2-LOW, Tacoma 2005+, possibly others.

Requires:
4 standard automotive SPDT relays.
3 1N4001 diodes
1 DPDT switch capable of switching 30A@16v
1 100 ohm resistor

References / Depends on: https://www.yotatech.com/forums/f166...-right-280412/

NOTE: I just finished designing this circuit and haven't slept more than about 4 hours in the last two days. It looks right through blurry-eyes, but I haven't yet implemented or installed it. If you don't actually *understand* what is going on, don't attempt it. At least not until someone (probably myself) actually implements it.


The 4WD ECU is located above the glove box, however even when you remove the glove box, it is not visible or easy to access. In the middle of the loop that the glove box latch hooks onto, there is a 10mm bolt. Remove it, and then pull the black plastic cover free, to reveal the 4WD ECU.

There are two plugs on the 4WD ECU. The "A" plug, and the "B" plug.
On the "A" plug, pin 5 and 6 are +12 and 0v input power. They are colored (PURPLE) and (WHITE w/BLACK TRACE) respectively.

On the "B" plug are ADD control and input signals.
3-B is RED, DM1
4-B is BLACK w/WHITE TRACE, DM2
12-B is PINK w/BLUE TRACE, DL2
13-B is LIGHT GREEN, DL1
26-B is LIGHT GREEN w/BLACK TRACE, ADD

DM1 and DM2 power the motor running the front driveshaft disconnect. Inside the 4WD ECU, there is a DUAL-SPDT RELAY -- this is NOT the same as a DPDT relay, since it is basically two entire SPDT relays that just happen to be inside the same plastic cover. One side of the relay switches DM1, the other side of the relay switches DM2. On both, normally connected is 0v. Energize the coil, and it switches to +12v.

So in a resting state, both DM1 and DM2 are tied to 0v. They do not float.

When you shift from 2WD into 4WD, it activates the DM1 relay, which connects DM1 (red wire) to +12v, and keeps it that way until either the input signals are consistent with 4WD, or it times out. When switching from 4WD into 2WD, it does the same, but with DM2.

Now the ADD has state sensor switches, called ADD, DL2, and DL1. In 2WD, DL1 is tied to 0v, the other two float. In 4WD, ADD and DL2 are tied to 0v, DL1 floats.

So what we want to do, is send the 4WD ECU the 4WD signal, without actually shifting the ADD into 4WD. Doing this the "old" way can accomplish this, but is timing sensitive since it will temporarily introduce inconsistent states.

What we do here, is use the DM1 and DM2 lines to create the 4WD and 2WD signals *at the moment they are expected*.

So, DM1 goes +12v temporarily while the input signals read 2WD, then goes back to 0v. DM2 goes +12v temporarily while the input signals read 4WD, then goes back to 0v.

Splice into 5-A and 6-A for +12 and 0v power supplies. Henceforth referred to as "s+" and "s-"

So, we clip the DM1 wire (red), connect the ECU side of it to the common pin on one side of the DPDT switch. Connect the ADD side of the red wire to the normally-connected pin. Take the normally-open pin, and that will be our activation trigger.

We use the activation trigger to power up one side of the coil of the first relay. The other side of the coil will be connected to A-plug pin 6. Because the DM1 line will return to 0v when the 4WD signal is recevied, we need to install a diode between the activation trigger and the coil, with the CATHODE pointing toward the coil. A diode is basically a 1-way electron valve, and it is needed to prevent the circuit from shorting out when the DM1 line returns to 0v.

Of course, we need the relay to REMAIN powered when DM1 goes back to 0v. s+ should be connected to the common pin of the unused side of the DPDT switch. The normally open side of that switch is our new power supply, "ss+".

What we do, is we create a latching circuit. If we connect ss+ to the common pin of the first relay, and the normally open pin back to its coil (directly, not through the diode), then when the DM1 activates the coil, it connects ss+ back to the coil to KEEP it turned on.

So now we have a latching relay, that turns ON when DM1 is switched to +12v. Only problem is that it will never switch off unless you manually flip the DPDT switch.

