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Ok, I'm going to cover some of the things I discovered with the AW4 TCU and manually controlling shifting. After I swapped the 4.0L and AW4 into my Comanche it didn't take long to get annoyed with the TCU deciding what gear I should be in. By disconnecting the TCU you are locked into 1st when the lever is in the 1-2 position, but can't get to 2nd. So I decided to wire in a switch to control 1st and 2nd gear and the torque contverter. Both the upshifts and downshifts seem a bit firmer when shifting manually, on the road I can bark the tires going into 2nd by flipping my toggle switch!
The AW4 has 3 internal solenoids that are controlled by the TCU for shifting. Soleniods 1 & 2 are used for determing the gear, soleniod 3 is used to lock the torque converter.
So to control shifting manually, all you have to do is provide power to the soleniods as needed.
There are several writeups and variations on how to wire in switches to control the shifting. I'll list some links at the bottom of this post. Many of these describe how to wire in a switch to lock 2nd, but not 1st.
I wanted to control both 1st and 2nd. What you need to do for this is kill power to the TCU when you want to control the shifting. You then have one toggle switch for 1st and 2nd gear.
So how to kill power to the TCU? There are several ways:
1) Unplug the TCU.
2) Unplug the 3 wire plugin on the Throttle Position Sensor, these wires provide the TCU input for calculating shift points. The TCU won't be able to do any shifting with it unplugged.
3) Wire in another toggle switch to kill power to the TCU when you want to manually control 1st and 2nd.
My Comanche doesn't see the road much, and I could care less if it shifted automatically... so I took out the TCU and hooked up a throttle postion sensor meant for a manual, it doesn't have the 3 wire plugin for the TCU.
Now if you want to easily switch between automatic and manual then obviously you want to do option #3 and wire in a switch. When you want to control shifting you would flip the TCU power switch off and use the shift lever to manually shift through the gears. When the lever is in the 1-2 position you would use the 1-2 toggle switch to choose 1st or 2nd.
The TCU is found under the passenger side dash:
There are several wires that plug into the TCU. You will only need to splice into two wires. The yellow wire (power) and the violet/white wire (soleniod #2). You can also rig up another switch to control the torque converter if desired. The torque converter is the black and white wire.
You may want to splice in a few diodes between your switches and the TCU for Soleniod 2 and the Torque Convertor. Why? Supposedly when you flip your switch to give power to soleniod 2, you don't want the power running back into the TCU and frying it. I spliced in some diodes on mine just incase I decide to hook the TCU up again someday, but not sure if they're needed. Some people don't use the diodes, but they're only a like a buck for a pack at Radio Shack.
Here is a wiring chart I drew up for wiring in three switches. (The torque converter switch is optional. The TCU power switch is also not needed if you take out the TCU, but then you loose your fully automatic shifting.)
With the TCU power switch in the on position, the transmissions works like normal. You will also need to have the 1-2 switch in the 1 position, and the torque converter switch in the off position.
With the TCU power switch off, you would control shifting with the shifter lever. When in the 1-2 position, you would then use your 1-2 switch to control 1st and 2nd. Also your 1-2 switch needs to be in the 1st position to reach 4th gear. This is because soleniod 2 has to be off for 4th gear. But you're not going to be using 4th very often anyways when in low range on the trails.
Now you can buy all kinds of fancy joysticks and paddle shifters to do the same thing... but why? Well I guess if you had money to blow it would be kinda cool, but I rather spend my money on something else.
Here is a stock wiring diagram of the TCU:
AW4 TCU Wiring Diagram
Here are links to other writeups on AW4 shifting:
http://www.go.jeep-xj.info/HowtoAutoSwitch.htm
http://jeepforum.com/forum/showthread.php?t=234510
http://www.fourwheeler.com/techarticles/drivetrain/129_0606_aw4_jeep_cherokees_transmission_lock/
http://wagoneers.com/XJ/tech/xj-tcu.html
http://www.naxja.org/forum/showthread.php?t=24701&highlight=hold+tranny
http://www.greatlakesxj.com/tech/aw4shiftmod.html