Allison 1000 & 2000 Gen 4 Fault Codes: P0703 Brake Switch Circuit Malfunction – GMT 800 Only
CIRCUIT DESCRIPTION:
A mechanical switch attached to the brake pedal sends a signal to the Transmission Control Module (TCM) indicating the service brake has been applied. This signal is either ignition voltage or a ground signal depending on the calibration installed in the TCM.
P0703 is a type C DTC.
CONDITIONS FOR RUNNING THE DTC:
1. The components are powered and ignition voltage is greater than 9V and less than 18V (12V TCM).
2. No DTC P0721 or P0722.
CONDITIONS FOR SETTING THE DTC:
The TCM sets a DTC when either of the following conditions occur:
1. The vehicle has accelerated 10 or more times with TCM input indicating brakes applied.
2. Decelerated 1 O or more times with TCM input indicating brakes released.
ACTION TAKEN WHEN THE DTC SETS:
1. No Grade Braking allowed.
2. Use the default assumption that the brake is off.
3. TCM inhibits TCC engagement.
CONDITIONS FOR CLEARING THE DTC/CHECK TRANS LIGHT:
A Scan Tool can clear the code 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 active.
DIAGNOSTIC AIDS:
Inspect the wiring for poor electrical connections at the TCM. Look for the following conditions:
1. A bent terminal.
2. A backed-out.
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.
