Honda Civic Questionable Alternator

[Daniel]

Hi All,

I apologize for the long post but I figured more information is better. I have a small amount of DIY experience with my car but I’m no mechanic. I’ll say this ahead of time, thank you SO much if you have the time to read this post and offer me any advice.

The TL;DR
– Car idles at battery voltage (12.42V)
– But voltage is steady at 14.4V under load or just with headlights on
– Not sure if low idle means alternator is bad

The Problem:
I have a few reasons to believe my alternator is bad, but after extensive research and testing, I’m not sure enough to spend $200 on a new alternator. I’m hoping someone can offer peace of mind as to whether I should replace it, or let it be.

The Car:
2012 Honda Civic LX (US)
1.8L / Automatic / Sedan
I bought it from a Carmax with ~23k miles, currently has ~50k
Done my own maintenance, so far mostly oil changes / tire rotation, once replaced all brake pads and replaced brake fluid

Background:
I’ve had no real problems with this car, except I’ve been a dunce and left the interior light on, draining the battery and requiring a jump… a total of 3 times in the last year. Just a few days ago I did it again and while the jump went fine, the car didn’t start on it’s own the next time I used it. I figured I finally killed my battery, so I got another jump and drove to Autozone. They did a battery test and confirmed it was done. They also did an alternator test (same tool) and said the alternator was bad as well. This last bit of news surprised me since I was only expecting a bad battery. There was an Oreilly next door so I had them do the same tests and they had the same results. They also did a starter test and said that at least that was good (great).

I bought a new battery so I could actually start my car and went home to do research and see if I could test the alternator myself.

Multimeter Results:
After some research, I whipped out my multimeter and did some extensive testing of the new battery in the car. Here are some results.

1. With the car off, the battery was at 12.42V. I was hoping it’d be at 12.6V since it was new, but I guess it lost some charge on the shelf. I definitely noticed that all of my interior lights were much brighter with the new battery so that made me happy enough.
2. With the car on and idling (~750RPM), the voltage didn’t move, it stayed 12.42V. I tried revving to 3,000 RPM, still no change in Voltage. However as the RPM returned to normal, I noticed a slight (0.2V) increase. It went away when RPM returned to idle. Now I’ve read conflicting things on the internet. Most websites/videos (including Eric’s) state the idle voltage should still be at least 0.5V higher than battery, so AT LEAST 13V. Some places however claim that some cars idle too low to really charge the battery or if the battery is full it won’t kick in. Anyways, on to the next test…
3. With the car on and under load (lights, brights, radio, AC, etc.) the Voltage went up to 14.4. What? It worked totally fine under load. In fact, I turned off everything but the headlights and found that just turning the headlights on and off caused the voltage to stay at 14.4

So it appears that my car isn’t charging the battery while idle, but under huge load, or just headlights, it works perfectly fine. If I had to guess, I’d say the idling voltage being so low is still a bad sign. But I hate the thought of having to replace it when it seems to work fine as long as the lights are on.

One more test that may or may not be significant.

• I watched the Voltage *as I started the car*. It starts at 12.42V since engine is off, then as I start it, it dips to as low as 9V, then slowly rises all the way to 14.4V, then slowly goes back to 12.42V where it stays while idling. Again this makes me frustrated since it seems like the alternator is capable, just not willing to charge at idle.

Oh, and I also had my dad, who owns a 2015 Civic, check his idle voltage and he had 14.0V. So unless there’s an expected difference in my 2012 model, I suppose this is a bad sign.

Some Questions:
1) Is 12.42V idle OK or is that definitely going to sap my battery?
2) Despite the alternator working under load (14.4V), should I still replace the alternator?
3) Can I be sure it’s the alternator? Does the 14.4V mean that maybe the ECU or some component that tells the alternator to charge is bad instead?
4) Is it worth extra money for an OEM / brand NEW part as opposed to aftermarket / re-manufacture?

For now I’m driving with the lights on until I figure out if the idle voltage is safe or not for the battery. I appreciate any and all advice and I’m of course willing to provide more detail or run some more tests if I can do them with tools I have or can borrow.

Thank you!

