Intermittent Dead Battery Problem Solved

Posted 7/13/2011
By Bob Mordorski

A shop called me recently about a 2003 Ford Focus. The customer's complaint was that some mornings the battery would be dead, but when they got it to the shop, it worked fine. It was noted that the customer had installed an aftermarket CD changer for the stereo system, but the factory stereo was still in place. The CD changer was disconnected to eliminate it as a source of the problem. The battery was also tested and found to be on the weak side, so it was replaced to try to rule out a failing battery.

When the car came back after a week with the same complaint, the shop asked the customer to leave the car so they could try to duplicate the problem. The car was left at the shop and continued to work flawlessly for several days. One day, however, while a shop technician was monitoring the draw on the system, roughly 5 amps showed up for a short time and then went away. We started questioning the ammeter since it was so inconsistent. At this point, another ammeter was used to verify the test results. With the second meter in place, again the same 5-amp draw was seen intermittently. There was no rhyme or reason as to when the draw would show up; it did not matter if the vehicle was still in its roughly "45-minute awake mode" or well after the 45 minutes when all of the modules in the car would be in "sleep" mode.

At least now we knew there was a 5-amp draw that would certainly pull a battery down overnight if the draw stayed present for several hours. Next up was narrowing down the draw. When the draw was present, the standard procedure of pulling all fuses one at a time was used. Most of us at Identifix recommend pulling each fuse and then leaving them out. Putting a fuse back in can wake up a sleeping module, causing erroneous results.

As each fuse was removed from the vehicle, the draw would still show up intermittently! At this point, about the only thing left that would cause a draw would be the alternator. 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 percent 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. 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 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. 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 ... wait it out until the problem presents itself.

Bob Mordorski is an Identifix Ford team leader. He is ASE master and L1 certified.

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