Owner's Problem Description
The owner reports that the 1988 Volvo 740 is experiencing a no-start condition due to lack of fuel reaching the engine. The issue occurs consistently when attempting to crank the engine. Previous actions include checking fuses #1 and #11, which revealed power to fuse #1 but not to fuse #11. The owner has also connected the in-tank pump wire to the main pump fuse, causing the pump to operate, indicating that the wiring to the pump is intact. However, they are uncertain about the power supply to the in-tank pump and suspect possible issues with the relay or ECU.
Professional Technical Analysis
The 1988 Volvo 740, equipped with a 2.3L inline-4 engine and an automatic transmission, has a reported no-fuel condition causing the engine not to start. Given that the vehicle is approximately 35 years old, it likely has around 150,000 miles, and environmental factors such as age-related wiring degradation may play a role. The owner has indicated that fuse #1 (main fuel pump) has power, while fuse #11 (in-tank pump) does not, suggesting a possible interruption in the circuit. The fuel pump relay must be checked for continuity and operation, particularly the voltage at the relay's female side, where the red wire should provide power when the ignition is on. A common failure point in this model is the fuel pump relay itself, which can fail to engage due to internal corrosion or heat damage. Additionally, inspecting the wiring harness for signs of wear or corrosion is crucial, particularly in older vehicles, where insulation may crack, causing shorts or open circuits. Current vehicle condition presents a safety concern as it is inoperable; therefore, diagnostics must prioritize fuel delivery integrity. Voltage checks should indicate 12V at the fuel pump relay output when the ignition is engaged. Furthermore, testing the radio interference suppression relay is warranted, as it may affect the fuel pump's operation. A systematic approach following the wiring diagrams for this vehicle is essential for isolating the fault.
Possible Causes
Most common causes (ordered by frequency):
- Fuel Pump Relay Failure: The fuel pump relay may fail to supply power to the in-tank fuel pump, which could be due to internal corrosion or heat damage. Relay testing should confirm if it clicks when energized and provides 12V output to the pump circuit. - No voltage at the in-tank pump fuse #11, relay does not click when energized, possible corrosion at relay terminals.
- Wiring Issues: Corroded or damaged wiring between the fuel pump relay and the in-tank pump could result in an open circuit. Visual inspection of wiring harness for wear, especially in high-heat areas, is necessary. - Signs of insulation wear or exposed wires, continuity test failing between the relay and pump.
- ECU Malfunction: An issue with the engine control unit (ECU) may prevent the fuel pump from receiving the necessary activation signal. Diagnosing with a scan tool can reveal any fault codes related to the ECU. - No stored fault codes in the ECU, but potential failure indicated by lack of fuel pump activation.
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 - Verify Relay Function: Start by checking the fuel pump relay for functionality. Use a multimeter to measure continuity across the relay terminals. The relay should click when energized and provide power to the fuel pump circuit. If it's faulty, replacement with OEM part number 3527358 is recommended.
- STEP 2 - Inspect Wiring Harness: Conduct a visual inspection of the wiring harness between the relay and the in-tank pump. Look for signs of wear, corrosion, or damage. If any issues are found, repair or replace the damaged wiring as necessary.
- STEP 3 - Check Voltage at Relay Output: With the ignition on, measure the voltage at the output side of the fuel pump relay. It should read around 12V. If no voltage is present, the issue may be with the relay or upstream components.
- STEP 4 - ECU Diagnostic Check: Use an OBD-II scanner to check for any fault codes in the ECU. This can help identify if the ECU is not sending the activation signal to the fuel pump. If any codes are present, address them accordingly.