Now we take our SECOND relay. Instead of going straight from the normally-open pin of the DPDT switch to the common pin of the first relay, we connect ss+ to the common pin of the SECOND relay, and run the NORMALLY CONNECTED pin of the second relay, to the COMMON pin of the FIRST relay. If you energize the second relay, it will BREAK the +12 latch circuit that keeps the first relay energized.

One side of the coil on the second relay should be connected to s-, the other side should be connected to DM2. We don't need a diode here, because we only need this relay momentarily to break the circuit.

When switching from 4WD to 2WD, we briefly activate the second relay and break the latching circuit that keeps relay 1 energized.


So what we have now, is a +12v power supply that is powered when the input signals are supposed to match "4WD", and unpowered when the input signals are supposed to match "2WD". This is available at the NORMALLY OPEN pin of the first relay.

We take that signal and feed it to one side of the coils of the 3rd and 4th relays. Connect s- to the opposite sides of those coils.

These two relays will manipulate the inputs to the 4WD ECU.

While in 2WD, DL1 is tied to 0v. This needs to be disconnected. Now, we can actually USE that 0v for the needed 0v signals to ADD and DL2. Without this source of 0v, we would actually need an additional relay. So cut the DL1 line (light green wire), connect the ADD side of it to the COMMON pin on relay 3, connect the 4WD ECU side of it to the normally-connected pin on relay 3. In 2WD state, DL2 is left floating, so we don't need to break its circuit -- we can just splice into it. We connect the CATHODE side of a diode to the normally-open pin of relay 3. Anode to DL2. Another diode cathode side to the same normally-open pin of relay 3, anode to the ADD input line of the 4WD ECU. The diodes are needed in this instance, because otherwise all three lines would be permanently tied to 0v through each other.

At this point, the modification should technically be functional, and I don't think it would throw any errors, but it is incomplete. The 4WD ECU's inputs will be as expected, but we are giving the incorrect signal to the skid control ECU -- telling the skid control ECU that we are in 4WD while we are actually in 2WD. That can and will cause problems involving VSC and ALSD.

So we are going to cut the ADD signal wire. That is 26-B and is light-green with black trace. I've already discussed splicing into that line with a diode. The diode should be connected to the 4WD ECU side of the wire. The other side of the wire runs to the junction box, the ADD, and the skid control ECU.

Connect the ECU side of the ADD wire to the NORMALLY CONNECTED pin of the 4th relay. Connect the junction box side of the ADD signal to the COMMON pin of the 4th relay. Connect a 100 ohm resistor from the NORMALLY OPEN pin to the switched +12v side of the relay's coil. This will supply the +12v signal to the skid control ECU, telling it that it is in 2WD. This is where it is necessary to have the correct TRAC-LOW modification implemented and activated, because the low range signal is contradictory to the 2WD signal.

Now technically, it should be possible to eliminate the second relay -- the one used to break the latch on the first relay. Instead of activating a second relay to disconnect the circuit, the DM2 signal can act directly on the 0v side of the first relay's coil. This would require installing an appropriately sized 3-4W resistor between that coil and s-, and a diode between the coil and DM2 with the cathode towards the coil. When both sides of the relay are +12v, the net voltage across the coil will be 0v, so magnetism will stop and the latch is broken. I think it would require another diode on the latch supply between the coil and the normally open pin in order to prevent a 0v from being supplied through the coils of the other two relays.


Schematic:


Summary:
- Cut the DM1 wire between the ADD and the ECU near pin 3-B. Connect the two sides where it shows "DM1/ECU" and "DM1/ADD".
- Cut the ADD wire between the ECU and the Junction Box near pin 26-B. Connect the two sides where it shows "ADD/ECU" and "ADD/JB".
- Cut the DL1 wire between the ECU and the ADD near pin 13-B. Connect the two sides where it shows "DL1/ADD", and "DL1/ECU".
- Splice in to +12v at pin 5-A
- Splice in to 0v at pin 6-A
- Splice in to DM2 at pin 4-B
- Splice in to DL2 at pin 12-B

I think that on a NON-VSC/TRAC/ALSD equipped unit, you can leave out the relay that disconnects the ADD signal wire, and just splice the diode in to it.