Masquerading transmission problems

A GM truck equipped with an Allison 1000/2000 transmission and a N.P. (New Process) 263XHD Transfer Case (figure 1) comes in to the shop with a complaint of harsh coast down shifts followed by a perceived no move condition. The speed signal seen in the instrument cluster intermittently jumps as well. Code P0746 for “Solenoid “A” Controlled Clutch Stuck Off performance code” and/or P0776 for “Solenoid “B” Controlled Clutch Stuck Off performance code” may be stored in memory.

Once the code sets, the computer aborts to a high gear failsafe. The abort also causes the transmission to have no reverse. With the vehicle being very heavy and stuck in high gear, it doesn’t move forward very well either.  This deliberate computer abort has at times been confused with thinking there is a problem inside the transmission causing a no move condition. As a result, the source of the problem being inside the transfer case is at times overlooked so the transmission is pulled. When the transmission is dismantled, no evidence of a problem related to the complaint is observed.  

The transfer case is made of magnesium. There is a steel bearing inside that supports the load of the output shaft and rear drive shaft yoke. This bearing wears into the case compromising the support due to a combination of load and dissimilar metals (steel and magnesium do not cohabit well with each other).

The torque load is then placed on the rear output shaft yoke bushing. With this bushing not being designed to support such a load, abnormal wear takes place. The wear can be at times so extreme the yoke wears into the housing (figure 2). Crazy as it sounds; there are occasions where under these extreme wear conditions, the rear seal will not leak. This too assists in diverting ones attention from the real problem area.  

This excessive wear coupled with damage to the bearing bore due to dissimilar metals (steel and magnesium), produces enough slop in the transfer case output shaft that the tone wheel is able to erratically rub into the tip of the vehicle speed sensor (figure 3).

Strangely enough, the computer doesn’t generate a code for a compromised speed signal as one might think. Especially since the irregular air gap of the speed sensor causes the speed reading in the instrument cluster to jump intermittently.

The damage to the transfer case in some instances is so severe, it is not worth rebuilding. But once the problem with the transfer case is corrected and the codes are erased, the vehicle works flawlessly.

But why did code P0746 and/or P0776 store rather than a VSS code? With these codes stored, along with a perceived no move condition and the rear seal not leaking, the real cause to the problem is certainly masqueraded.

The best answer I have is a system description from GM. It says:

The transmission has 2 pressure control solenoids that controls the flow of transmission fluid to engage clutches. The flow is directed through shift valves that allow it to engage different clutches. The vehicle speed sensor and automatic transmission turbine speed sensor contains a permanent magnet surrounded by a coil that gives off a continuous magnetic field. As the vehicle is driven, a speed sensor rotor located near the magnetic pickup of the speed sensor coil also rotates. This rotation produces a variable AC voltage signal. The frequencies of the signals are proportional to the transmission turbine and output speeds. The transmission control module (TCM) uses the signals to calculate the gear ratio. The TCM compares the computed ratio to the commanded gear ratio.

From this description it is clear that the vehicle speed signal plays an intricate roll in determining gear ratio. My guess is that although the speed sensor dug in to the tone wheel, the signal was not bad enough, long enough to trip a code. Yet the readings were confusing enough to produce an incorrect gear ratio calculation thus blaming the solenoids for not doing their job.

Heavy duty trucks with diesel engines are prone to frequent failure in this area due to the heavy work load they continuously undergo.  As well as for those who use an aftermarket tuner or any other method of boosting turbo power. Rockland Standard Gear has designed a beefy aluminum case half that prevents continued failure in this area (figure 4). This will also eliminate another well known problem this transfer case suffers from. That is, the lubrication pump (figure 5) wearing into the case enough to dig a hole through it. 

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