The other day a Nissan came into the shop. The battery and the alternator had been replaced. The car would start if you drove it, and let it sit a few hours, but would need a jump start if left over night. The battery tested good, and the alternator was charging plenty of amps, with no A/C ripple, and 15 volts. Sounds like a classic case of a draw.

I hooked up a test light and found the draw is on the fuse for the alternator charging signal. (For information on how to find a draw please visits here ) Occasionally, you will have an alternator that is charging fine, but back-feeds when the car is off and thus results in a draw. Given the fact that the alternator has already been replaced, it is highly unlikely that the alternator is the cause of the draw. So where do we go from here?

Upon inspection of the alternator, I found the following

poor alternator repair

Clearly this is not the factory connector. Someone has damaged the connector and repaired it, all be it poorly. This connector contains two wires, the charging signal wire to the alternator and the charging warning light wire that goes to the gauge cluster. The question remained, if these two wires where switched, what would happen?

Being that we did not want to simply swap the wires and see what happens, we called Nissan for a pig tail. Nissan does not offer a pig tail for this specific vehicle, but they do offer a $400 engine harness. Next call went to a junkyard who not only had the pigtail but offered it to us at “whatever it was worth to us”. The rapair looked a little like this. ……

alternator properly repaired

Note the wires were indeed backwards. Having fit the new pigtail, the draw went away, and charging returned to a normal 14.3 volts. The moral here is that no one in the shop knew positively wether swapping those wires would result in the complaint we encountered. Everyone in the shop however, knew that spade connectors in lieu of a proper factory connector was not right. Fixing that known problem fixed all the problems.

More on theories of automotive repair, including fixing know problems, go here