AutoFocus: Ford’s intermittent dead battery problem

This vehicle uses a powertrain control module (PCM) controlled alternator. These alternators from the factory are typically designed to be ‘self exciting,’ so if there is a wiring problem to the voltage regulator connector on the alternator, once the engine is revved up the alternator will still start charging at about 13.5 volts. Now, I make that statement somewhat loosely because even though Ford says that is how the alternators are supposed to work, we have not found it to be true 100% of the time.

The voltage regulator had already been disconnected with no change in the draw, so my suggestion to the technician was to very carefully take the output wire off the alternator when the draw was present. Obviously, this wire was still live, and since we did not want to disturb the ammeter and affect the draw, extreme caution needed to be taken in order to not provide a circuit to ground. When the tech did this, he struck gold! He found the answer we were looking for.

When he applied pressure on the retaining nut while trying to loosen it, he saw the draw go away. He then applied pressure the other way, as if trying to tighten it, and the draw would come back. The pressure was so slight that the nut was never actually loosened to the point that the wire was loose on the alternator. So something inside the alternator was shorting out under the right conditions and allowing current flow, thus draining the battery. Replacing the alternator resolved the problem, making both the technician and vehicle owner very happy.

Ultimately, when trying to diagnose an intermittent problem, the best tool any technician can have is perseverance -- wait it out until the problem presents itself.

Source: Identifix Inc.

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Related Topics: Auto Service, Exhaust systems, Ford Motor Co.

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