Diagnosing a Wiring Issue on a VW MK4 with an Ignitron ECU ME7.5
A customer reached out with a list of trouble codes after installing our ME7.5 engine harness on their VW MK4 running an Ignitron ECU. The codes reported were:
X0074 - Fuel injection shutdown activated due to overboost.
P0118 - Engine coolant temperature circuit high input.
P0113 - Intake air temperature circuit high input.
P1338 - Camshaft #1 position sensor circuit open or short to battery.
Diagnosing the Issue
After reviewing the codes, we devised a plan to assist in diagnosing these issues. Generally, when an ECU shows a high input for a sensor, it typically indicates one of two things:
1. A missing sensor ground.
2. A faulty sensor sending incorrect resistance data to the ECU.
In this case, we focused on the coolant temperature sensor, intake air temperature sensor, and MAP sensor. The overboost condition appearing with the ignition on and the engine off indicated a high input for the MAP sensor as well. What do all these sensors have in common? They all share the main sensor ground located at Pin 108 of the ECU.
Step 1: Checking Continuity
We instructed the customer to check continuity for each sensor from its connector at the engine to the ECU pin locations:
Engine Coolant Temp Sensor → Pin 93
Intake Air Temp Sensor → Pin 85
MAP Sensor → Pin 101
Continuity for each sensor was confirmed, so we moved on to the next step.
Step 2: Adjusting the Sensor Ground Connection
Next, we checked Pin 108, the main sensor ground. We had the customer remove each of the affected sensor pins from the ECU connector and use needle-nose pliers along with a pick tool to slightly narrow the receptacle for better contact. After this adjustment, the codes for the three sensors disappeared.
Step 3: Diagnosing P1338 – Camshaft Position Sensor Circuit
The last remaining code, P1338, indicated an open circuit for the camshaft actuator. This meant the ECU was not seeing a loopback ground signal from the sensor. We guided the customer through these steps:
1. Check continuity between the sensor connector and the ECU harness connector. Continuity was confirmed.
2. Reconnect the sensor and ensure it was fully seated. After doing this, the issue was resolved.
It turned out that the connector was not fully seated, causing the ECU to detect an open circuit.
Conclusion
After reassembling everything, the car started successfully, and all sensors functioned properly. With all codes cleared, the vehicle was back in operation.
We've encountered this issue before with certain ECUs, whether new, aftermarket, or even stock. Over time, some ECU pins can become slightly worn or oxidized, preventing proper contact with the terminals in the harness. In this case, gently crimping the ends of the terminals allowed for a better connection, resolving the issue and ensuring the sensors communicated correctly with the ECU.
By following these troubleshooting steps, the customer was able to diagnose and fix the wiring issue efficiently, avoiding unnecessary part replacements. Proper pin seating and continuity checks remain essential in diagnosing electrical issues in engine management systems.