We all know that intermittent problems are the worst thing to try to diagnose, mainly because when the problem is not present, there is very little we can do to pinpoint it. We get these types of calls everyday, and usually the technician is just hoping we’ve run into something common that fits their symptoms. In a few cases we get lucky, and it is a common problem and we can point them in a good direction; but, more times than not, all we can do is try to help with the diagnosis once the vehicle decides to act up.
A shop called me recently on a 2003 Ford Focus. The customer’s complaint was the battery would be dead some mornings, 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 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, the same 5-amp draw was seen intermittently again. 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, which would certainly pull a battery down overnight if the draw stayed present for several hours. The next challenge 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 test 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.
[PAGEBREAK]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.
