Always in AWD

The yellow wire comes from the module in the truck. Diagram attached.

Roy

You were going to check the yellow wire as per the last conversation?

Roy

Okay, Did you get this information?

Roy

Transfer Case-Automatic Four-Wheel Drive (A4WD)

Features and Operation
The Automatic Four-Wheel Drive (A4WD) transfer case uses an electronic shift 4x4 system that allows the operator to choose between four different operational modes. The operator can switch between 2WD, A4WD and 4WD HIGH modes at any speed. However, to engage or disengage LOW Range, the 4WD Control Module requires that the vehicle speed is less than 5 km/h (3 mph), the brake is applied, and the transmission is in NEUTRAL.
In A4WD, the 4WD Control Module varies the torque split between front and rear drivelines by controlling the transfer case clutch. Under most conditions, the 4WD Control Module activates the transfer case clutch at a minimum duty cycle (percentage of time the clutch is turned on) which allows for a slight speed difference between the front and rear drivelines which normally occurs when negotiating a corner on dry pavement. When the rear wheels are overpowered, the 4WD Control Module detects this slip condition, and the duty cycle to the transfer case clutch is increased until the speed difference between front and rear drivelines is reduced. In this manner, the 4WD Control Module can redirect engine torque to the front wheels when the rear wheels lose traction yet still allow operation in the A4WD mode on dry pavement.
When the ABS system is active, the 4WD Control Module will default to a minimum duty cycle for the transfer case clutch.
Feature Inputs:
Shift range selector switch.
ABS active input (battery voltage when ABS is active, grounded when inactive).
Contact plate position inputs A, B. C, D grounded when closed, open circuit when open.

Feature Outputs:
Solid state clutch relay (pulse width modulated signal: grounded when relay is on, battery potential when relay is off).

Shifting Between 2WD, A4WD and 4WD HIGH
The operator can switch between 2WD, A4WD and 4WD HIGH mode at any speed.

Shifting Into/Out of LOW RANGE
When shifting into or out of LOW RANGE, the 4WD control module requires that the vehicle speed is less than 5 km/in (3 mph), the brake is applied, and the transmission is in NEUTRAL.

I read it as well. If it was the fuse, the ABS light would be on.

The hose is a vent line to atmosphere.

Roy

Here is the complete 4wd wiring diagrams with a guide to help you test the connection:

https://www.2carpros.com/articles/how-to-check-wiring

Check out the diagrams (below). Please let us know what you find. We are interested to see what it is.

I sent you the same diagram Ken sent you as well. There is no simple relay in the circuit.

Can you get a scan tool that can do bi-directional testing and see if you can command this thing into two wheel?

Roy

It is a full blown scan tool, not a code reader.

It may be helpful to take it to a shop and ask them if they can check it for you.

Roy