C1150

What Does Code C1150 Mean?

DTC C1150 signifies a detected fault within the Two Speed Rear Axle Input Switch Circuit, specifically indicating a Short to Vbatt (Vehicle Battery Voltage). This code is primarily relevant in vehicles equipped with a two-speed rear axle system, commonly found in commercial vehicles, medium-duty trucks, or specialized heavy-duty applications. The two-speed rear axle allows the driver or vehicle control unit to select between a ‘high’ (overdrive) ratio and a ‘low’ (reduction) ratio, optimizing performance for varying loads, speeds, and terrains, thereby enhancing fuel economy and pulling power. The Input Switch, which can be a pressure switch, position sensor, or Hall effect sensor, is responsible for communicating the current operational state (high or low range) of the rear axle to the Powertrain Control Module (PCM), or in some architectures, a dedicated Axle Control Module (ACM). The PCM monitors the voltage signal on this circuit. When a C1150 is set, the PCM has detected a voltage on the input switch’s signal circuit that is consistently at or near battery voltage (typically 12V-14V) when it should not be. This indicates an unintended and direct electrical connection between the signal wire and a constant power source, bypassing the switch’s intended signal pathway or internal resistance. This abnormal voltage prevents the PCM from accurately determining the rear axle’s selected range, potentially impairing shift control, vehicle speed calculations, and overall drivetrain efficiency.

Common Symptoms

• Inability to shift the two-speed rear axle between high and low ranges, potentially becoming stuck in one gear ratio.
• Incorrect or non-functional display of the rear axle range on the instrument cluster (if equipped).
• Reduced engine performance or activation of a “limp home” mode to prevent potential drivetrain damage.
• Illumination of the “Service Engine Soon,” “Check Engine,” or a specific axle system warning light.
• Abnormal vehicle operation, such as excessively high engine RPMs at cruising speed (if stuck in low range) or lack of pulling power (if stuck in high range with a heavy load).

What Causes the Code C1150?

• Damaged Wiring Harness: The most prevalent cause is a short circuit within the wiring harness leading to or from the two-speed rear axle input switch. This often occurs due to chafing, pinching, or insulation breakdown, allowing the signal wire to come into direct contact with a fused 12V power supply wire or an exposed B+ (battery positive) source.
• Faulty Two-Speed Rear Axle Input Switch: An internal short within the switch itself can cause its signal output to be continuously at battery voltage, regardless of the actual axle position. This could be due to internal component failure, corrosion, or manufacturing defect.
• Corrosion or Water Intrusion: Extensive corrosion within the electrical connectors for the input switch or the wiring harness can create conductive paths, potentially shorting the signal circuit to an adjacent power wire or creating a direct path for stray voltage.
• Incorrect Aftermarket Wiring or Accessory Installation: Improperly installed aftermarket components or repairs can inadvertently create cross-circuits, causing the input switch signal wire to become shorted to battery voltage.
• Powertrain Control Module (PCM) Internal Fault: While less common, an internal failure within the PCM’s input monitoring circuit for the two-speed axle switch could cause it to incorrectly interpret or report a short to Vbatt.

How to Diagnose and Troubleshoot

Diagnosis of C1150 requires a systematic approach, leveraging visual inspection, a diagnostic scan tool, and a Digital Multimeter (DMM).

