A Mazda 6 with a V6 3.0L DOHC using the front wheel drive six-speed AW6A-EL transmission is stuck in high gear fail safe. When referring to Mazda’s symptom related diagnostic procedures for no shifting, it states that when the gear position is fixed in 3GR or 5GR due to failsafe operation, malfunction is in the ATX (automatic transaxle). This should be accompanied with a transmission related code or codes but none were stored in the system. Only a P0328 for a failed Knock Sensor. Since a knock sensor is used to detect engine knock or ping (pre-detonation), a complete transmission failsafe condition is not suspected. Past history of knock sensors related to transmission concerns tend to eliminate this code as the cause. Knock codes in some Toyota vehicles have prevented a shift into high gear and some GM vehicles the code elevated line pressure in the transmission causing harsh shifts.
Typically, when a knock sensor indicates a problem, the PCM will retard the spark timing to avoid pre-detonation. In this case, engineers decided to write a program to failsafe the transmission should it think there is a hard failure with the wiring or the sensor itself.
This two-wire sensor is being monitored by the PCM. Should it see voltage go above 4.79 volts, the PCM considers this a hard circuit malfunction. Failure could be related to the sensor itself or related wiring from the connector to the PCM. When the car is raised on the lift, this sensor is located on the engine block below the rear cylinder head area. Inspect the condition of the knock sensor, connector and wiring and correct or replace where necessary.
1998 to 2005 Mercedes Benz “M” Class vehicles may come into the shop with the transmission in limp mode and the transfer case inoperative. The “Low Range” lamp illuminates and turns off in five second intervals indicating a fault is present (Figure 2).
Trouble codes 51 and 147 are set in the TCM pointing to an implausible gear ratio error. P1831 may also be set for a temporary loss of communication between the Transfer Case Control Module and the Transfer Case Shift Motor (Figure 3).
Additionally, the “Low Range” Lamp will illuminate when the Transfer Case Control Module is disconnected but not when depressing the Low Range button.
Beginning in October of 2015, the 8L90 had the #7 checkball in the control valve solenoid body eliminated (Figure 4).
Additionally, the lube override enable valve was eliminated from the upper valve body (Figure 5).
The lube override enable valve bore will be cast shut as well as the channels feeding the valve. No modification was made to the spacer plate.
The lube override enable valve was found to have shown minimal effect on improving fuel economy.
- Control Valve Solenoid Body.
- Upper Valve Body.
The modified valve body will back service all 8L90 transmissions.
Order Valve Body By Vehicle Application & VIN Number.
November 2019 Issue
Volume 36, No. 11
- Mazda 6 – AW6A-EL: Stuck in High Gear Failsafe
- Mercedes Benz M Class: Transmission in Limp Mode
- GM 8L90: Elimination of the #7 Checkball