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Thread: Modifying your AW4 Transmission to lock-in 1st or 2nd gear

  1. #1
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    Modifying your AW4 Transmission to lock-in 1st or 2nd gear

    The only thing I don't like about my XJ is the AW4 Tranny. It doesn't give you an option to be in 1st or 2nd gear. Both gears are on the same position on the shifter. In my XJ, 1st is to slow to go down hills unless they are real steep. Sometimes climbing a steep hill the tranny goes from 1st into 2nd and then the motor lugs because of the higher gear. That just POs me! Yeti showed me his FOUR WHEELER magazine that talked about the tranny problem. :book: Here is the on-line link and what they had to say:
    -------------------------------------------------
    http://fourwheeler.com/techarticles/...129_0606_weak/

    1st or 2nd Gear, You Decide
    Modifying your AW4 Transmission to lock-in 1st or 2nd gear.

    By Dion Fotinakes
    Photography: Ken Brubaker, Dion Fotinakes

    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.

    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.

    You'll need:
    -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)
    -Electrical Tape

    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.
    Pacific Northwest 4 Wheel Drive Association Region 4 PR

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  2. #2
    Rimrock Moderator benw's Avatar
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    Here is another more elaborate option. Gives you the option of automatic or full manual control of your aw4. http://www.awshifting.com/

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    Quote Originally Posted by benw
    Here is another more elaborate option. Gives you the option of automatic or full manual control of your aw4. http://www.awshifting.com/
    Thanks for the info.
    Pacific Northwest 4 Wheel Drive Association Region 4 PR

    1951 CJ3A (project)
    1995 Dolphin 34' Motor home, 460, very big gas tanks
    2003 Yukon XL
    2005 Arctic Cat 400 4x4 ATV, Auto-trans, push button 2/4 wheel drive, Front cable locker, 2" receiver, Warn winch, Tusk MTS Rear Trunk.
    2007 JKU Rubicon 3.8L, auto tranny, D44s with 5:38 Nitro Gears, 33x12.50 MT/Rs, 3" lift, Rough County Stubby bumper, 12,000 Mile Marker.
    2013 JD 1023E, diesel, 4x4, Hydrostatic, Rear locker, D120 loader.

  4. #4
    Adventurer Fatboy's Avatar
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    Here is another link. This guy swapped an AW4 into his 5spd. TJ. http://www.stu-offroad.com/engine/awshift/awshift-1.htm
    "No Rest For The Wicked"
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  5. #5
    I plan on fixing my 1-2 shifter issue real soon. I've only been on three runs since I put in the 4.0L and AW4 and it is so annoying when going down hill and it decides to shift into 2nd, can be a little dangerous too. I was going to do this at the time I was doing the swap, but I figured it could wait, wish I would have just done it then. I don't know why they ever designed it like that.
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    Quote Originally Posted by commando14
    I plan on fixing my 1-2 shifter issue real soon. I've only been on three runs since I put in the 4.0L and AW4 and it is so annoying when going down hill and it decides to shift into 2nd, can be a little dangerous too. I was going to do this at the time I was doing the swap, but I figured it could wait, wish I would have just done it then. I don't know why they ever designed it like that.
    Did you ever do the mod? I still haven't got around to it. I found I need to lock it into 2nd for down hill on the trail so I don't have to ride my brakes. It stays in 1st which is to slow for most trails but is cool for the steep stuff. It would also be nice to lock it into 1st so I can use the power band on steep hill climbs. I have not had that problem of it shifting into 2nd since I went to 3.73s. It sucks when that happens.
    Pacific Northwest 4 Wheel Drive Association Region 4 PR

    1951 CJ3A (project)
    1995 Dolphin 34' Motor home, 460, very big gas tanks
    2003 Yukon XL
    2005 Arctic Cat 400 4x4 ATV, Auto-trans, push button 2/4 wheel drive, Front cable locker, 2" receiver, Warn winch, Tusk MTS Rear Trunk.
    2007 JKU Rubicon 3.8L, auto tranny, D44s with 5:38 Nitro Gears, 33x12.50 MT/Rs, 3" lift, Rough County Stubby bumper, 12,000 Mile Marker.
    2013 JD 1023E, diesel, 4x4, Hydrostatic, Rear locker, D120 loader.

