2003 Ford Crown Victoria 4.6L no start wiring problem

This car quit and when I tested it, it has no spark, no fuel which I can see when we tested the injector terminals for each cylinder using a noid light tester. The computer of this was removed from the left side of the engine compartment and with the terminals connected and measuring the voltage for power and ground supplies, I got normal power and ground reading. However, when I tested the 5 volt vref wire, it only read 1 volt instead of 5 volts. Is my PCM due for replacement?

ATS Adviser:

The PCM supplies a controlled 5 volt supply to most sensors in the engine circuit. This happens as soon as you turn your key on and start the engine. If disrupted, it would do this exact symptom. To test this vref voltage from all circuits of the engine, get 4 copies of the performance wiring diagrams of this engine. The wiring must be for this specific model year and engine size. Once you have the print out, go to the wiring diagram which says location of the PCM and find the terminal pin called "ref volt". That is the vref wiring you are looking for. Put a yellow mark or high light on all the lines pertaining to that ref volt until you find all the sensors connected to it.

Once all vref lines are identified, it just a matter of checking continuity or disconnecting the sensor to test were the short is located. It is shorted because the vref is now reading 1 volt which should be 5 volts. It shows the wire is touching a ground frame or wire somewhere in the circuit or a sensor has failed.

Another quick way to test it is to use the bus DLC wiring diagram which you can see below. This takes 5 minutes to test and it will tell you if the PCM bus wires is OK or not. You can check if the PCM is getting adequate power and ground signals thru pins 4, 5 and 16. Once you confirmed bus wires is OK that is when you start checking the vref wirings which need opening of the engine hood.

Shown below is a typical 2003 Ford Crown Vicoria Bus Module Wirings. Kline bus wire (pin 7) is 5-7 volts at key on engine off (KOEO). Both Can bus wires are + or - 2.5 volts at pin 2 and pin 10 terminals at DLC.

2003 Ford Crown Victoria Bus Wirings

For more details, you can get a complete wiring diagram of this vehicle from most parts store when you buy parts or get it free online when you join ATS.

The rest of the bus wires like pin 7 ISO, pin 9 SCP (-) and pin 13 SCP (+) is only needed if your scanner will not give you any fault code or read them under "no communication" article problems.




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