Expert: Carl Brandt - 1/10/2012

Question
I have a 1997 chevy Z71 and when i put it in 4x4H the lights flash for 4x4H and 4x4L and nothing happens. 4x4L is working just fine, but 4x4H will not engage at all.

Answer
Lp,

It sounds like a faulty motor/encoder or transfer case control module.

OPERATION
The Transfer Case Control Module (TCCM) operates the motor/encoder in one direction by energizing one relay while the second relay is de-energized. The TCCM operates the motor in the reverse direction by energizing the second relay while the first relay is de-energized. The encoder part of the motor/encoder sends shift positioning signals to the TCCM.

The motor/encoder converts a shaft position, representing a mode of range selection, into electrical signals for use by the TCCM. By interpreting the four channels (A, B, C, D) of the encoder, the transfer case control module can detect what position the transfer case is in 2HI, 4HI, 4LO, or in transition between any of the two.

Four hall effect sensors of the motor/encoder are used for channels (A, B, C, and D). These sensors provide a path to ground when a magnet passes over them. A rotating magnetic ring causes the hall effect sensors voltage to drop from 5 to 0 volts. The TCCM detects the voltage of all the channels and interprets the current transfer case shift position.

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The Transfer Case Control Module (TCCM) checks the encoder signal to verify that the transfer case is in either 2HI, 4HI, or 4LO.

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While the transfer case is shifting, the encoder within the electric-shift motor is monitored for the proper operating sequence. If during a shift, the encoder changes to a incorrect position, an error counter in the TCCM starts to count the number of times the encoder fails. If the encoder fails eight times, the Transfer Case Control Module (TCCM) produces a DTC of 2 indicating a (Encoder Fault). When this happens, the TCCM outputs a signal to default the rail shift pattern in the encoder to allow for only shifts into 2Hl and 4LO.

I hope that this answers your question,

Carl