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. Omix-ADA 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.
On our transmission, we started by unbolting the engine bellhousing and the 4WD adapter housing at the tail end. The adapter houses the electronic speed sensor that reacts to a magnet spinning on the output shaft.
With the oil pan and filter removed, we disconnected the three electrical solenoid connectors and began unbolting the valve body assembly. This exposes the accumulators and a few other parts that need to be removed from the bottom of the case.
The pump body is the first component to be removed from the large end of the transmission case. A plate or bridge-type puller may be helpful here if the pump proves to be stubborn about releasing from the case.
With the pump out of the way, it’s possible to start pulling subassemblies from the case. Many are held in place with a large snap ring that must be removed first. Some portions come out with multiple pieces, and we laid all the parts on a table in consistent up/down orientation in their order of removal.
Several planetary, clutch, and piston assemblies need to come out. Between these components are often plastic thrust washers and metal thrust bearing/race sets that need to be kept in order and orientation.
There are a handful of stacked clutch packs within the transmission. These consist of alternating steel plates and friction clutch discs, and sometimes thicker steel retaining rings.
Here is the complete Omix-ADA rebuild kit, plus a new neutral safety switch for the shifter assembly. With all the components stripped from the transmission case, it got an inspection and thorough cleaning. Then we started the rebuild process for each of the subassemblies.
A number of the subassemblies have piston-return springs that are under tension and secured with a snap ring. There are various methods to remove these, including using deep-throat pliers. We quickly made a couple of tools in our shop press to help us.
When we got to the forward clutch assembly, it was obvious that this area had seen high heat. Metal parts showed discoloration, and we found significant debris throughout the clutch pack.
Here is one of the piston-return springs. They can be measured and checked against a height specification in the service manual. The manual also lists other inspection measurements for wear components.
Prior to the assembly of the clutch discs, they should be soaked in automatic transmission fluid for about a half-hour. We are using Royal Purple Max ATF synthetic fluid for this purpose and for the complete fluid fill upon reinstallation in the Jeep.
Commonly a subassembly will have a clutch pack and an associated piston assembly. Splined steel plates are alternated in the stack with friction clutch discs.
The piston assemblies can usually be popped open with a quick shot of compressed air. An air nozzle is also used during reassembly to apply compressed air to fluid transfer ports to check the sealing and operation of the rebuilt pistons.
The pistons take one or more new rubber O-rings, which we coated in transmission fluid before assembly.
There are also square-shouldered sealing rings used. These need to be well lubricated with transmission assembly lube or petroleum jelly to ensure that don’t catch or tear going into their mating bores.
Plastic thrust washers are keyed and placed between some subassemblies. There are also 10 thrust washers and their races distributed through the transmission. These also need to be prelubed and placed in the correct orientation.
Once the subassemblies were all rebuilt, it was time to replace all the components in the cleaned case. The second brake drum was installed by aligning it to the slots inside the housing, and we proceeded from there.
Here’s one of the large snap rings that fits inside the case. It’s necessary that some be oriented in a certain position as noted in the service manual.
The oil pump was also disassembled and the internal clearances checked for signs of wear. We then reassembled it, and installed new sealing rings and input seal on the pump housing. The pump was the final subassembly to be installed back into the front the case.
It was time to address rebuilding the valve body. The upper and lower (shown here) halves were separated. Each was disassembled and cleaned and the components checked. This is where you'll find a number of small balls, oil strainers, and other pieces that all need to be reassembled in their exact order.
The transmission has three electrical solenoids on the valve body. Continuity of these should all read between 11 and 15 ohms. Fortunately, all of ours were within spec so we reinstalled them with fresh O-rings.
With the valve body reassembled with new gaskets, it was bolted back onto the bottom of the transmission after the accumulators were placed back into their bores. On our transmission, we also reconnected the throttle cable to the throttle valve cam on the valve body. Then a new filter and the oil pan were installed.
Omix-ADA also offers a replacement neutral-safety switch that mates to the shift shaft on the side of the transmission. The switch body is slotted at this bolt hole so it can be adjusted as needed.
Automatic Transmission Service Group