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AW4 TCU Shift Mod


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I wanted a way that would allow me to completely control what gear my transmission will be in on my AW4. Knowing that my XJ had a Transmission Control Unit lead me to believe that it would be possible to "fool" it into thinking other things. I also did not want to spend the money on the AW4Shift kit as I think it's way over priced for what it is.

 

NOTE: This mod works on ALL year XJ's or MJ's, however on the '97+ it sets off your check engine light on your gauge cluster. This can be easily removed with a code reader or I believe with 50 key on-key offs and it goes away.

 

Parts Needed:

 

3 Pole Switch

Wire

Wire Tap

 

After taking out the Transmission Control Unit (TCU) and doing a few test, I figured out what wire did what. The TCU is located under the dash on the passenger side under the glovebox. It's behind the plastic panel there. You'll only be using 2 out of the some 16/18 wires coming from it. First off you need to snip the yellow wire, this is the power wire for the TCU. You'll also need to locate the purple/white wire and tap into it. I used a wiretap that goes over the wire found from Radioshack. You than will send the TCU side of the yellow wire to position "A" on the 3 pole switch. The other half of the yellow wire will go in position "B". The purple wire that you tapped into will go to position "C" on the switch. I added wire to the ends of the yellow wire as I located my switch in a switch panel by the shifter.

 

Here is a diagram:

 

TCU1.jpg

 

Just remember where A, B, and C are in relation to the switch. This is how your switch will now work along with your shifter for the desired gear.

 

TCU3.jpg

 

When the switch is left in the position C, your transmission will act like it would stock. Also, if as long as the switch is NOT in the position C, the TCU will remain OFF and your torque converter will never lock up.

 

So now you want a switch to control the torque converter?

 

You can rig up another switch to control that.

 

Parts Needed:

 

2 Pole Switch

Wire

Wire Tap

 

You will now need to locate the white/black wire. You need to tap into it. I again used a wire tap from Radioshack. You will also use the yellow wire again. You need to tap into that wire again, but make sure you take power from the yellow wire BEFORE the splice for the first switch or it will not work.

 

Here is a diagram:

 

TCU2.jpg

 

Be careful not to stop with the converter locked up, or you may stall your engine. I don't think the converter locks up when the transmission is shifted in 1, but I havn't tried to stall it yet (need to go out and wheel and figure it out). Also, make sure if you're wiring up both switches (one above) that you take power from the yellow wire BEFORE the splice for the first switch (this is key!).

 

Now you can engage the lockup at redline and delay your shift! I LOVE IT!

 

EDIT: A bit more info on the AW4 and doing this.

 

Two things control AW4 shifting:

 

1) Shift lever (lever position controls tranny valves mechanically)

2) Transmission Control Unit (TCU)

 

The shift lever makes a very positive move in control, the only weak point being there is no separate position for 1-2, as they are combined. Position 1-2 holds in 1, provided there is no TCU connected. If you have the TCU connected, you'll get a 1-2 upshift at about 4800rpm. The shift lever does do one thing that the TCU cannot accomplish however - ENFORCING compression braking via transmission's internal overrunning clutches and sprags. But besides this, all other transmission controls are performed by the TCU.

 

The TCU monitors throttle position, ground speed, and shift lever position (switches in the console) to select the gears appropriately. It also monitors position of the POWER/COMFORT switch (for those of you who have it) to change upshift and downshift points and TCU lock parameters.

 

The TCU's outputs are very simple, only three wires. One for Tranny Solenoid 1, one for Tranny Solenoid 2, and one for Tranny Solenoid 3. Solenoids 1 and 2 perform ratio selection as follows:

 

TCU5.jpg

 

Solenoid 3 is very simple. Put power to it, and the torque converter will lock. (what we did above with the 2nd switch).

 

As far as the torque convertor Lock goes. Solenoid 3 is unique, in that it only controls torque converter lock functions. Being able to lock up a torque converter awards you with improvements in fuel mileage, a cooler-running transmission, less cabin noise from the tranny, AND MOST IMPORTANTLY, it has the ability to multiply torque from the engine.. It's in esance like having one more gear.

 

Although it's possible to lock the torque convertor in EVERY ratio, it really only works consistantly in 2nd, 3rd, and OD. Anyways, Locking it in 1st is pretty much worthless, and is not too good for the torque convertor.

