you'll have to refer back to the article for some of the Letter references from the diagrams
http://www.supermotors.net/registry/media/724137_1
DIAGNOSIS
Nothing Happens (Dead System)
The obvious first step in diagnosing a dead system is to check the power sources, fuses and grounds. If these check out, you should run the shift control module self-test. Figure 22 shows the module with pigtail "A," "B" and "C" attachments. The self-test button and self-test LED are located on the side of the shift control module.
Disconnect the "B" and "C" connectors, turn on the ignition and allow 4 seconds for the module to power up. Then push the self-test button. If the LED doesn't illuminate, the module is dead and must be replaced. If the LED comes on and stays on for 30 seconds, there is an error condition and the module will have to be replaced. If the module is OK the LED will flash 4 times.
This test, however, is not 100 percent accurate. During the test, the vehicle isn't operating, so the shift control module isn't receiving data and initiating shifts. If the unit passes the self-test, it's probably a good module. Go on to check for other probable causes, but don't rule out the module entirely. If your search doesn't turn up any other cause, it may be the module after all. You may have to try replacing the module even though it passed the self-test.
Check Sensors
A logical next step is to check the three sensors with the ignition on. The transmission sensors (manual or automatic) should be closed with the clutch in, or the automatic shift in neutral. The speed sensor should show 225-275 ohms with the vehicle stopped. Check at the module connection.
Check both the motor position sensors and the wiring harness at B4, B5, B6 and B7 and the input from B8 (see Figure 21). Make sure the harness is OK first, then check the contacts against the chart in Figure 23. Each valid combination of open and closed switches indicates a different position of the motor. For example, at the 4H position, B7 is closed, B6 is closed, B5 is open and B4 is open. Besides the three main positions (2H, 4H and 4L), intermediate positions are also shown - three between 2H and 4H and three bewteen 4H and 4L (see Figure 20 and the chart in Figure 23). Combinations other than those shown in Figure 23 indicate a defective motor sensor assembly.
Check Transfer Case Motor
To check if the transfer case motor is actually functioning (because it may be hard to hear it in a noisy shop) attach a voltmeter to A4 and A5. Have someone activate a shift and look for a brief (one second) increase in voltage and listen for the relay to click on, then off.
If there is power present, then the control module is trying to run the motor. At this point, unbolt the motor and have someone activate a shift again. If the motor doesn't actually turn, it is malfunctioning and must be replaced.
If there is no power to the motor, check both the power circuits again and the sensors. If there is incorrect sensor data, the module won't power the motor.
Check Shift Switches
If the problem isn't in the sensors or the motor, you should next check the control panel switches. Make this check with the ignition on. Disconnect the "B" and "C" connections to the sensors, so that their data won't confuse the diagnosis. Now check power at the C1 connection at the module. There should be 5 volts coming from the shift control module to the 4H and 4L switches. Check C1, C2, and C3 for short to ground.
You can check the switches themselves by disconnecting C1 and checking continuity across C1 to C2 and C1 to C3. If current passes when the switch buttons are pushed, the switches are OK.
Finally, check for a short between C2 and C3 by bridging between them and activating the 4H and 4L buttons. There are situations where wiring harnesses get crushed in such a way that two wires are crushed together and short, even though there is no short to ground.
Check Lamps
To check the lamps, turn the ignition on and ground C4 and C5. The lamps should light.
edited by Seabronc to remove inappropriate comment 11/5/2009
