The JH-K/M/A/B equipped with an Allison transmission has a common issue where the sprocket and wrench light up on the dashboard and the vehicle will stay in 3rd gear and exhibit lack of power from a stop and loss of top speed.
The common failure mode here is a fault with VSS or speed sensor located at the tail shaft of the transmission. The wires to the VSS can bend and break or have a failed previous repair which will cause the powertrain control module to lose the signal. Sometimes the wires can break inside the protective insulation which can be a little tricky to find.
Inspecting the wires leading to the vehicle speed sensor on top of the output shaft may reveal damaged wiring. Locate the sensor on the back of the transmission, pull back the protective tubing and inspect the wires. Look for broken insulation or failed repairs. If the wires look OK, gently try to stretch small sections of the wire. If there is a broken wire inside of the insulation, you will be able to stretch the insulation. Age, heat, and stress cause these circuits to fail. If the dash light is flashing, the larger feeder harness may have damaged wires. Repair using a heat shrink connectors and tape, then secure the harness to prevent damage. Reinstall all protective tubing and clamps.
After repairing the wiring, test drive truck over 15 mph so the computer can reset and cancel malfunction indicators. This fix should be a road-side, 10 minute operation. Look at this sensor and wiring before calling for a tow truck.
Please see the video below describing the issue and repair. You could get the
customer back on the road with little hassle.
Allison Transmission Repair Video (Click Here)