make sure your transfer case and differential fluids are not low.

 

Thank you!

Well, in the end, I figured it must be the actuator inside the diff. I couldn't get the diff to lock with 12V on the coil. I asked the owner, and he figured well take the diff apart some other time. Currently, I installed a 3.3 Ohm resistor to get rid of the SERVICE 4WD.

What puzzles me is why it reads the error only going backwards, when the diff lock isn't actually working never. So yeah, should've started with checking everything with the wheels in the air.

I don't think the fluid levels affect the actuator anyhow, as it's all electrical?

Thank you for everything!

 

might be a break in the actuator coil wires that gets separated in reverse when the actuator shifts

 

If, then it must be really quick. I was driving around with the multimeter connected. I didn't see anything wrong. Would get a better understanding with an oscilloscope of course.

 

When did you last change fluids in your rear and transfer case?

 

Not sure, it's a customers car.
The coil surely should have an effect even with a dry diff.

 

DTC C0388Circuit/System Description

The rear differential control circuit consists of an actuator which engages and disengages the rear axle differential lock. The rear differential actuator consists of a coil assembly, and a locking assembly with springs and pins to lock the rear differential, all within the rear differential assembly.

The rear axle differential lock actuator consists of the following circuits:

• The rear differential lock control circuit, which is connected to the transfer case shift control module and supplies battery voltage.
• The rear differential lock return ground circuit, which is also connected to the transfer case shift control module.

Whenever the rear differential lock has been requested the transfer case shift control module provides battery voltage through the rear differential lock control circuit, and provides ground through the rear differential lock return circuit through a current limiting driver.

Whenever a rear differential lock is requested the transfer case must be in 4 LO and the vehicle speed must be below 5 km/h (3 mph) for the request to be accepted. The rear differential lock will remain locked up to 32 km/h (20 mph) after which the rear differential lock will disengage.

This DTC detects a short to ground, short-to-voltage, or an open circuit.

Conditions for Running the DTC

• The ignition is ON.
• The system voltage is 9-18 volts.

Conditions for Setting the DTC

• The transfer case shift control module monitors the state of the rear axle differential lock actuator. The transfer case shift control module reads the rear differential lock control, and the rear differential lock return circuits for a high and low voltage. The DTC will set if these circuits are high when a low voltage is expected or when the circuits are low when a high voltage is expected.
• The DTC will set if this circuit is high when a low voltage is present or when the circuit is low when a high voltage is present.

Action Taken When the DTC Sets

• All rear differential lock requests will be disabled. Transfer case mode shifts will not be affected.
• SERVICE 4WD indicator will be latched on for the remainder of the current ignition cycle.

Conditions for Clearing the DTC

• The Transfer case shift control module will clear the DTC if the condition for setting the DTC is not currently present.
• A history DTC will clear after 33 consecutive ignition cycles without a fault present.
• History DTCs can be cleared using a scan tool.

rotation direction may impact the wiring as the coil floats/rotates on the carrier,

I would think that if rotational forces were high enough that the coil could move enough over time to cause wiring issues

Neal