[phattyduck]

Hondas do have dual stage charging so the drop you're seeing could be low charge mode?

Low charge mode should never happen if the headlights are on or AC is on.

-Charlie

 

[oldskewel]

your post prior to this one suggests your battery is OK.

Here is the update: my multimeter does not have 10 AMP for parasitic draw test,

Even if you had one with 10A, there would be a good chance it would blow the fuse during the current surge when the car powers up after reconnecting the battery circuit. Measuring current on cars can be difficult. Many cool ways to do it though. My favorite is with a DC clamp meter. You clamp it around one of the battery cables and it measures current based on the induced magnetic field. There is no way to blow any fuse in that. For chasing a parasitic draw, best would be a pretty small unit, maxing out at around 100A, with a small clamp.

so I found on website a protocol to test alternator (below):
How to test the alternator
The easiest method to check the alternator is by measuring the voltage at the battery terminals when the engine is running.

1. Start your Odyssey.
2. Connect the black cable of the multimeter to negative terminal of the battery, and the red cable to the positive terminal.
3. The voltage should be between 13.5 and 14.8 volts with the engine running. (My battery is 14.3 v)
4. Switch on the accessories in your Odyssey, for example, headlights, AC fan at full speed, and interior lights.
5. The voltage should remain around 14 volts. If it drops and remain below 13.5 volts, it indicates a problem with the current supply from the alternator. (My battery started with 14.03 v and slowly dropped to 12.08 v and seems to be stable there. When I turned off all the accessories, it came back to 14 v.)
6. If the voltage drops below 12 volts with the engine running and the accessories turned on, it means the current supply from the alternator is completely cut off and all the electronics are solely powered by the battery.

The conclusion based on this test: the alternator is not completely bad, but intermittent bad. Is that right? Is the method reliable?

This definitely sounds like something is wrong with the alternator. If it is repeatable (i.e., you can repeat that careful test with identical results), I would not call it intermittent, but that is just terminology. The difference there is that if something is intermittent, I think of the failure as being GONE, not just undetected; in your case (if it is as you think), the fault (that voltage drops when loaded, after a while) is always present, just only detected under certain conditions. But it does sound like something an in-store bench test might miss.

Honda's ELD - Electric Load Detector - system is supposed to reduce alternator output in certain situations, when power is not needed and the purpose is to be more fuel efficient. But that would not make sense in this case unless the logic (it is PCM / ECU controlled, like most things) is broken, since when power demand increases, the alternator should not be throttling back.

On most cars there is a way to bypass the ECU's charging control - making the alternator charge at all times. Like back in the good old days, when if the engine was turning, the voltage regulator would be controlling the field current to hold system voltage steady at 14.4V. So reliable, and maybe slightly inefficient once in a long while.

If you're looking for a quick solution, I'd just replace or repair the alternator. If you really want to try harder on the diagnosis, I'd see if you can figure out how to hot-wire the charging control, and repeat your test to see if it holds or drops voltage. It could be that you just need to set a wire to ground or B+, or it could be that you need to set a pulse width modulated 5V signal.

And if you do take the alternator out, I'd just then start carefully taking it apart and observing. Oil? Debris? Loose screws? Loose 4P connector or B+ wire? Worn out brushes? Loose screws after the cover is off? Then there are a series of electrical tests you can do on it. I've posted a few times on this forum on alternator repairs, so if you search with my name, you should find some info.

 

[Sun_odyssey]

Load on the test might not be enough to stress the alternator enough. I would guess a bad diode, not regulator (the regulator seems to be working when there isn't much load). That could also explain the parasitic draw.

-Charlie

your post prior to this one suggests your battery is OK.


Even if you had one with 10A, there would be a good chance it would blow the fuse during the current surge when the car powers up after reconnecting the battery circuit. Measuring current on cars can be difficult. Many cool ways to do it though. My favorite is with a DC clamp meter. You clamp it around one of the battery cables and it measures current based on the induced magnetic field. There is no way to blow any fuse in that. For chasing a parasitic draw, best would be a pretty small unit, maxing out at around 100A, with a small clamp.


This definitely sounds like something is wrong with the alternator. If it is repeatable (i.e., you can repeat that careful test with identical results), I would not call it intermittent, but that is just terminology. The difference there is that if something is intermittent, I think of the failure as being GONE, not just undetected; in your case (if it is as you think), the fault (that voltage drops when loaded, after a while) is always present, just only detected under certain conditions. But it does sound like something an in-store bench test might miss.

Honda's ELD - Electric Load Detector - system is supposed to reduce alternator output in certain situations, when power is not needed and the purpose is to be more fuel efficient. But that would not make sense in this case unless the logic (it is PCM / ECU controlled, like most things) is broken, since when power demand increases, the alternator should not be throttling back.

On most cars there is a way to bypass the ECU's charging control - making the alternator charge at all times. Like back in the good old days, when if the engine was turning, the voltage regulator would be controlling the field current to hold system voltage steady at 14.4V. So reliable, and maybe slightly inefficient once in a long while.

If you're looking for a quick solution, I'd just replace or repair the alternator. If you really want to try harder on the diagnosis, I'd see if you can figure out how to hot-wire the charging control, and repeat your test to see if it holds or drops voltage. It could be that you just need to set a wire to ground or B+, or it could be that you need to set a pulse width modulated 5V signal.

