This link has good information:
http://www.asashop.org/autoinc/oct2007/techtips.htm. Here's a copy of the link:
How to Diagnose Ford 4WD Problems.
Posted 10/8/2007
By Bill Strehle
“Most of the calls concern diagnosing a dead system or flashing 4WD indicator lights. The 4WD systems will vary by vehicle application, but the diagnostics for all systems are basically the same.”
Almost every year, we get the seasonal rush of phone calls on Ford four-wheel drive (4WD) problems. Most of the calls concern diagnosing a dead system or flashing 4WD indicator lights. The 4WD systems will vary by vehicle application, but the diagnostics for all systems are basically the same.
When the 4WD lights flash on and off, this indicates that there are 4WD-related trouble codes in the 4WD control module. Sometimes 4WD lights never illuminate.
The first step in diagnosing the system is to establish communication and retrieve trouble codes from the control module. After retrieving the codes, write them down, clear the codes, and retest to see which codes are regenerated. False codes can be set if a component is disconnected when the key is turned on.
If the 4WD lights flash on and off, and no codes are found in the 4WD control module, try clearing the codes anyway. After a certain amount of key cycles, the codes will normally clear from the 4WD control module, but the module will not respond to commands until it is re-initialized by clearing the codes or disconnecting the battery to erase its memory.
If your scan tool will not communicate with the 4WD module, first verify that the scan tool has the proper software, and the latest updates. If it does, then try another scan tool. If it still has no communication, the power, and grounds that are supplied to the module will need to be tested. See if the scan tool will communicate with the other modules on the shared communication network. If they're OK, check the harness from the data link connector (DLC) for an open circuit to the 4WD module.
Most 1997-2001 4WD systems were controlled by the generic electronic control module (GEM). This system monitored the 4x4 mode select switch, and controlled external relays to power up the shift motor to move it into the commanded position.
Ford then decided to use a stand-alone 4WD control module in some 2002-2004 vehicles. The system works basically the same, but instead of using external relays, it has drivers in the control module to apply power, and ground to the shift motor. Just like a power window motor, the rotational direction of the shift motor is controlled by forward or reverse polarity from the drivers that are integral to the shift module.
Starting in 2004, for some applications, Ford moved the 4WD commands to the powertrain control module, and reverted back to using external relays to control the shift motor.
Regardless of which module is controlling the system, the module relies on four contact plates that are integral to the shift motor to tell it what position the shift motor is in. The system will fail, and be unresponsive if the control module does not know the exact position of the shift motor. Usually if it is a contact plate problem, the 4WD module will output code P1867 for contact plate circuit failure.
There are four contact plate sensors in the shift motor. The 4WD module sends a reference voltage (V-REF) to these contact plates, and monitors this voltage. The actual REF voltage will vary depending upon the calibration of the module, but the REF voltage should read the same on all of the contact plates.
There is a fifth wire that is a ground. It will ground the V-REF depending upon the position of the shift motor. The contact plate parameter identification descriptions (PIDS) are usually labeled A, B, C, and D on the scan tool data. Using this data, the PIDS can be monitored to see if they read correctly. The PIDS will read open or closed, and change correctly when the shift motor moves. The PIDS are not universal to all Ford vehicles, so you will need to compare your actual readings to Ford's PID chart for your specific vehicle.
If the PID chart is unavailable, monitor the contact plate PIDS, and simply unplug the shift motor. When this happens, all four contact plates should read the same, either open or closed. Depending upon your scan tool software, these PIDS may differ from the PID chart, reading the exact opposite. This is usually a software glitch, but as long as they read the same, the wiring harness from the shift motor is usually OK. If one or more contact plates read differently, check the wiring from the shift motor to the 4WD module for shorts or opens.
If the contact plates read correct unplugged but read incorrect with the shift motor plugged in, the shift motor will need to be replaced, as the contact plates are not serviced individually.
If all of the contact plates read the same, each circuit can be tested individually by providing the ground (the common fifth wire) to that contact plate circuit. Monitor your scan tool data, and ground each contact plate wire in succession looking for the scan tool data to change from open or closed. The ground is usually identified as "position return" or "encoder ground" on the electrical schematics. Since this article is for generic applications, you will need to access the correct wiring diagram to identify the contact plates A, B, C, and D wiring colors and the common ground. If the wiring harness tests OK, replace the shift motor, and retest.
