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Allison 1000 & 2000 Gen 5 Fault Code:P0837 Four Wheel Drive (4WD) Switch Circuit Range/Performance

CIRCUIT DESCRIPTION:
The 4WD low signal circuit notifies the TCM that the vehicle is in 4WD low range. When this function is enabled, the TCM adjusts specific shift calibration values to accommodate for changes in vehicle performance which result from the shift to low range at the transfer case.

Wires used for this function can vary depending on the vocational package used, therefore it is important to check the diagnostic tool and/or the calibration summary to see which wires are implemented. Wires 102 and 103 (signal ground return) should be used in part time high N/V input function (CC1) applications along with either wire 122 or 101, depending on the vocational package used. Full time high N/V input function (CC2) only uses wire 103 wired directly to wire 122, leaving wire 102 open.

ACTION TAKEN WHEN THE DTC SETS:
1. DTC P0837 is stored in the TCM memory.
2. The CHECK TRANS light illuminates.
3. The TCM inhibits GPI function.

CONDITIONS FOR CLEARING THE DTC:
Use the diagnostic tool to clear the DTC from the TCM history. The TCM automatically clears the DTC from the TCM history if the vehicle completes 40 warm-up cycles without the DTC recurring.

DIAGNOSTIC AIDS:
Switch Status Displayed in The Diagnostic Tool:
Transfer Case Range: Low
Input-High NN Switch State 1 (Wire 122 or 101): On (closed)
Input-High NN Switch State 2 (Wire 102): On (open)

Transfer Case Range: Neutral
Input-High NN Switch State 1 (Wire 122 or 101): Off (open)
Input-High NN Switch State 2 (Wire 102): On (closed)

Transfer Case Range: High
Input-High NN Switch State 1 (Wire 122 or 101): Off (open)
Input-High NN Switch State 2 (Wire 102): On (closed)

Inspect the wiring for poor electrical connections at the TCM. Look for the following conditions:
1. A bent terminal.
2. A backed-out terminal.
3. A damaged terminal.
4. Poor terminal tension.
5. A chafed wire.
6. A broken wire inside the insulation.

When diagnosing for an intermittent short or open, massage the wiring harness while watching the test equipment for a change.

You may have to drive the vehicle in order to experience a fault. Use the data obtained from failure records to determine transmission range and/or certain vehicle operating variables such as temperature, run time etc. This data can be useful in reproducing the failure mode when the DTC was set.

Check the diagnostic tool or calibration summary for specific input wire numbers.