 

When you come to a stop, either the torque convertor unlocks itself (by nature of how the hydraulic plumbing works) or it will bang a bit, and then stalls the motor. I'd just flip the switch and not worry about it.

 

NOW, giving it to much throttle with the torque convertor locked will give you a quick wake-up call. The torque convertor lock is not intended to handle full engine torque (remember where your peak torque occurs, fairly low) and it will give you a quick jolt if you forget to unlock before feeding too much throttle. Besides, the torque converter is your friend, it multiplies torque output in exchange for higher engine RPM, so only lock up when you know you don't need much "oomph".

 

Also, The TCU is programmed to force torque convertor lockup when engine RPMS's are above a certain point (not sure exactly what RPM though?), and you're at wide open throttle (WOT). Why? easy, top speed. The AW4's torque convertor has a very high stall speed (translates to high torque multiplication factor, and higher RPM off the line). If you allow 2500rpm of stall, that means the transmission's input shaft speed will be quite a few RPMs lower than crankshaft speed when you're under one "quick launch runs". At WOT, you're going down the highway at 75, and the torque multiplication factor really isn't required, thus you need more tranny RPMs.

 

And lastly, remember how I said the torque convertor lockup mechanism wouldn't handle full engine torque? Well, when you're screaming down the highway at WOT, engine RPMs are usually well above the engine's peak torque point (by nature) it's not developing peak torque output (although it may be generating peak HP). Locking the torque convertor here doesn't put it in jepoardy, but it does eliminate slippage that may be costing you MPH and/or MPG. (and we all know with big tires and a lift we need all the MPG we can get!).

 

For those of you interested, here is a wiring diagram of the AW4 I based the switches off of. http://www.greatlakesxj.com/tech/TCU4.jpg

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  • 3 weeks later...

I hope LEAD doesnt mind if I add this to his thread.

 

I took it a step further and made the whole transmission shiftable with switches.

 

 

Just to add to this thread, I did this in my MJ. I couldnt get the computer to shift the stupid thing....so I made my own set of switches. I really wanted BrettM's control box, but it was too espensive. I think I have $15 into this, for wire, and switches.

 

 

 

 

Here is a wiring diagram, which I drew up in MS Paint.

 

 

The first pic looks much more confusing than the diagram, because I had to add power to the switches, which light up. The blue one is solenoid 1, and the red is solenoid 2. The metal 3 pole switch locks and unlocks the torque converter (and the little green light goes on when it is locked). This works great for me. There is no gear "hunting" on the highway, because the tranny isnt shifting its self.

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that's sweet

 

 

I'd do it with relays and set it up so that you tap one button up (spring-loaded button so it only works one way) to shift up, and another button down to shift down. that way you have alot less fooling around while driving.

 

do you daily drive the thing? if so just one suggestion...put the switches near the shifter, or even on the steering wheel so that they're near at hand.

 

GREAT JOB though! jamminz.gif

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  • 2 weeks later...
that's sweet

 

 

I'd do it with relays and set it up so that you tap one button up (spring-loaded button so it only works one way) to shift up, and another button down to shift down. that way you have alot less fooling around while driving.

 

do you daily drive the thing? if so just one suggestion...put the switches near the shifter, or even on the steering wheel so that they're near at hand.

 

GREAT JOB though! jamminz.gif

 

I did DD it when my XJ broke down, and I drive it to the trails. I don't really mind it. But if I was to do it again, I would move the switches closer to make it more convenient (like you suggested).

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that's sweet

 

 

I'd do it with relays and set it up so that you tap one button up (spring-loaded button so it only works one way) to shift up, and another button down to shift down. that way you have alot less fooling around while driving.

 

do you daily drive the thing? if so just one suggestion...put the switches near the shifter, or even on the steering wheel so that they're near at hand.

 

GREAT JOB though! jamminz.gif

 

Swap in a paddle shifter from a Formula 1 car!! :brows:

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  • 8 years later...

hey is this posible? the AW4 can be controlled with this? if so i MUST get one...

 

also check out http://www.radesignsproducts.com/rail-controller.htmlor some of their other shifters, if i ever get around to swapping in an auto this will be what i use (or one similar), and ill have a switch i to set manual or automatic shifting.

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