This article details the diagnostic process using VCDS (Vag-Com Diagnostic System) to pinpoint the cause of a no-start condition in a 2008 Volkswagen Beetle. The focus is on verifying power supply to the Engine Control Unit (ECU) and associated components.
Initial Checks and Fuel System Verification
The initial troubleshooting steps involved confirming the functionality of the crankshaft position sensor and fuel pump. A strong fuel flow from the fuel rail when the driver’s door is opened or the engine is cranked indicated that the fuel pump, fuel pump relay, and crankshaft position sensor were operating correctly. Basic checks included verifying ground points, fuses, and battery voltage (around 12.7V). A preventative measure was taken by replacing the old black coolant temperature sensor (CTS) with the updated green top version known for its reliability. The secondary air injection (SAI) pump relay (J299), often referred to as the “100” relay, was also replaced with the updated “644” light-green relay (part# 4H0951253).
ECU Relay (J271) Testing and Fault Codes
The focus shifted to the ECU relay (J271). Manually grounding pin 5/85 of the relay (the ground trigger) did not activate the relay. With the ignition on and the relay manually grounded, VCDS reported the following fault codes:
- 17987 – Throttle Actuator (J338) Adaptation Not Started
- 17070 – ECM Power Relay Control Circuit Short to GND
- 17925 – Power Supply Relay for ECU Electrical Malfunction
- 17953 – Throttle Valve Controller Malfunction – Intermittent
The key fault code, 17070, pointed to a short to ground in the ECU power relay control circuit.
Voltage Measurements at Relay Socket
Voltage measurements at the J271 relay socket revealed:
- Pin 1/30 (Constant 12V): +12V present with ignition on or off.
- Pin 3/86 (Switched 12V): +12V present with ignition on or off. This voltage was looped from Pin 1/30.
- Pin 5/85 (ECU Ground Trigger): Only 0.5V with ignition off and 0.35V with ignition on, significantly lower than the expected 12V.
- Pin 2/87 (Output to ECU): No voltage due to the lack of ground trigger.
The low voltage on Pin 5/85 suggested a problem with the ECU’s ground trigger signal. However, direct measurement at the ECU harness connector (T121/21) showed a correct 12V output with the ignition on or off, confirming the wiring between the ECU and relay was intact.
Further Testing and Potential Causes
Bypassing the J271 relay by jumping pins 1/30 and 2/87 and measuring voltage at the ECU harness (T121) with the ignition on revealed:
- T121/3 (Ignition 12V): No voltage present, indicating a potential issue with fuse 29 (S229) or its wiring despite a previous check showing it as okay.
- T121/21 (ECU Ground Trigger): No voltage, further confirming a problem with the ECU’s ground trigger signal despite the direct measurement contradicting this.
- T121/121 (Output to Ignition Coils): +12V present due to the relay bypass, allowing the engine to start.
The lack of voltage on T121/3 suggests a potential issue in the ignition switch or wiring related to the main power supply to the ECU. The conflicting results regarding the ECU’s ground trigger signal warrant further investigation, focusing on potential intermittent faults or issues within the ECU itself.
Conclusion
Using VCDS in conjunction with manual voltage measurements helped narrow down the no-start issue to a problem with the ECU’s power supply. While the initial diagnosis pointed towards a potential fault in the ignition switch or related wiring, the inconsistent readings related to the ECU ground trigger signal require further investigation to definitively identify the root cause. This case highlights the importance of systematic troubleshooting and the value of using VCDS for accurate fault code retrieval and guided diagnosis.
