Allison LCT1000 Does Not Engage During High Idle Without Brake Pedal Input - kw 6.6 8.1 LB7 LLY L18 M74 no drive DTC reverse

When servicing the vehicle with the VIN you entered, the following diagnosis might be helpful if the vehicle exhibits the described symptoms.

A customer may comment that the Allison LCT1000 transmission will not engage without applying the service brake during high idle. If the high idle then disables because it has warmed up, the transmission will still not honor the gear request and hold the gear state at neutral for the rest of the key cycle. This applies to the PTO idle up, manual high idle (button on dash), or the Elevated Idle warm up feature (LLY diesel only).

The calibration does not recognize a PRNDL change for disabling high idle, and the transmission controller will not honor the requested gear while the idle is high. The calibration will disengage high idle if the service brake is depressed, but since the C4500/5500 trucks do not have a BTSI (brake transmission shift interlock), it is possible to put the vehicle in gear during high idle activation if the service brake is not depressed.

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Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance. This diagnostic approach was developed for the vehicle with the VIN you entered and should not be automatically be used for other vehicles with similar symptoms.

(03 - 04 Chevrolet Kodiak) and (03 - 04 GMC Topkick )

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