1. Visual Inspection: Begin by thoroughly inspecting the entire wiring harness associated with the two-speed rear axle input switch, from the switch connector back to the PCM. Look for obvious signs of physical damage such as chafing, cuts, pinching, or melted insulation that could indicate a short to a power wire. Examine the switch connector and the PCM connector for corrosion, bent pins, or loose connections. Pay close attention to areas where the harness passes through frame members, near exhaust components, or alongside other high-voltage circuits.
2. Scan Tool Live Data Analysis: Connect an OBD-II scan tool capable of reading live data. Monitor the parameter for the “Two Speed Rear Axle Input Switch” or similar (e.g., “Axle Range Status,” “Axle Ratio Input”). With the ignition ON (engine OFF or running), observe the displayed value. If a C1150 is present, the scan tool will likely show a constant “High,” “On,” or an abnormally high voltage reading (e.g., 12V-14V) regardless of actual axle selection or manipulation of the axle shift mechanism.
3. Digital Multimeter (DMM) Testing – Circuit Integrity:
   • Isolate the Switch: Disconnect the electrical connector from the two-speed rear axle input switch.
   • Test Harness for Short to Vbatt: With the ignition ON (engine OFF), use the DMM set to DC Volts. Probe the signal wire terminal at the harness side of the disconnected switch connector. Place the black lead on a known good chassis ground. If the DMM reads battery voltage (12V-14V), a short to Vbatt exists within the wiring harness between this point and the PCM, or from an external power source. If it reads a much lower reference voltage (e.g., 5V) or 0V, proceed to the next step.
   • Test Harness for Short to Power (PCM Side): If the previous test did not show Vbatt at the switch connector, it’s possible the short is closer to the PCM or an internal PCM fault. Disconnect the PCM connector that houses the two-speed axle input circuit. Using the vehicle’s wiring diagram, identify the specific signal wire pin for the input switch at the PCM harness connector. With the ignition ON (engine OFF), measure voltage between this specific signal wire pin and chassis ground. If battery voltage is present, the short is within the harness between the switch and the PCM. If no voltage is present, the PCM itself may be faulty, or the initial test was misinterpretated.
   • Test Input Switch: If the harness tests confirm no short to Vbatt in the wiring, the fault may lie within the switch itself. With the switch still disconnected, use the DMM to test the switch. For a simple ON/OFF switch, check continuity across its terminals in both axle positions (if mechanically actuated). For a voltage-producing sensor, apply appropriate power/ground and check its output voltage in different states. A continuously high output or an internal short within the switch would indicate replacement is necessary.
4. PCM Pin Check: Carefully inspect the corresponding pin at the PCM connector for damage, corrosion, or pushed-back terminals. Ensure good contact.

Recommended Repairs and Solutions

Once the root cause of DTC C1150 has been identified through thorough diagnosis, the following repairs are typically recommended:

• Repair or Replace Damaged Wiring Harness: This is the most common resolution. If a short to Vbatt is found in the wiring harness, the damaged section must be repaired. This involves carefully locating the precise point of the short, cutting out the compromised wire section, and splicing in new wire using heat-shrink butt connectors or soldering for a durable repair. Ensure proper routing and securement of the harness to prevent future damage. In cases of extensive damage, replacing a segment of the harness or the entire component-specific harness may be necessary.
• Replace Faulty Two-Speed Rear Axle Input Switch: If diagnostic testing definitively points to an internal fault within the input switch, such as an internal short causing a constant Vbatt output, replace the switch with a new OEM-equivalent part. Ensure proper torque specifications are followed during installation, especially for pressure-type switches.
• Clean and Repair Corroded Connectors: If corrosion is found at the switch or PCM connectors, use specialized electrical contact cleaner and a small brush to remove all traces of corrosion. Ensure all pins are straight and make good contact. Apply dielectric grease to the connectors to prevent future corrosion and seal out moisture.
• Address Aftermarket Wiring Issues: If incorrect aftermarket wiring or accessory installation is determined to be the cause, disconnect or re-route the offending wiring to eliminate the short. Ensure all aftermarket electrical connections are made professionally and protected from chafing or environmental exposure.
• PCM Replacement (As a Last Resort): Only consider replacing the PCM if all other diagnostic steps have been exhausted, the wiring and switch have been verified as good, and strong evidence points to an internal PCM fault related to the input switch monitoring circuit. This is a rare occurrence for this specific code. Always confirm the PCM is the fault before replacement, as it is a costly component and often requires programming.

After any repair, clear the DTCs using a scan tool and perform a thorough test drive to confirm the C1150 code does not return and the two-speed rear axle system operates correctly across its full range of functions.