TRANSFER CASE STUDIES: DIAGNOSING GM's 4WD SYSTEM

Access to DTCs can simplify any system diagnosis. But as two separate cases on GM's 4WD system reveal, these codes are not always 100% accurate.

GM has used many different four-wheeldrive control systems in its vehicles over the years, ranging from pure mechanical to electronically controlled. Each has its own diagnostic procedures, and the best source for understanding how these systems work will always be your information system.

Some later model 4WD systems have scan tool capabilities along with diagnostic trouble codes (DTCs) to help locate and diagnose many of the problems associated with them.

We looked at twu GM K 1500 trucks that have the NVG 246 two-speed transfer case. One is a 1999 model, the other a 2000. Both use the same controls and happen to have stored the same DTC. The basic system consists of a transfer case motor and encoder, Front differential actuator, trans for case module, speed sensors, switches and park input and output to the PCM. It seems logical to use case studies to show how these systems can he repaired.

The transfer case module data can be accessed with a scan tool such as the Tech 2, which is GM's official scan tool. To get to the proper screens, you must choose the Powertrain selection, then navigate your way to the ATC selections. ATC is the GM acronym for acMoe transfer case. As with any module/computer-based diagnostic strategy, it's always a good idea to make checking DTCs one of the first steps in your diagnosis. These two trucks have 19 possible DTCs, including those for input, motor, encoder and even VIN codes.

Selecting "F1" on the menu got us where we needed to be. Both trucks had one DTC stored: B2725 (active transfer case mode switch malf'unction). A Tech 2 information screen will also let you know whether certain 4WD systems do not allow scan tool access.

I could hear the encoder motor at the transfer case working, which confirmed my scan data and helped me find a diagnostic direction. Since it seemed like the problem was in the front axle area, I decided to check the front axle motor actuator next. Using a voltmeter, I determined O voltage was present on terminal C, (circuit 241, brown wire). I chose this wire based on the wiring diagram, which showed fuse 24 in the IP fuse box feeding voltage to the front axle motor. Going this far without first checking the fuse could turn out to be pretty embarrassing if the problem was simply a blown fuse. However, I knew the fuse also powered the transfer case control module. Since I was getting scan data and encoder motor operation, the fuse had to be good.

The only culprit left was the wiring harness between the fuse and front axle motor. It's always best to start checking for wiring problems by inspecting the harness in areas where it's most likely to be damaged by moving engine components, rotating parts or road objects striking the underside of the vehicle. I located the problem within minutes. The wiring harness near the right front axle where it enters the differential housing was damaged. At least two wires were broken, including the brown wire, which can be seen in the photo lower left. A harness repair was in order.

I had found the problem, but still could not explain why the system had stored code B2527. Was there another, intermittent problem just waiting for me to turn my back? I decided to look closer at the description for B2527 and found something interesting, "...the transfer case shift control module will set this DTC if a button is held down or sticks for a period longer than 5 minutes."

It seemed very possible that a driver might hold down a 4WD button for five minutes if he got stuck and the 4WD system wasn't working. To test my theory, I held the button down for several minutes to create the 4WD warning light visible in the photo on page 33. The system also set a code B2527. Maybe, just maybe, the DTC was actually set by the driver due to the broken wire to the front axle. I may never know for sure.

It's easy to fall into the habit of replacing common problematic components without verifying that they actually are defective. As this second case study of GM's relatively simple truck 4WD control system proves, this approach can get you into trouble. It always pays to test and verify before replacing a component.