The Motronic ECM Power Supply Relay, often diagnosed using VCDS tools like the 2.5 1K0 907 379 P, can be a source of frustration for VW Jetta owners. This article explores a common wiring issue found in 2007 and 2008 Jetta 2.5 engines that can cause communication failure with the Engine Control Module (ECM).
Identifying the Problem: A Hidden “Green Puff”
A frequent culprit behind ECM communication problems lies within the wiring harness connected to the Motronic ECM Power Supply Relay (J271). Specifically, the red/black wire responsible for powering the relay is prone to damage. This damage often manifests as a “green puff” or blister approximately 10 inches from the ECM connector. While it might appear to be a simple rub spot, the damage likely originates during the manufacturing process. A possible theory suggests the wire may have been inadvertently punctured, creating a weak point vulnerable to corrosion and eventual failure.
Seam Ripper for Wiring Harness Inspection
A seam ripper can be used to carefully open the wiring harness tape for inspection.
This internal corrosion expands, creating the visible bulge. As the corrosion worsens, resistance in the wire increases, generating heat and further accelerating the damage. This can lead to intermittent or complete loss of communication between the ECM and the relay.
Diagnosis and Repair: Beyond Ohm Testing
A standard ohmmeter test might not reveal this issue. While the wire may show acceptable resistance under low-load conditions, it can fail to carry the higher current required to activate the J271 relay. This explains why the ECM might appear unresponsive despite seemingly good continuity readings.
To pinpoint the problem, a visual inspection of the red/black wire is crucial. Carefully remove the wiring harness tape using a tool like a seam ripper, ensuring you avoid nicking any wires in the process. Look for any signs of bulging, discoloration (green corrosion), or damage to the wire insulation.
Volkswagen and Audi recommend against soldering repairs on wiring harnesses. The preferred method for addressing this issue is to use crimp connectors for any necessary repairs or replacements of the damaged wire section.
Conclusion: Effective Troubleshooting with VCDS
Diagnosing electrical issues, especially intermittent ones, often requires specialized tools like the Vcds 2.5 1k0 907 379 P. Understanding the common failure points, such as the red/black wire in the J271 relay circuit, allows for more targeted troubleshooting. By combining VCDS diagnostics with careful visual inspection, you can effectively identify and repair this prevalent issue in 2007-2008 Jetta 2.5 engines, restoring reliable communication with the ECM.
