How do you defeat the 4-high safety lockout that disallows locking the differentials on an 08 Rubicon?

Answer 1

This mod allows you to use the lockers in all transfer case positions and should allow the sway bar disconnect to operate as well. It will also allow the ESP to operate in all three modes. With this option the ECM will simply think the transfer case is always in 4 low.

Background: The Totally Integrated Power Module (TIPM) provides sensor ground to the transfer case and many other components. This is on pin 28 of connector C1. The transfer case switch then returns a MODE SENSOR SIGNAL back to the TIPM on pin 3 of connector C1 (Yellow/Light Green). This signal varies in resistance as follows:

Operating Mode Versus Resistance SENSOR POSITION OPERATING MODE SENSOR RESISTANCE (ohms)

Position/mode/ohms

1/2H/1124-1243

2/4H/650-719

3/NEUTRAL/389-431

4/4L/199-221

5/NOT USED/57-64

During normal vehicle operation, the Powertrain Control Module (PCM) monitors the transfer case position sensor return voltage to determine the operating mode of the transfer case. Refer to the Operating Mode Versus Resistance table for the correct resistance (1-6) for each position.

So if one simply connects this return signal to ground, which is the equivalent of 0 ohms, causes the TIPM to assume the transfer case is in 4 low. A good modification would be to install a SPDT switch to connect the signal return circuit through a 220 ohm resistor to ground.

Description of the Transfer Case Position Sensor from the Factory Service Manual:

The transfer case position sensor is an electronic device whose output can be interpreted to indicate the transfer case's current operating mode. The sensor consists of a five position, resistive multiplexed circuit which returns a specific resistance value to the Powertrain Control Module (PCM) for each transfer case operating mode. The sensor is located on the top of the transfer case, just left of the transfer case centerline and rides against the sector plate roostercomb. The PCM supplies 5VDC (+/- 0.5V) to the sensor and monitors the return voltage to determine the sector plate, and therefore the transfer case, position.

There are two wires going to this sensor. One is Yellow with a Light Green stripe and goes to pin 1 on the connector. This is the wire you want to connect to ground with a new switch. The other is Dark Blue with a Yellow stripe and goes to pin 2 on the connector. This wire should show +5 volts with the ignition turned on.