Other Articles - June - 2018

4×4 Mayhem: Diagnosing DTC C0306 for the American Axle (AAM) Transfer Case

Change happens even when you don’t realize it. Change in this industry is happening quickly and that trend will continue for several years to come. All too often, you’re expected to repair problems you’ve never seen before, often part of an unfamiliar system or component.

The lack of familiarity and understanding with the system or component can be frustrating as you fight your way through to diagnose and repair something you know little or nothing about.

One such experience deals with a little-known AAM transfer case system (RPO NQV) used in 2013-2014 GMC Canyon/Chevrolet Colorado mid-size pickups. The 2012-and-earlier applications used an Isuzu T-150 transfer case, while the 2015-and-later applications use a Magna Powertrain (MP) transfer case.

The transfer cases used in the 2013- 2014 applications were manufactured by AAM, better known as American Axle Manufacturing, located in Detroit. The AAM transfer case is a little different from some transfer case systems, especially when it comes to electronic operation and what you can expect to see on your scan tool.

At this point, you may be wondering why I chose to write about a transfer case that was only used for a little while. The reason? I’ve been asked to help diagnose several vehicles that were having transfer case issues. Before we get into the actual issue, let’s take a bird’s-eye view of the AAM transfer case.

The AAM transfer case is a part-time, 2-speed, electronically shifted unit very similar to some of the New Process and MP transfer cases used in many truck applications. The transfer case is controlled by a standalone transfer case control module, which is located behind the glovebox.

The bidirectional shift motor mounts to the back of the transfer case and controls a mode and a range fork inside the unit. 4WD low range achieves a 2.62:1 ratio, while high range is 1:1.

As with many other transfer cases, you can make “on the fly” shifts between 2WD and 4WD, but shifting into or out of 4WD low range will require that you place the transmission shifter into neutral, with road speed less than 2 MPH. If the shift fails to occur, the indicator lamp on the dash-mounted rotary switch will flash for 30 seconds before lighting the previous range LED.

To shift the transfer case to neutral, rotate and hold the switch past 4WD low position. With the transmission in neutral or the clutch pedal depressed, the red, neutral LED on the switch will light.

To shift the transfer case, you’ll need to rotate the switch to the desired position. There are several fixed resistors inside the switch. The TCCM provides five volts to the switch and simply monitors the voltage drop across the switch resistors to determine the range the customer desires.

Once the TCCM receives the voltage signal from the switch, it commands the TCCM motor drivers to reposition the motor to the desired position. The TCCM knows the motor’s position based on values received from an incremental sensor mounted in the motor.

In addition, the TCCM energizes a clutch synchronizer coil to make the 2WD-to-4WD shift, preventing gear clash during the shift. The synchronizer coil mounts on the output shaft and takes the place of the conventional synchronizer cone or ring and shift sleeve used in many other transfer case applications.

Now that you understand how the system is supposed to operate, let’s look at an issue you’re likely to run into: DTC C0306. One thing to remember regarding DTCs on today’s vehicles is that many, including GM and Toyota, display more than just basic DTC information.

This DTC is a good example. The DTCs are displayed as 7-digit codes rather than the old 5-digit codes. The last two digits are symptom codes and can be quite helpful. In this case, you may find something that looks like this:

  • C0306-01 — short to battery
  • C0306-02 — short to ground
  • C0306-04 — open or high resistance
  • C0306-F1 — performance issue

So what does it take to set these DTCs?

  • The TCCM supplies voltage to each of the motor circuits and reads the return voltage on the other circuit (voltage drop).
  • If the TCCM detects incorrect voltage drop, it’ll set a DTC.
  • The fault must be present for at least four seconds to set the DTC.

On the suspect vehicles, the DTCs that set were C0306-04, indicating we had high resistance somewhere. As I always point out in seminars, start with the basics and keep it simple. The basics start with a visual inspection.

Since we were dealing with a high-resistance issue and the motor is exposed to the elements, I started with the motor connector. A simple visual inspection of the connector is all it may take to diagnose the issue.

We replaced the connector with a pigtail, which solved the issues. Many times a simple repair is all that’s necessary to solve your problems, so try not to get “lost in the forest,” which is easy to do when working on something you’ve never seen before.