Owner's Problem Description
The owner of a 1990 Lincoln Town Car reports no power from the PCM to the fuel injectors. They have checked fuse number 13 and fuse T in the engine compartment, both of which are confirmed okay. After running a hot wire directly to the injectors, the vehicle runs fine. However, when reconnecting the sensors, the car dies as if the key was turned off. The owner is confused because there is spark and fuel present, but power to the injectors is inconsistent, showing 12.8V with the key on and dropping to 0.84V during cranking.
Professional Technical Analysis
The 1990 Lincoln Town Car, equipped with a 4.6L V8 engine and approximately 150,000 miles, is experiencing a critical issue where the PCM is not supplying power to the fuel injectors. The symptoms include the vehicle running only when the injectors are powered directly and stalling when reconnecting sensors. This indicates a potential failure within the PCM, EEC relay, or associated wiring. Given the voltage readings of 12.8V with the key on and a drop to 0.84V while cranking, it suggests a significant voltage loss or interruption in the circuit supplying the injectors. The EEC relay, which powers the injectors, must be tested under load, as it may be faulty or have poor connections. Additionally, the condition of the wiring loom and connectors should be inspected for corrosion or damage, as environmental factors can lead to intermittent failures. Testing should also include the integrity of the diode in the relay box that could affect relay operation. This case highlights the interconnected nature of the vehicle's electrical systems and the importance of systematic diagnostics when dealing with intermittent issues.
Possible Causes
Most common causes (ordered by frequency):
- EEC Relay Failure: The EEC relay controls power to the fuel injectors, and if it is faulty, it may not supply the necessary voltage. Testing should include verifying relay operation under load and checking for proper voltage at the relay terminals. A failed relay may present issues such as voltage drops or complete power loss. - Confirmed that the EEC relay was checked; however, intermittent problems can occur even when the relay appears functional.
- PCM Malfunction: The Powertrain Control Module (PCM) could be failing internally, leading to irregular power output to the injectors. Given the vehicle's age and mileage, PCM failures are not uncommon. Testing should include checking for any stored fault codes and performing a bench test if necessary. - The owner has replaced the ECM, indicating previous issues with the control module; further testing may reveal additional faults.
- Wiring and Connector Issues: Damaged or corroded wiring or connectors between the PCM and injectors can cause significant voltage drops, leading to injector failure. Continuity tests and voltage drop tests should be performed on the wiring harness to pinpoint any problematic areas. - Symptoms of stalling and inconsistent voltage readings suggest possible wiring issues that need thorough inspection.
- Sensor Failure: A faulty sensor connected to the PCM may cause the vehicle to shut down when reconnected. This could indicate a short circuit or incorrect input signal, which the PCM interprets as a critical error. Further diagnostic work is needed to isolate the faulty sensor. - The vehicle dies when sensors are reconnected; identifying which sensor is causing this issue is crucial.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial OBD-II Diagnosis: Begin with a comprehensive scan using the GeekOBD APP or a professional OBD-II scanner to retrieve any stored fault codes. This will help identify potential issues with the PCM or connected sensors that could be affecting injector operation. Pay attention to any fault codes related to the engine management system.
- STEP 2 - EEC Relay Testing: Test the EEC relay for proper operation by checking for voltage at the relay terminals while the vehicle is cranking. Use a multimeter to measure voltage at the injectors to confirm that power is being distributed correctly. If voltage is absent or low, the relay may be faulty or have poor connections.
- STEP 3 - Wiring Inspection: Inspect the wiring harness and connectors between the PCM and injectors for any signs of damage or corrosion. Perform continuity tests on the wiring to ensure there are no breaks or shorts that could affect the injector voltage supply. This step is crucial to rule out wiring issues as a cause.
- STEP 4 - Sensor Evaluation: If the previous steps do not reveal any faults, evaluate the sensors connected to the PCM. Disconnect each sensor one at a time and observe if the vehicle continues to run without stalling. This can help identify any faulty sensors that may cause the PCM to shut the injectors down.
