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Jeep Cherokees from 1987-2001 came from the factory with the Aisin Warner Transmission model AW4. The AW4 is an electronically controlled, 4 speed transmission. It is operated with three solenoids, solenoids one and two control the gear selection, while solenoid three controls the lock up torque converter. The AW4 has a 450lb-ft torque rating, and with after market clutch packs, a 600+lb-ft torque rating, making it a fairly bulletproof transmission. There are a few downfalls to using the AW4 as a performance transmission. The shifter is set up in a “1-2â€, “3â€, and “OD†pattern, giving you NO control in the first two gears. The TCU (PCM in the 99-01 models) controls when the transmission shifts from first to second and from second to first when in the “1-2†mode. Also, the computer program shifts the transmission very slowly for a more comfortable shift. The rest of this paper will explain how to completely control the transmission, and make it shift hard and fast.
Electronically, the AW4 is a very simple transmission. From 87-98 the transmission was controlled by a separate transmission control unit, or TCU. In 99 the TCU was integrated into the power train control module, or PCM. The TCU has inputs from the vehicles sensors as well as outputs to the sensors and transmission. Some of the inputs include, 12 volt power, a signal from the throttle position sensor, and in the 87-89 years, input from the “power/comfort†switch on the dash. The TPS and the "power/comfort" switch tell the computer how hard to make the transmission shift. If the computer sees that the butterfly is wide open, it will bring the engine to red line before shifting. Likewise, if your accelerating normally, the computer will see the lower signal from the TPS, and shift sooner and slower.
For performance applications most people prefer a manual transmission, but a well built automatic can be strong, shift faster and more reliably than a standard. To start building an AW4 for performance, the first thing to do is lose the computer. For performance driving the predictability of a manual transmission is almost a necessity, so removing what tells the transmission to shift is a good start. Now the transmission can be controlled by the shifter, but because of the “1-2†set up, you lose second gear, having first, third and overdrive. To fix this problem a new shifter must be made. As far as the shifter, the style is up to the driver, toggle switches, push button switches, a rotary switch, or even an arcade joy stick can be used. In this case, a one pole five position rotary switch will be used. Only 4 wires from the transmission harness are going to be used, the rest were for the TCU adjusting how the transmission shifted. Pin C1 is a 12 volt ignition wire, it is only powered when the key is on. Splice this wire into the input of the switch. D14, D15, and D16 are the wires for the the three solenoids. Solenoid one is D16, two is D15, and three is D14. For the first position on the rotary switches output connect a wire to D16, this will provide first gear. For second gear, power needs to be supplied to D15 and D16. Third requires power to just D15. For over drive nothing needs to be connected to the switch, fourth gear is both of the solenoids off. The fifth position can be used as a fifth gear by supplying power to D14, this will give fourth gear with the torque converter locked. Having the torque converter locked is usually only used when theres light engine load like highway use. Also, if the vehicle is stopped with the torque converter locked, the engine will stall, just like stopping a vehicle with a manual with the clutch being disengaged.
Now that the TCU has been removed, the transmission will shift much faster than before because there is no pulse width modulation easing the clutch packs together. For most people the transmission will shift fast enough like this. It is possible to increase positive engagement though. The AW4 is equipped with a throttle valve cable, or TV cable. This is a cable that runs from the throttle plate arm to the transmission, the more that the throttle is open, the more the transmission line pressure is increased. A properly adjusted TV cable on a jeep is extended about 65-70% at WOT. Removing the cable from the throttle plate arm, and extending it all the way using a crimp on connector to hold it open will keep line pressure at a maximum giving very firm shifts. Because the TV cable can be extended past the WOT setting, it also increases WOT shifting, where its most important. Performing this modification to the transmission can cause premature failure. The clutch packs slam together with positive, instant engagement instead of being feathered together by the pulse width modulation of the TCU. Upgrading the clutch packs to after market, performance clutches would reduce the risk of failure.
Although there are many other modifications that can be made inside the transmission, these are much simpler and accomplish the same thing. Shift kits are not readily available for the AW4 transmission, but if they were, they would also increase line pressure in the transmission, but at a progressive rate, not a fixed rate like the TV cable modification does. There is a kit available to manually shift the AW4, it is a cut and splice in kit, and ranges from $90 to $200 depending on what style shifter is purchased. A simple shifter can be made for about $5 in parts and wires following the directions in this paper. This is how to get good performance, fast and controlled shifts, and more power to the wheels from an AW4 transmission.