Topic

[Robby Villabona]

There’s this component called the Electrical Load Detector (ELD) in your under-hood fuse box which sends data to the ECU which (along with other info) tells the alternator how much charging voltage to put out. It decides not to have the alternator put out a battery-charging voltage if it senses that the battery doesn’t need charging. What you describe appears to be normal. Here’s a write-up on how the ELD works:

Honda’s Dual Mode Charging System

On cars without an ELD I’d say the “normal” charging voltage would be within 13.5 to 14.5 volts, depending on how many accessories you have running. OTOH, I’ve read an extensive article on how batteries work and it says that they don’t really start charging until 13.8V, and even then they charge v-e-r-y slowly at that voltage. Me I just go by checking every day if my battery is getting fully-charged or not.

However the fact that your car is only five years old and has an alternator fail would be worth investigating. Like in this thread, I would start by doing voltage drop tests on the main engine and battery ground wires, as well as the actual charging system. Bad connections may be causing your alternator to work at full load for extended periods thus shortening its life. That’s what happened to me for years as shop after shop repeatedly threw new and remanufactured alternators at my problem until I decided to study the problem on my own.

The other thing that I find a cause for concern is that you saw the battery voltage drop to 9V while starting. Below 10V while starting is a sign of a battery going bad. Were all your voltage tests done at the battery posts? I’m also curious why your battery doesn’t get fully charged. A normal drive should’ve already charged it to around 12.6V even if that new battery lost some charge sitting on a shelf.

Like you I’ve left my headlights on too long on several occasions which drained my battery so just last week I finally decided to do a DIY warning buzzer. Got a small, $1 piezo-electric buzzer and connected it to the appropriate OPTION connectors in the under-dash fuse box. Now I get a warning buzz whenever I inadvertently leave my headlights on.

[Daniel]

Hi RobbyV,

Thanks for the reply. I read through the Dual Mode Charging document and it seems hopeful that it explains my test results. I did some more research on ELDs and found other sources/forums that confirm it keeps the voltage low (12.4-12.8V) to save gas and purposely avoid working the alternator. I suppose I’ll do more research to make sure my car is equipped with one. Although my dad’s 2015 Civic idles at 14V which doesn’t agree with this. I would think his car should have the same ELD behavior as mine. Maybe it’s related to my battery being new? Or his newer model?

To answer your question, all Voltage tests were at the battery.

It’s worth noting that the battery voltage did get up to 12.6V after the drive. It seems to be about 12.45V in the morning again.

I did the Voltage drop tests and found that positive side (alternator output to battery positive) was 0.03V drop and up to 0.08V when I turned on the lights (to get the alternator to increase total Voltage). On the negative side (battery negative to alternator exterior) I got no drop at all under either condition. My understanding is these readings are good (although I’m surprised there was no drop at all for negative, it flickered to 0.01 occasionally so I think I was in the right place).

I ran the startup test again a couple times and actually the Voltage only dips to 11V. It is also a super quick dip so I may have gotten the 9V wrong the first time I watched it.

I also did the load test again because I think I forgot defrosters and this time got some notably different behavior. Under the full stress test, this time the idle voltage was NOT 14.4V but was back at 12.5~12.6V (slightly higher than idle under no load). Just turning off one of the big power sinks (lights, defroster, or AC) was enough to make it go up to 14 again. Also, still with load, if I upped the RPMs at all the Voltage quickly jumped back up to 14. This being different from idle where RPMs didn’t make a different.

These new load test results seem a little scary since it was barely at 12.6V, but then again maybe it reinforces the fact that it’s normal ELD behavior. It seems pretty advanced though that even under load it outputs just enough to keep the battery voltage at ~12.6. But then revving or partial load puts it at 14V.

As for leaving lights on, it’s actually been the interior overhead light each time. They were especially dim with my old battery so I guess I could hardly tell they were on. Perhaps your solution is applicable to those too, that’s something I can look into.

I will do some more research on ELDs when I have time today after work (late tonight probably). Let me know what you think of these new results and if there are any more useful tests I can do. Thanks for your time!

