1. Transmission Range Sensor (TRS) or Shift Solenoid: This sensor tells the transmission's computer (PCM) what gear you've selected. A faulty TRS can cause erratic shifting, and the flashing light is a common symptom. The shift solenoids themselves are also electrically controlled and could be failing. These are relatively common failure points on older Ford Explorers.
2. Wiring Harness Issues: The wiring harness connecting the transmission to the PCM can degrade over time, especially in areas subject to heat and movement. Look for frayed wires, broken connectors, or corrosion. This is often overlooked but crucial.
3. PCM (Powertrain Control Module): While less likely than the others, a faulty PCM can certainly cause electrical problems within the transmission system. This is a more expensive repair as it requires replacement.
4. Transmission Speed Sensor (VSS): This sensor provides the PCM with information about the vehicle's speed. A faulty VSS can lead to incorrect gear selections and cause the overdrive light to flash.
5. Low Transmission Fluid: While seemingly unrelated to an electrical problem, low transmission fluid can cause excessive wear and heat, which in turn can damage the electrical components within the transmission. Check the fluid level *and* condition. Dark, burnt-smelling fluid is a bad sign.
6. Transmission Solenoid Pack: This contains multiple solenoids that control shifting. A single faulty solenoid within the pack can trigger the flashing light.
Troubleshooting Steps (in order of increasing difficulty and cost):
1. Check Transmission Fluid: This is the easiest and should be done first. Low or dirty fluid is a problem that needs to be addressed regardless.
2. Inspect Wiring Harness: Visually inspect the wiring harness connecting the transmission to the PCM, looking for any obvious damage. Pay close attention to the connectors.
3. Test the Transmission Range Sensor (TRS): This can be tested with a multimeter. You may need a wiring diagram to identify the sensor's connector.
4. Test the Transmission Speed Sensor (VSS): Similar to the TRS, this can be tested with a multimeter.
5. Professional Diagnostic: This is where you likely need a qualified mechanic with experience in Ford transmissions. A more advanced scan tool may be needed to identify precisely which solenoid or sensor is causing the problem.
Important Note: A flashing overdrive light generally indicates a serious transmission problem. Continuing to drive the vehicle could cause further damage. It's best to limit driving until the issue is diagnosed and repaired. Ignoring the problem can lead to a complete transmission failure.
