Ever try charging up a hill in your Jeep Cherokee only to have your AW4 shift into first just before the top leaving you with no more momentum to make it? Ever try to crawl at a slower pace than second gear will allow? All because the lowest gear selection is 1st and 2nd with the Transmission Control Unit, (TCU), doing the deciding on which one to use at which moment. We can fix that. This article only applies to '96 and earlier Jeep Cherokees with the AW4 4-speed automatic transmission. WARNING! '97 and later Cherokees use a different control unit and damage can result if you try to do this to those year vehicles.
TCU solenoid chart
The idea is easy. There are two solenoids controlled by the TCU that select which gear to use by which is off and which is on at any given time.
We are going to control solenoid number 2 with a Single Pole Single Throw, (SPST), switch thereby overriding the TCU.
-A SPST switch (it can be a toggle, rocker or any style switch you like)
-6 feet of 18 gauge wire (any color OTHER then red or black, I used purple)
-A Diode (go to radio shack and ask for a 1N4002 diode, or anything close to that will do)
-Some wire splice connectors
-Wire connectors for connecting to the switch (appropriate for the switch you choose)
Find a location to mount the switch that is comfortable for you. Most people mount them somewhere on the center console near the shift lever. I recommend you mount it where it won't easily get bumped into the wrong position. This picture shows the switch location I chose.
I removed the center console cover from the transmission shift lever to access the area where I would be mounting the switch. I also removed the lower dash cover on the passenger side to access the TCU wiring connectors. (The "cubby hole" shelf is an excellent access point if you remove that). Next I cut my 6 foot wire in half to get two 3 foot lengths and ran them from the TCU area, behind the dash, and under the center console to the area I mounted my switch. Now you decide which gear configuration you want. 1st gear override or 2nd gear override.
If you're looking to lock out 2nd and lock in 1st gear for crawling purposes you just connect the two wires to the two contacts on the switch. At the other end of the wires you are going to connect them to the solenoid #2 control wire so that the switch blocks the solenoid voltage when it is open and allows normal operation when it is closed. Solenoid #2 control wire is wire position C15 on the TCU connector. It should be a violet colored wire. Cut this wire leaving about 4 inches worth on the connector end. Now just connect your two 18 gauge wires you ran, one to each end, with wire splices (solder or crimp, either works) and tape them up. That's it. Flip the switch open and 2nd gear is disabled. Flip it closed and it operates as normal.
Suppose you would rather go the other route and lock in 2nd gear. This method is very similar but a little more involved. Mount the switch and connect the two wires to it as in the above procedure. The difference is in the TCU connections. We are going to use the switch to supply 12v to the solenoid whether the TCU does or not. This is done by connecting one of the switch wires to the yellow wire on terminal D16 of the TCU (or any other 12v source that is only on when the ignition is on works just as well). The other wire will connect to the Violet wire on terminal C15 of the TCU. Same as before, cut the violet wire with at least 4 inches on the terminal side.
This is where we use that diode. A diode is an electronic device that allows current to flow only one way. The diode is a black cylinder with a silver band printed on one end. In simple terms, the voltage enters the dark end of the diode and exits the end with the silver band, but can not go the other way. We don't want to switch 12v to the solenoid and the TCU terminal at the same time. The TCU is meant to put 12v OUT, not get 12V in! This could damage the unit. So we use the diode as a voltage "blocker" to prevent this. Connect the dark end of the diode using a wire splice to the wire connected to the TCU terminal. Connect the other wire from your new switch and the solenoid control wire to the end of the diode with the silver band. Tape up the connections to prevent shorts and put the covers back on the console and dash. You're done.
Now, when you put the gear selection lever into 1st/2nd position and flip the switch to CLOSED, the transmission will stay in 2nd gear and avoid that nasty shift into 1st at the wrong time. Put the switch in the OPEN position and everything operates as normal. Say you're feeling pretty proud of what you've done and want an extra bonus. I used a red lighted rocker switch so I wouldn't forget if I left the switch in the "override" position. This requires a ground wire connected to the switch and the Jeep chassis and another diode, otherwise the light will be on every time the TCU energizes the solenoid. Instead of connecting the second wire from the switch to the silver band end of the 1st diode, connect it to the black end of a 2nd diode. Then connect the silver band end of the 2nd diode to the silver band end of the 1st diode with the solenoid control wire. This will prevent the switch from lighting unless you are using the switch to energize the solenoid.
With a little electrical ability you can modify your AW4 automatic transmission to do all sorts of things that it was never intended to do. Like choosing between 1st or 2nd gear manually. You can also lock up the torque converter, operate the transmission as a manual three speed, or have full manual control by using switches. But those are projects for another day.