Four wheel drive Jeep vehicles that use the NAG1 transmission may suddenly have a no move condition. The presence of a noise may or may not be heard when the transmission is placed into gear. Sometimes you can feel the engagement, you can hear a slight noise yet there is no movement. There are no pressure taps to do any reasonable diagnosis and there may or may not be gear ratio codes stored in the TCM. When the transmission is removed and disassembled all the internals look good and all air checks well. The filter and valve body also checks well. By this time the transfer case becomes suspect. When this is disassembled it too all looks well.
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What causes the problem which can be easily missed is a partially stripped output shaft spline where it fits into the transfer case adapter (Figures 1 through 5). Besides the output shaft spline stripping out the internal spline of the adapter also strips out. There have been other occasions on our help line when this adapter is left out causing this no move condition after a rebuild. The adapter rolled down the bench and was out of site. Of course it was then out of mind and the transfer case was bolted up without it.
In other situations where the output shaft can spin yet is unable to drive the transfer case, a vehicle speed signal can be seen in the instrument cluster. This is a clue that something is wrong between the transmission and transfer case. With this NAG1, there is not an output speed sensor in the transmission assisting in the elusiveness of the problem. Another clue that could work would be to engage park when at a lift throttle low rpm state and you can hear the park pawl ratcheting across the park lugs. This will inform you that the output shaft is spinning. If you listen closely, while you are on throttle, you may even hear the transmission shifting. This too would point to a problem between the transmission and transfer case or the transfer case itself.
Since we are talking about the NAG1 and transfer cases, this transmission is known to have converter clutch vibrations due to valve body wear issues. And if you replace the valve body with a remanufactured one you may find that the problem still exists afterwards. Some 2011 Jeep Grand Cherokees with a 5.7L and 3.6L engine using the 4WD Quadra-Trac (II) Transfer case may exhibit shudder during acceleration coast or deceleration in both Auto and Sport modes. In these situations the DTCM software was configured to reduce the 4WD torque to the front axle during nonslip events under normal driving conditions (NUMBER: 21-004-10 REV A).
Earlier Commanders and Grand Cherokees may also experience a transfer case electrical failure may result in an unintentional shifting of the transfer case into the neutral position. There is a campaign to reflash of the final drive controller which governs the transfer case. The Chrysler recall campaign number is N23 should this be a customer complaint that comes up in your shop.