4 wheel drive not engaging - recent transfer case swap (NP246)

Discussion in 'GM Powertrain' started by Family Force 6, Dec 26, 2012.

  1. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Haha, we have a test. OK, jack up one rear wheel, with tranny in PARK and try to turn that wheel. Please make sure you block the other wheels before you jack the car.

    Before we go to the jacked up wheel, let's look at why the wheel with lest resitance get's all of the power. First of all, the driveshaft provides the rotation (not the wheel). The pinion on the end of the driveshaft is engaged with the big ring gear, turning the ring gear will cause the planetary assembly to revolve. A gear on the assembly is free to engage one of the two sun gears (which are connected to each axle shaft). This planetary gear will engage the sun gear with the least resistance.

    Ok, back to the jacked up wheel. The answer, is that the wheel in the air will not turn.

    It will not turn, because it is the wheel with the least resistance and it is connected to the driveshaft which is prevented from turning because the tranny is in PARK. Take the tranny out of PARK and you will turn the driveshaft.

    That is the reason I suggest blocking the other wheels, because the wheel on the ground is the wheel with the most resistance and the wheel not connected to the driveshaft and not being stopped from rolling by the tranny being in PARK.

    Now, to the OP, sorry, we seem to have gone a little :sign0018:
  2. a.graham52

    a.graham52 Rockstar 100 Posts

    thats why i feel that the right front wheel should not spin if jacked up. and if it does need to check and make sure the drive shaft isnt turning too. its possible if the actuator is partially engaged it could spin when both wheels are lifted and the drive shaft is spun by hand.
  3. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    You are absolutely correct, my apologies for posting incorrectly in post #7.

    You are right, if the truck is in 4WD and the right front wheel is jacked up, the wheel should not turn. If it turns, the two axle shafts are not connected.

    Which by default, means the OPs test, which I answered in post #7, means the truck was not in 4 wheel drive. I will go back and fix that post. Thank you.
  4. Family Force 6

    Family Force 6 Rockstar 100 Posts

    OK, I'm a little confused. Are you saying that in 2wd if jack one front wheel up and I turn the front drive shaft the front wheel will turn, but if I'm in 4wd and I do the same thing the front wheel will NOT turn? (BTW I did have the vehicle in drive with the engine off and fairly sure that 4wd was engaged on the transfer case- at least the 4hi light was on). It would seem to me that the front wheel would not turn when the front drive shaft is turned when it's in 2wd. I know the front drive shaft turns in 2wd put I assumed the front axle would not be engaged.

    Oh, and when I put it into 4lo it does move forward at very low speeds. Based on an earlier post that tells me that the transfer case is working properly?

    Anyways, what should I do to test if its the front axle that is not engaging or a transfer case problem.
  5. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    OK, look at it this way, in 2wd the front driveshaft is not connected to the rear driveshaft, in 4WD, the job of the t-case is to connect them. If, as you stated in an earlier post, you are able to turn the front driveshaft when your truck is in 4wd and the front is in the air (one wheel, or two), then the clutches are not compressed in the t-case.

    The 4LO test tells me that the t-case is receiving the commands to switch into 4LO (that is a gear change inside the t-case), however you may not have the complete set of commands to lock the clutches. I'll bet if you place it into 4LO, move it to make sure, put it into PARK and turn off the truck, and then jack up the front pass side wheel, I'll bet the front driveshaft will turn.

    If it turns, you have a problem in the t-case, or in the signals going to the t-case.

    When the switch is moved to any 4x4 position, the TCCM (transfer case control module) commands the encoder motor to engage 4WD, after this happens, the TCCM commands the front pass axle actuator to engage the two axle halves. A return signal will tell the TCCM that the front axle is connected, the TCCM then tells the encoder motor to lock the clutches, sending power to the front driveshaft (note: the 4auto is slightly different).

    Somewhere the sequence is breaking down.
  6. Family Force 6

    Family Force 6 Rockstar 100 Posts

    Finally got a chance to get under there pulled the fill plug. The oil is black and has a strange smell to it (maybe that's the way the Autotrac fluid smells). Anyways, it only has about 3-4K miles on it since I put the new unit in so something is obviously wrong.
    It is now occassionally making an intermittent noise when coasting that can get extremely loud. As I said earlier it does go in to 4Lo range but the 4wd (front wheels) will NOT engage.
    I'm going to drop the case and crack it open.
    Does anyone have any ideas on what might be the problem? I'm thinking maybe the shift fork.

    Also, any good ideas on cleaning the case? What chemical do you guys all use? Kerosene? Brake cleaner? Mineral Spirits?
  7. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    I have had luck with simple green when cleaning large parts. Your grinding noise in reverse could be a cracked sun gear in the trans. When my sisters went out it would grind in reverse occasionally, then it did it all the time in reverse, eventually she lost reverse completely. She drove it like that for 3 years! She just pulled into parking spots where she could drive forward out of.
  8. Family Force 6

    Family Force 6 Rockstar 100 Posts

    OK, final update.
    Apparently the rebuilt case I purchased off of craigslist had been dropped or knocked hard enough to cause the oil line to become disconnected (or so said the mechanic that cracked open the case). No oil to components = short life for transfer case. The case was so bad that almost none of the components could be used. He said it wasn't that uncommon, but I've never heard of that. He also says that he uses a little lock-tite on the oil line fitting to keep it in place.
    Anyways, he found another case that was recently rebuilt and installed it for $950. Ouch! Still have oil leaks everywhere and 198,000. This thing has been one repair after another over the last 6-12 months and is starting to wear thin on me. I'm actually looking at the 04 Expedition as an upgrade. They're a little cheaper and look nice. (let the flame throwing begin!)
    Thanks everyone who responded to this post.

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