Auto Care Advice: GM 4WD Warning Light Issues
GM Push Button 4WD Systems
These systems use dash-mounted mode switches and electronic controls to perform the functions of the 4WD system. They allow shift-on-the-fly from 2WD to 4WD high range. This is accomplished by locking the front differential using a remote actuator.
Why Are My 4WD Lights Flashing?
Here are some of the reasons why the mode light on the 4WD switch might be flashing:
- The mode light for a selected shift will flash until the shift is complete as indicated by a switch in the front differential. If the shift is not completed, the mode light will flash for a short time and then go out (no shift competed).
- If a shift is attempted that is impossible (for example, attempting to shift from 2WD high to 4WD low while the vehicle is moving), all the mode lights will flash for approximately 30 seconds and then go out (no shift completed).
- In the first generation systems, mode lights will also blink to display fault codes. The timing of the flashes indicates the fault code stored in the transfer case control module (TCCM).
In the mid 1990s, the push button system was introduced on GM T series trucks and SUVs (Chevrolet S10/Blazer and GMC Sonoma/Jimmy). The system was introduced on the full-sized trucks and SUVs shortly after. Some common problems with these systems are:
- Logic lock-up within the transfer case control module. Removing the 4WD fuse for a few minutes may correct this issue (fuse location varies according to the model). Resetting the TCCM may only be a temporary repair. If the system fails again, further diagnoses may be required to isolate the fault.
- Faulty encoder motor on the transfer case.
- Sticking actuator in the front differential on the full-sized models.
The second generation systems were introduced during the model changeover in the early 2000s. The T series (Chevrolet Colorado/Trailblazer and GMC Canyon/Envoy), along with the K series (Chevrolet Silverado/Tahoe and Sierra/Yukon), began using the second generation system at this time. Common problems for the full-sized trunks and SUVs are:
- Erratic operation of the dash-mounted mode selector switch.
- If the 4WD system is not working and the code C0327 is stored in the TCCM, you most likely have a faulty position sensor in the encoder motor.
- Various software updates depending on vehicle year and model.
Floor Mounted Shifters
Way back when, the 4WD shifters were on the floor; some of these systems used the same style of locking front differential as the current push-button systems. These systems were used prior to the push-button style and came online in the mid 1980s. Since there is no TCCM, there are no diagnostic fault codes available. The actuators are vacuum operated; the vacuum is controlled by switches that are controlled by the shift lever. Common problems are:
- Vacuum leaks from any of the various vacuum lines
- Faulty vacuum controlled actuators
The Bulb Check
When the ignition key is cycled from OFF-RUN-START and is released back to the RUN position after the vehicle starts, the 4WD mode lights should illuminate for 1 to 2 seconds and then go out. If they stay on, there is a problem with the 4WD system. Fault code(s) should be stored in the TCCM, which displays stored codes by flashing the mode lights. If one or more of the mode lights do not illuminate during the bulb check, the problem could be a burned out bulb or something more serious requiring proper diagnoses.
When the ignition key is cycled from OFF-RUN-START and is released back to the RUN position after the vehicle starts, the 4WD warning light should illuminate for 1 to 2 seconds and then go out. If it stays on, or the "Service 4WD System" message is displayed, there is a problem with the 4WD system. Fault code(s) should be stored in the transfer case control module and must be read with a scan tool. Properly diagnosing and repairing any problems found should restore the system to proper operation. The "Service 4WD System" message does not mean it is time for routine maintenance of the 4WD system; rather, it indicates a problem with the 4WD system that should be properly diagnosed and repaired.
The operating characteristics of the first and second generation systems are the same. The mode switch on the dash will send a signal to the transfer case control module depending on which button is pushed. If possible, the TCCM will send the necessary commands to complete the shift requested. The shift-on-the-fly operation is made possible by the fact that the transfer case is engaged and the front driveshaft is turning in the 2WD mode. The front differential is not "locked." When the switch to 4WD high is requested, the TCCM commands the actuator in the front differential to "lock" the front differential in order to send power to the front wheels. The shift is completed when the TCCM receives a signal from the front differential engaged switch.
The shift to 4WD low range is a bit more complicated and requires the vehicle to be stopped with the transmission in Park or Neutral. When the command for 4WD low is sent to the TCCM, it will then perform two functions.
- Command the encoder motor to move the transfer case to low range
- Command the actuator in the front differential to its locked position
This shift is complete when the encoder motor position sensor sends a signal to the TCCM that the transfer case is in low range and the front differential switch signals the TCCM that the front differential is locked. The vehicle must be stopped in order to perform this shift because the transfer case must be shifted from high to low range.
It is not advisable to use the 4WD mode on dry pavement. Unlike all-wheel drive (AWD) systems, these 4WD systems do not have a front to rear differential. Due to the speed variation of the four tires, operating the vehicle in 4WD mode on dry pavement will cause the drivetrain to bind during sharp turning maneuvers.
Some of these systems incorporate an "Auto-4WD" function. These systems use the wheel slip information from the anti-lock brake system to automatically shift between 2WD and 4WD high as necessary.