  7. #7
    Quote Originally Posted by Ceg
    Did you ever do the mod?
    Yes I did, super easy and works great. I can lock in 1st or 2nd. There's several ways to do it... I'll make a write up later of how I did it, I think this method gives you the most control.
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  8. #8
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    Quote Originally Posted by commando14
    Quote Originally Posted by Ceg
    Did you ever do the mod?
    Yes I did, super easy and works great. I can lock in 1st or 2nd. There's several ways to do it... I'll make a write up later of how I did it, I think this method gives you the most control.
    That would be cool if you could. Scuba called me and asked how to do it. He wants to do it after that last snow run. I also want to do it to the Molly Mule as soon as the 4.0 L swap is done.
    Pacific Northwest 4 Wheel Drive Association Region 4 PR

    1951 CJ3A (project)
    1995 Dolphin 34' Motor home, 460, very big gas tanks
    2003 Yukon XL
    2005 Arctic Cat 400 4x4 ATV, Auto-trans, push button 2/4 wheel drive, Front cable locker, 2" receiver, Warn winch, Tusk MTS Rear Trunk.
    2007 JKU Rubicon 3.8L, auto tranny, D44s with 5:38 Nitro Gears, 33x12.50 MT/Rs, 3" lift, Rough County Stubby bumper, 12,000 Mile Marker.
    2013 JD 1023E, diesel, 4x4, Hydrostatic, Rear locker, D120 loader.

  9. #9
    Senior Adventurer colemancooler's Avatar
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    I looked at the link Ben posted pretty sweet set up
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  10. #10
    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 back up 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/techartic...smission_lock/
    http://wagoneers.com/XJ/tech/xj-tcu.html
    http://www.naxja.org/forum/showthrea...ht=hold+tranny
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  11. #11
    Main Admin
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    Thanks.
    Pacific Northwest 4 Wheel Drive Association Region 4 PR

    1951 CJ3A (project)
    1995 Dolphin 34' Motor home, 460, very big gas tanks
    2003 Yukon XL
    2005 Arctic Cat 400 4x4 ATV, Auto-trans, push button 2/4 wheel drive, Front cable locker, 2" receiver, Warn winch, Tusk MTS Rear Trunk.
    2007 JKU Rubicon 3.8L, auto tranny, D44s with 5:38 Nitro Gears, 33x12.50 MT/Rs, 3" lift, Rough County Stubby bumper, 12,000 Mile Marker.
    2013 JD 1023E, diesel, 4x4, Hydrostatic, Rear locker, D120 loader.

  12. #12


    Now to find the time and patience!!!!!!
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  13. #13
    Senior Adventurer colemancooler's Avatar
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    i liked the push button shifter the guy made in the vid I watched. it could be a fun worthwile project
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  14. #14
    thanks im gonna do it this weekend.
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  15. #15
    well i did it . i only went with holding first gear. but im really need some info on what the advantage is having the converter locked.
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  16. #16
    HELLO...........................
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  17. #17
    I wouldn't recommend a switch for the torque converter. I wired one up, but always leave it off. You might read that it helps to turn it on when going down a steep hill, but I can't really tell any difference.
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  18. #18
    thanks...i have read that and they said it was like compresion brakes and that it wasnt good for it....nuf said i wont event screw with it. i just wanted to hold first as long as i can.......thanks again....SCUBA
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  19. #19
    Adventurer Fatboy's Avatar
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    I'm pretty sure the locking torque converter is for highway use; mine doesn't lock until about 40 mph. No slippage = less heat, better mpg.
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  20. #20
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    WOW that is some cool stuff, make me almost wish I ran a JEEP :crybaby:
    printers are more graphic

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