* PIP Sensor (Camshaft Position Sensor): This is the most likely culprit. The PIP sensor tells the computer the crankshaft's position, which is crucial for determining injector pulse timing. A faulty PIP sensor can prevent the computer from sending the signal to fire the injectors.
* Ignition Control Module (ICM): The ICM receives signals from the PIP sensor and other sensors and sends signals to the ignition coil and injectors. A bad ICM could be preventing the injector pulse signal from getting through.
* Computer (PCM/EEC-IV): While less common, a faulty PCM could be the source of the problem. It could have a malfunctioning output circuit for the injectors.
* Wiring Harness: Look for damaged, broken, or corroded wiring between the PCM, ICM, injectors, and PIP sensor. A short or open circuit in the injector circuit will prevent pulsing.
* Injector Relay: Check the injector relay to make sure it's functioning correctly and not stuck closed.
* Fuel Pump Relay (Though less likely given the symptoms): While the fuel pump runs continuously, ensure the fuel pump relay itself isn't faulty. A faulty relay *could* indirectly cause injector problems in some rare cases but it's less likely with the other symptoms.
Troubleshooting Steps:
1. Check for Codes: Use an OBD-I code reader (for a '93 Mustang) to see if any diagnostic trouble codes (DTCs) are stored in the computer's memory. This can provide valuable clues.
2. Check the PIP Sensor: Inspect the PIP sensor for damage and test its signal with a multimeter or an oscilloscope (oscilloscope preferred for a more accurate reading).
3. Check the ICM: Testing the ICM requires more specialized equipment and knowledge. It's often easier to replace it as a suspect part after checking other components.
4. Check Wiring: Visually inspect the wiring harness, paying close attention to connections at the PCM, ICM, injectors, and PIP sensor. Look for breaks, corrosion, or chafing.
5. Check Injector Circuit: Test the injector circuit for continuity and proper voltage.
6. Test the Injectors Directly: You can use a "noid light" or a multimeter to verify that the injectors are receiving a pulse signal from the PCM.
7. Check the Fuel Pump Relay: While less likely, ensure the relay is working as it should and not stuck.
Because a constantly running fuel pump usually indicates a problem with the fuel pump control circuit, and no injector pulse is a separate problem, addressing the PIP sensor or ICM is the most logical starting point. A mechanic with experience in these older Mustangs would be very helpful in diagnosing and resolving the problem.
