My sister has a 2009 Honda Pilot that's got some issues with the electrical system.

The battery frequently drains overnight (every other day or so).

When driving a lot (several hours or more) the car will start the next morning. When driving not much (like yesterday about 10 miles) the next day the battery was so dead it couldn't even unlock the car doors. Medium timed drives may or may not start the next day.

They took it to an Autozone and the guy plugged in some type of computer that said it was the alternator. However, we just started it this morning and my voltmeter read 13.5-13.7 volts. It was reading about 13.7 volts for about 10 minutes and then it dropped to about 13.5. Though running the engine steady at 2,000 rpms made the volts stay at a steady 13.7.

They've been having intermittent problems with the air conditioning. At lower speeds it seems to work fine, but at higher speeds it stops blowing cold air. Occasionally when they turn off the AC condenser it will actually sometimes blow colder air. They took it to someone who pulled the dash off and found that the expansion valve(?) was stuck and was causing freezing (literally, ice inside the dash) to occur. They think that problem has kind of gone away since coming to much warmer climate (Arkansas, we've been in the 90ºF mostly), but they weren't sure if that was related to the electric. They were told that alternator problems could cause issues with AC & other things.

Is it likely that the alternator is causing the battery drain? If not, what other things could we check? The battery is 2-3 years old.

No lights are left on at night :)

  • Having the same issue with my 09 pilot. I thought it was the battery. Okay, I hoped it was the battery, so I replaced it. When I replaced the battery, I noticed it was the exact brand I purchased less than two years ago. And the truck died three days later. I left nothing on. When I had the Took it to the Honda service center. $125 just for them to look at it. I'll let you know what the deal is according to Honda.
    – acm
    Feb 17, 2017 at 19:43
  • 1
    You should have just spent the $10-20 to replace the AC clutch relay first. Worst case you have a spare relay ;) Feb 19, 2017 at 4:09

5 Answers 5


There are too many possibilities when it comes to draw. The compressor clutch could be staying on, alternator voltage regulator, a glovebox light, radio, computer etc...

You're going to need to use an inductive amp probe and a multimeter to voltage drop the fuses. I'd avoid disconnecting battery terminals to hook an ammeter inline with the battery ground. This can sometimes cause the draw to disappear while testing. Make sure the battery is fully charged 12.5v or better, before testing for draw or it can lead to false positives.

13.5-7v on the alternator is kind of weak I'd expect 14v or better. You may want to have the charging system load tested.

  • It looks like it was an issue with the compressor clutch - Honda Pilots apparently are notorious for having a bad relay that keeps that on. That causes the A/C pressure to build up, at which point the pressure relief valve blows and it vents refrigerant. That happened to them 2-3 times. Jun 15, 2016 at 11:52

an easy way to check current draw without an amp meter or any tools: go to the car after dark. pull and replace each of the fuses, 1 at a time. Watch for any kind of spark. If there is a spark then that circuit is drawing power. It could be one thing or a combination of multiple things. Another possibility could be bad diodes in the alternator, which would not show up this way, but may spark if the alternator wire is disconnected. The battery could be bad also. To check it, you could disconnect the ground connection as soon as you shut the vehicle off, then reconnect in the morning and see if it starts - that may confirm whether the issue is a current draw.


13.5 volts doesn't indicate a weak alternator. As long as you get 50+ amps or so under load, the alternator will be just fine. 14 volts is nice, but the regulator may be only allowing 12.5 since the car was new.


Check your AC relay they go bad get stuck on overnight and drain the battery.


Same vehicle same symptom, it was the GPS, we just disconnected it and the problem was solved. Very annoying, took a lot of trial and error to find it.

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