And if you do take the alternator out, I'd just then start carefully taking it apart and observing. Oil? Debris? Loose screws? Loose 4P connector or B+ wire? Worn out brushes? Loose screws after the cover is off? Then there are a series of electrical tests you can do on it. I've posted a few times on this forum on alternator repairs, so if you search with my name, you should find some info.

Thank you for very thoughtful input! This morning, I brought a new cheap battery from Walmart and installed it, it did not clear out the "red battery light" and "check charging system", so yes, it is not the battery problem. Then I run on the highway with the old battery for 50 miles or so. Before running on high way, the old battery has 12.3 v on it, and after run, it has 12.6 v. So the alternator is still charging the battery from what I can tell. But the "check charging system and battery light" was on all the way while I drove. The only test to show the alternator is defected was this one:

5. The voltage should remain around 14 volts. If it drops and remain below 13.5 volts, it indicates a problem with the current supply from the alternator. (My battery started with 14.03 v and slowly dropped to 12.08 v and seems to be stable there. When I turned off all the accessories, it came back to 14 v.)

I redid the test again this morning. It is repeatable. So the alternator is somewhat functioning, but is not as good as it should be at full load? I do not know how long the alternator will completely fail. I may drive it in local with a new battery in the back of the van just in case. To take out the alternator and take it apart is probably too much a challenge for me. I would leave it to pro mechanic tot he point that I have to fix it. Any suggestion? It has been very helpful to get so many comments here and narrow down where the problem could be. Thanks again!

 

[oldskewel]

Yes, you are right that having the voltage increase from 12.3 to 12.6 at the end means the alternator was doing some charging. Risky to drive it that far with the charging light on - if it had truly not been charging at all, a battery would probably have run down low enough to stop the engine.

When I had the problems discussed in the link back in post #9 above, I quickly installed a digital voltage gauge so I could confirm to myself (and my wife when she was driving) whether things were charging, even when the light was on.

Before installing that, I used a little eBay gauge plugged into the power outlet. If you have neither of those, but do have an OBDII reader, you may be able to read system voltage on that, as a live reassuring measurement that things are OK and not about to die (as you drop to about 10V, probably).

I doubt you would find a pro who would disassemble and troubleshoot the alternator after removing it. You might find an alternator rebuild shop to do that, and maybe the pro would send it there.

But at that point, once you have pro's involved, they'd probably just want to do a replacement, which I think would be a fine decision for you.

I can't speak from experience since I have never bought a Honda replacement alternator, but there is are a lot of stories on here about people having their replacement alternator fail before too long, when it was not a new Denso (the OE supplier on your car) or a Denso-rebuilt Denso. So I would go for a Denso.

To be clear, a Neverlast branded rebuild of a Denso alternator would not be likely to last long, yet the guy at the parts counter or service desk might assure you it is a Denso.

Your logic and care in measurements and observations gives me confidence that you would be able to figure out what is going on with the old alternator once you got it out. Some people seem to think electrical stuff is black magic, but I'd say that opening up the alternator is a small fraction of the difficulty of removing it from the car, and even that is pretty easy for some people (but apparently difficult for many).

 

[Sun_odyssey]

Yes, you are right that having the voltage increase from 12.3 to 12.6 at the end means the alternator was doing some charging. Risky to drive it that far with the charging light on - if it had truly not been charging at all, a battery would probably have run down low enough to stop the engine.

When I had the problems discussed in the link back in post #9 above, I quickly installed a digital voltage gauge so I could confirm to myself (and my wife when she was driving) whether things were charging, even when the light was on.

Before installing that, I used a little eBay gauge plugged into the power outlet. If you have neither of those, but do have an OBDII reader, you may be able to read system voltage on that, as a live reassuring measurement that things are OK and not about to die (as you drop to about 10V, probably).

I doubt you would find a pro who would disassemble and troubleshoot the alternator after removing it. You might find an alternator rebuild shop to do that, and maybe the pro would send it there.

But at that point, once you have pro's involved, they'd probably just want to do a replacement, which I think would be a fine decision for you.

I can't speak from experience since I have never bought a Honda replacement alternator, but there is are a lot of stories on here about people having their replacement alternator fail before too long, when it was not a new Denso (the OE supplier on your car) or a Denso-rebuilt Denso. So I would go for a Denso.

To be clear, a Neverlast branded rebuild of a Denso alternator would not be likely to last long, yet the guy at the parts counter or service desk might assure you it is a Denso.

Your logic and care in measurements and observations gives me confidence that you would be able to figure out what is going on with the old alternator once you got it out. Some people seem to think electrical stuff is black magic, but I'd say that opening up the alternator is a small fraction of the difficulty of removing it from the car, and even that is pretty easy for some people (but apparently difficult for many).

I just ordered a digital Voltmeter. It is a good idea to monitor the real-time voltage while driving. In the mean time, I will try to find out what is the problem. I have read the link you provided which is very informative. I will see what i can do to find out and post the result here to share. Thanks a lot for all the suggestions.

 

[phattyduck]

So the alternator is somewhat functioning, but is not as good as it should be at full load?

Is the alternator original or a replacement that happened at some point? If a non-Denso rebuild was used (as explained by oldskewel above), it could just be a poor quality part that can't supply enough power. It could also be a bad diode in the OEM alternator that could cause decreased max output (and the diode can fail "partially" shorted, which would also drain the battery when off).

So, a couple possible explanations for your current situation.

-Charlie