[Daniel]

Just did a little more research during lunch. Found this article which talks some more about the ELD and how the alternator only charges under load. Not much new from the one you provided but I wonder if it’s worth disconnecting the ELD for one test to make sure the alternator charges when there’s no limit.

Honda Electrical System Diagnostics

[Robby Villabona]

Your voltage drop test results are great. I get 0.18V and 0.05V on my (+) and (-) sides respectively, but it IS a 20 year-old car.

Honestly I’m at a loss as to why two shops said your alternator was bad when by everything else your saying, it seems to be working fine — your battery is getting charged and isn’t getting overcharged. Your volt meter readings are within normal range.

I don’t know what bypassing/removing the ELD will show. Your alternator outputs correct charging voltage with load, so your alternator is good. If it was bad then it won’t be putting out the right charging voltage under any condition (or your accessories won’t be getting the power they need).

What I do find curious is how your interior lights ended up draining your battery on 3 separate occasions. AFAIK those lights automatically shut off after some time to prevent this sort of thing from happening. The only way those lights would’ve been on long enough to drain your battery is if the keys were also left in the ignition.

[Daniel]

If I recall correctly, even once my battery reached 12.6V with the engine off, starting it and even increasing RPMs didn’t boost the voltage past 12.4~12.5 yet again. Hopefully that small difference is negligible and it’s not really pulling .2V from the battery itself or something like that.

As for the interior lights, it’s been the push-button one on the ceiling above the center of the dash that was the culprit. I’m pretty sure those do not turn off on their own (at least in my model) because this last time I killed the battery I could see one of them was still very dimly lit.

Also, I called a couple of mechanics to get quotes on replacement (just in case) and after describing the voltages to them, they said that behavior sounded ok and basically described to me how the ELD behaves (without specifically mentioning it). I’m not implying your opinion wasn’t good enough, but to really get peace of mind I wanted to check with multiple sources.

For kicks, I think I’ll go back to the auto parts store and just have them run their darn test again to see if replacing the battery changes the alternator test results (its free for whatever its worth). I should’ve done that right when I bought the new battery but I was too upset about hearing the alternator was “bad” to think it through.

Even if their test fails again, maybe it’s because their testing tool doesn’t account for the fact that it’s normal for cars to idle low. If your opinion is still the same, then I think I’ll just continue to drive my car like normal (no more lights on during the day) and I’ll just pay attention if anymore bad signs come up.

[van to]

Hello Daniel,
I have the same issue with my Honda Civic EX 2012 and wonder if you found out what was the culprit? There is a service bulletin for such issue, PCM software must be updated to 3.10.40. I took my car to the dealer, they said I had the latest PCM software, battery and alternator are good and don’t know why my battery drained gradually if I’m driving with the headlights off.
Thanks.

[SonJa]

I see this is quite an old post (original), but thought I would chime in for you, “van to”. This experience I had was well over 20 years ago, but perhaps this type of situation can still happen on “modern cars”:
My car would sometimes start, sometimes not. Battery tested fine, alternator tested fine, replays replaced. Not starting was a minor issue compared to what ensued…I would be driving down the highway at 70, in the dark, and my lights would go out (no check engine light, no warning, nothing to precede the lack of lights. I would pull over and wait…and wait, and wait. Eventually it would start…except the last time. Now, being as it was in the days before cell phones, I FINALLY got a cop to pull over. He drove me to headquarters where I called my dad to come and get me (2 hours away at 11 at night) and wouldnt you know, car started right up. Made it ALMOST all the way home 5 miles away) with my dad following me and the headlights died again (car was running fine, just no lights), so he got in front of me, turned on his high beams and I followed him home…and the next morning, car started just fine…grrrrr. TWO Honda dealers could not figure it out so I finally decided to give my dads small time country hick mechanic a try. His diagnosis was an intermittent alternator-which I did not expect as I had just installed it six months prior-however, I was racking up about 2500 miles a week…so, I changed it out and never had the problem again…so, it could maybe be that? No mechanic here, just a chick that likes to work on cars in the famous “installation is the opposite of removal” world. I always figured that as long as I could get it off, I could get it put back on…
Good luck!

[Author]

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