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  Topic Review (Newest First)
02-02-2009 10:24 PM
bustednuckles using the switch in position B would set the tranny into "od" because the tcu has no power, right?

so how does the OP claim that its 1st gear since there is no power to the solinoids?
08-26-2008 09:37 AM
XJ AW4 TCU Shift Mod


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 belive that it would be possiable to "fool" it into thinking other things.

NOTE: This mod works on ALL year XJ'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 belive 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:

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.

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:

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!

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:

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. LINK

The original post on JF can be found at

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