2007 Ford F150 Battery Light Stays On

This 2007 Ford F150 came in with the complaint that the battery light would come on about 30 seconds after the engine was started. The voltmeter in the dash indicated the alternator was charging. Checking the system with my charging system analyzer found the charging voltage was good at 14.23 volts and the amperage was at 14 amps. The battery and the alternator had been replaced before it arrived at my shop. I checked the codes and it had a code P0620 stored.  In most cases the code P0620 is caused either by a faulty or incorrect alternator. Since the alternator on this truck had just been replaced the day before, I was suspecting an incorrect or at least a non compatible alternator. The alternator for this vehicle interacts with the PCM or engine computer. A used alternator with the same mounting configuration but the wrong electrical circuits will not communicate with the PCM and would be incorrect.  The correctly listed replacement alternator with a low quality voltage regulator will also not communicate with the PCM therefore making it incompatible.

To test the basic communication lines between the PCM and the alternator is quite easy. Ford engineers must have anticipated some problems and installed an inline connector to aide in this process. It is located at the front of the passenger side engine cylinder head. It has two wires. Light Green/Orange and Light Green/Red. It is kind of difficult to tell the two wires apart so you have to look closely.  I used backprobe pins to access the circuits and a voltmeter to take readings. In this case it was enough. The Light Green/Orange wire has 12 volts applied to it from the PCM and the alternator’s voltage regulator toggles that signal to ground in a regular pattern creating a square wave. I found 12 volts present at all times, looked at the alternator wiring and found the harness disconnected from the voltage regulator connector. Turned the engine off, plugged it in and retested. All okay. I was alittle surprised that the PCM did not set any other codes such as a P0622. The signal must be at about a 50% dwell rate since the voltage with the system operating correctly was a nominal 6 volts. Since I was already here I wanted to do some more testing. Monitoring the Light Green/Red wire with a voltmeter was not as productive as I had hoped. I found a nominal zero volts with an occasional flash to .2-.4 volts. This was at about five second intervals so I did realize something was happening. I remembered afterwards that I could have set the min/max feature on the voltmeter and captured the signal but I had already moved on to looking at the signal with a lab scope.


The green lead of the scope is connected to the Light Green/Orange wire and the signal can be seen on the green trace. This circuit is known as the GEN-MON (generator monitor) and it is a feedback circuit from the regulator to the PCM. Remember that the PCM outputs 12 volts and the regulator toggles that signal to ground at a nominal 50% dwell rate resulting in an average voltmeter reading of 6 volts.


The yellow lead of the scope is connected to the Light Green/Red wire and the pattern can be seen on the yellow trace. This circuit is known as the GEN-COM (generator command) and it is a signal sent from the PCM to the regulator. The signal varies with the electrical load determined by the PCM. If the PCM determines a higher electrical load from the headlights , blower or etc being turned on it will flash a series of rapid battery voltage pulses. In the images that I captured there were two pulses flashed from the PCM. At this point I can only assume if more electrical loads were applied by turning various systems on that there would have been more pulses. If true, the signal could also be captured more readily on a voltmeter by applying the higher electrical loads on the system.


The GEN-COM pattern repeats in five second intervals.


In hindsight I wish I would have applied more loads and captured that image. The next time I get one of these truck back in I will try to capture that image and voltmeter readings.


The Ford alternator used on this vehicle was in the 6G 110 amp IF/IR family with a 337 plug code.

To check the regulator basic circuitry you must measure the resistance between each terminal and the case of the alternator with an ohmmeter. The resistances should be as follows.

Pin 1 to ground should be greater than 1000K ohms.

Pin 2 to ground should be greater that 125K ohms.

Pin 3 to ground should be greater than 125K ohms.

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