AW-4 TRANSMISSION REVIVAL
FIXING A SLIPPING SLUSHBOX
Fixing a slipping slushbox.
THE AISIN AW4 IS A FOUR-SPEED automatic transmission used in some Jeeps and as the A340 in various Toyotas. It is electronically controlled in the forward gears and has been used in numerous variations in 2WD and 4WD models. It uses a transmission control unit (TCU) along with valve body solenoids to control shifting based on inputs from a speed sensor, throttle position, and brake pedal actuation. Electronic torque-converter lockup is also controlled by the system.
This transmission has proven itself over the years to be highly reliable. It may last several hundred thousand miles with proper care. History seems to indicate that its biggest enemy, as with many fluid transmissions, is excessive heat, which damages the hydraulic fluid, leading to internal parts damage. Excessive heat can occur from running with too low a fluid level, from straining the transmission by running too tall a tire with poor match of axle gearing, or from simply working the transmission too hard too long and generating sustained high fluid temperature.
In our case, we had a 1989 Jeep Cherokee that had been acting oddly for months despite having a proper supply of good fluid. It was a high-mileage transmission of unknown history, and its behavior was continuing to degrade. A quick check of the fluid at this point revealed it looked and smelled as if it had overheated.
We decided we would tear into this transmission and perform a rebuild. OmixADA offers a full rebuild kit for the AW4/ A340 that includes all seals, gaskets, filter, clutch discs, and steels, plus other wear items. After removing the transmission from the Cherokee, we began a rebuild on the bench. We did the work over several days using mostly common mechanic’s tools. We found that the biggest challenge of the project was dealing with the many snap rings of various sizes. We made good use of both pointed snap-ring pliers and flat-billed retaining ring pliers, plus some long screwdrivers or pry bars to extract snap rings deep inside the case.
What follows is a basic overview of the rebuild using the Omix-ADA parts kit. Space here does not permit us to provide a lot of details. We used the very complete AW4 factory service guide from Automatic Transmission Service Group (ATSG). It contains detailed information on the transmission operation along with full disassembly, inspection, and assembly procedures for it.
There are a lot of parts involved in the build, and success takes good attention to detail. Once our project was done, we also added a supplemental cooler to the system as a further safeguard against future overheating.