Ford Voltage Regulator Test Summary

I've been working as a diagnostic automotive tech for a while now and one of the things that I have seen that stumps many techs, is the Ford P0620 diagnostic trouble code.

Let me tell you of a recent diagnostic experience I had at work:

Not Charging Complaint (2004 Expedition 5.4L)

Where I work, the owner has a huge small time used-car dealer/lot clientele. These are cars that never get new parts (for the most part) since the used-car lot owner wants to maximize his profit as much as he can.

Well, one of these used-car lot owners sent a 2004 Ford Expedition that had had the alternator replaced with a used one but the check engine light was on with DTC P0620: Generator Control Circuit Malfunction and the warning center was flashing the charging system malfunction warning across its little screen on the instrument cluster.

To make the long story short, the very first thing I did after retrieving the diagnostic trouble code P0620, was to start the engine and check the voltage output of the alternator (at the battery).

The alternator was outputting 14.3 Volts! One of my fellow techs came over and when he saw the test result, mentioned that the PCM was probably bad since the alternator was obviously charging. But, as you'll soon see/read, it was the alternator that was bad.

Repair Solution

Having seen this problem so many times before, over the course of my repair career, I knew that I needed to immediately check the resistance between the alternator's pin 1 and Ground (preferably on the alternator's case).

The resistance registered was 60+ K ohms. This told me I didn't need to spend any time trying to diagnose a bad PCM or anything else, I needed a new alternator.

Well, my boss (and owner of our little repair shop) ordered the alternator, of course he didn't get a new one. Well, he ended up getting me 4 used alternators (over the course of 3 days) and they were all bad.

The good thing was that I didn't have to install any of them to test them since I was able to test them on the bench (specifically checking the resistance of pin 1 and Ground). It was finally the fifth one (and a new one) that solved the problem.

In Conclusion

I remember another similar experience (many years ago), that involved the same issue, the Ford vehicle got the alternator replaced with a new one from O'Reilly Auto Parts. The alternator would only last about a week and the owner of the vehicle would bring it back with to the shop.

About three alternators later (the Ford Van ended up staying about a week till we figured it all out), what solved the issue was buying an alternator from a different parts supplier.

Now, I wasn't the tech troubleshooting this problem, but I did get involved with it (everyone was stumped, including myself) that what helped us to avoid replacing the PCM was knowing about testing the resistance of pin 1 and Ground.

Now, in concluding, I hope this info has helped you. If it has, dropped me a line and share your diagnostic and repair experience with the rest of us. You can use the contact form below:

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Ford Vehicles:

  • Expedition 4.6L, 5.4L
    • 2004, 2005, 2006
  • F150 4.6L, 5.4L
    • 2004, 2005, 2006
  • F250 5.4L
    • 2007, 2008
  • F350 5.4L
    • 2007, 2008
  • Mustang 4.6L, 5.4L
    • 2007, 2008, 2009

Ford Vehicles:

  • F350 5.4L
    • 2007, 2008