
     
    Last edited: Jan 31, 2010
  18. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    It's just a generic image that's covering several different generations of trucks. Yours is not equipped with a separate switch, it's internal to the actuator as you suspected.

    The encoder and actuator can be purchased just from the year, make, and model of the vehicle, however the TCCM p/n is a little more critical. Either have to get the number directly from the one in the truck, or give the dealer the VIN and have them look up the correct one as they're not all the same and there were a few revisions over the years and model lines. But in all honestly in my personal experience the TCCM's are seldom the cause. The switch assembly's and encoders are by far the more replaced components in the system. Second in line is a poor electrical connection.

    And as I mentioned previously you're diagnosis was worthwhile (just in education alone) but a good scan will tell all...
     
  19. rjpjnk

    rjpjnk Junior Member
    from NJ
    Messages: 14

    It's fixed!

    I am very happy to report that today I fixed the problem! It turned out to be a bad wire.

    While listening to the sound of the encoder motor I noticed it sounded strained, like it was stuck, yet when I connected a battery to it manually it moved and sounded different. So I got out the meter and using some test wires I attached probes to the 2 motor leads, and also the 2 brake leads. (As you may know the encoder motor has a built in brake. It is solenoid operated and is normally in the stop position. The motor shaft can not move unless powers is applied to the brake) The brake can be seen in B&B's second schematic above. It is inside the box labeled "Transfer Case Encoder Motor" at the lower right.

    The motor voltages were good, but there was 0 volts across the brake terminals. As seen in the schematic, the break receives 12V by the orange wire continuously, and the other lead (Tan wire) is pulled to ground by the TCCM (Pin E1) when the system wants to release the brake. Well the 12V was good on the orange wire, but the tan wire was not connected to ground.

    I then traced this tan wire back to the plug on the TCCM, and tested its resistance, which was about 300 K ohms. Should have been zero. When I wiggled the bundle it went out altogether. So I cut the tan wire at both ends and simply spliced in a new wire.

    Now everything works perfectly.

    It's a little strange that one wire should break like this. It's in a bundle with many other wires and I see no signs of abrasion or anything. How weird.

    Well, I couldn't have done it without those schematics. Thanks a million B&B!

    It feels great to not have to buy a single part to fix this. Makes it all worthwhile ;-)
     
    Last edited: Jan 31, 2010
  20. B&B

    B&B PlowSite Fanatic
    Messages: 12,777

    Makes perfect sense and as I stated a faulty connection is the second most common issue next to the components themselves. It would have been an easy find to discover the brake wasn't releasing with a scanner, but it would have still ultimately lead to the brake ground wire and an OHM test to finish the job, would have just been ten times faster. However think of the knowledge you gained in the process of working with it yourself. Congrats :drinkup: