A Twist of Fate
Aisin Seiki’s six-speed transmission used in medium-duty trucks is called by various names. It is the AS68RC in Dodge incomplete-chassis packages using L6 6.7-liter diesel engines. It is also called the AS6 in the Mitsubishi Fuso, the M036A6 in Nissan UDs and the A465 in Isuzu NPR trucks.
One of the common failures with this transmission is the stator turning in the pump (figures 1 and 2). A similar problem occurs with Allison 1000/2000 transmissions, in which it causes a TCC overheat and/or C1-clutch failure. You can quickly determine this by pulling the return cooling line. Oil comes out of the case fitting instead of the cooling line.
Isuzu NPR/GMC Tiltmaster
The Isuzu NPR Commercial Truck and GMC Forward Tiltmasters use an exhaust-brake system that assists in engine braking during downhill coasting while the trucks are carrying heavy loads. This system is used instead of the familiar Jacobs Vehicle System known as the “Jake Brake.” When the exhaust brake is activated, it restricts the flow of exhaust with a flapper valve in the exhaust pipe similar to that of a heat-riser assembly (see Figure 1). When the flapper valve closes, exhaust pressure increases, causing air intake to decrease. This increased backpressure and slower air intake reduce torque transfer to the geartrain, producing the desired on-demand engine braking needed during downhill coasting with a payload.
