How to Test Wiring in Under 1 Hour - 2CarPros How to Test Wiring. Need to test your wiring? All cars use wiring harnesses to connect various sensors and accessories to the computer ECU in order for the systems to work. This guide will help when you have replaced a particular sensor such as a camshaft angle Blown Fuse · Vehicle Repair Guides · Ask a Car Repair Question How to Test a Wiring Harness | It Still Works Testing Voltage: Disconnect the end of the harness that feeds the device. For example, pull out the Testing Continuity: Ensure the power to the harness feeding the device is disconnected.