Yesterday, my wife's car (a 2009 Toyota Sienna AWD) wouldn't start, and this came clear out of the blue. A jump fixed it. We let the car sit idle for a good 30 minutes, and it was able to start up later in the day. Today, it needed another two jumps, and I took it to AutoZone to get the battery tested. It was an old battery and it came up bad, so I bought a new one.

We ran the tester again, and it said that the alternator was good but that the voltage regulator was bad. He said it was odd, because it was still showing 13.48V at load. I took it home and ran some tests:

  • Battery voltage with car off: 12.7V
  • Car idling: 13.49V, and voltage increased steadily over time
  • Car idling with all accessories on: 13.2-13.3V
  • Car revving at 2000 RPMs with all accessories on: 13.4V

Any idea if this would indicate a bad voltage regulator? The steps I saw online made it sound like this would check out, but I'd welcome feedback.

1 Answer 1


Sounds to me more an alternator problem: I ran with a problem like that precisely this year, and was the alternator brushes being dirty making a poor electrical contact inside the alternator. That will make the alternator not provide enough juice. My experience is that regulators "work or won't work". Also check the alternator belt. Under load or sudden revving would make the alternator not spin enough because a loose belt.

Failing or dirty brushes/carbons contacts can't be detected by a multimeter; if they touch their tracks even dirty, the meter will show some reading, but when you load the electric consumption they will fail to let the alternator produce enough. You can only check diodes and contacts in general, with a meter, but not the brushes.

  • Hmm, I don't get it. If the alternator is not providing enough juice for the load, shouldn't the voltage drop significantly? Commented Jul 13, 2016 at 10:35
  • Depending the load, and depending on how bad the inside contacts are. I suffered in one of my cars this problem: all worked well but as soon as I turned the lights on, or the wipers when driving at night I couldn't crank the next morning. That car had a voltimeter and it never show low volts when driving, since the battery + alternator were supplying current, but not enough to run on alternator only, ie, the load was split between alternator and battery, draining the battery off too. Commented Jul 13, 2016 at 15:18
  • Interesting. How would one troubleshoot in such a case? Is disassembly the only option? Commented Jul 14, 2016 at 8:06
  • Before taking everything appart, loosen the belt enough to be able to turn the alternator by hand, and spray WD40 in the brushes area while turning it both ways, to try to clean the brushes and the tracks. Hopefully it may do. If wasn't the case, then you will need to disassemble it, which is a pain in the neck...although you only need access to the butt side, since there is where the conflict is. Once opened, use 200 grits or whatever near sand paper and carefully clean the tracks, use WD40 for the brushes, don't sand them, then WD40 everything, dry with a rag and put all together. Commented Jul 14, 2016 at 15:51
  • I meant how would one know if brushes are to blame in this case? Would suck taking the thing out and disassembling to see that the brushes are sparkling new. Commented Jul 15, 2016 at 9:09

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .