* Wiring Issues: This is the most common culprit. Damaged, corroded, or loose wiring harnesses related to the ABS system can cause intermittent problems with both the ABS and cruise control. These systems share some common components and circuits, particularly speed sensors. Check all wiring connections, especially those near the wheel speed sensors, the ABS module, and the brake pedal switch. Look for chafing, breaks, or corrosion.
* Wheel Speed Sensors: These sensors provide crucial information to both the ABS and cruise control systems. A faulty sensor (or its wiring) can cause intermittent issues. Even if the rear ABS module was replaced, a faulty sensor (front or rear) will still cause problems. Have the sensors tested, not just visually inspected.
* ABS Module (again): While the rear module was replaced, there's a possibility of a faulty front module or a problem with the communication between the modules. A faulty module can cause erratic behavior in the ABS system and potentially impact the cruise control. Consider having the system properly scanned for codes again to see if there are any module-specific errors.
* Brake Switch: The brake switch is a crucial component and activates safety features. A faulty switch can send false signals, preventing the cruise control from engaging or causing it to disengage unexpectedly.
* Vehicle Speed Sensor (VSS): The VSS provides speed input to both the cruise control and the ABS. A malfunctioning VSS can cause both systems to fail intermittently.
* Low Battery Voltage: A low battery voltage can cause intermittent problems with electronic systems, including the cruise control and ABS. Check the battery's voltage and charging system.
* Powertrain Control Module (PCM): The PCM is the "brain" that manages many systems in the vehicle. A fault in the PCM can affect multiple functions, including cruise control and ABS. This is less likely but possible.
Troubleshooting Steps:
1. Have the system professionally scanned: A diagnostic scan tool will read any fault codes from the ABS and other relevant modules (PCM, etc.). This provides the most direct indication of the problem area.
2. Inspect all wiring: Carefully examine all wiring harnesses related to the ABS and cruise control, paying close attention to areas that experience vibration or movement.
3. Test the wheel speed sensors: These should be tested with a multimeter, not just visually inspected. A professional mechanic is best equipped for this.
4. Check the brake switch: Ensure the brake switch is operating correctly.
5. Check battery voltage: Measure the battery voltage with the engine running and at rest.
It's highly recommended to have a qualified mechanic, preferably one experienced with motorhomes and the E350 chassis, diagnose and repair this problem. The intermittent nature of the fault makes troubleshooting more challenging. Ignoring it could lead to more significant safety issues, especially regarding the ABS.
