Hey guys,
I'm having a strange issue where the voltage on my 2006 S2000 sometimes drops down dramatically for a couple of seconds. So far it only acts up for a short period of time, (30 minutes or so I'd say), but it's been getting more frequent. A couple of times it has been bad enough to get the car to die, but most times it just restarts my electronics (radio & radar detector). I have started monitoring it through obd2 and an ios application. Voltage with the car on is consistently above 14v. 14.2-14.6, but the spikes bring it down all the way to like 9v, though usually just down to low 12's. The drops in voltage always happen when I'm off throttle (shifting gears, going to neutral), but I can also idle fine. The drops happen as the revs are falling.

Since the car can make 14v I assumed the alternator was fine, and that it was a bad battery. But I went to get it tested and the guy at Autozone said the tests passed fine. I also measured the voltage and it read 12.6 off, and ~14.2 on. But, I measured it again today and it was only 12.2 off and the same on. Is this amount of variation normal? Could the battery not be getting charged at the correct times? Next time the car acts up I'll turn the car off and measure the voltage then too. I wouldn't be surprised if it's in the low 11's. (Hopefully I can start it up again lol). After 30 seconds of Googling my best guess is that it's the voltage regulator since that determines when the battery should be charged, but I'm not positive. Can this part be replaced without replacing the entire alternator?

When the car starts acting up it seems like it's running off the alternator, so I'm guessing that if it could run off the alternator alone, the voltage rectifier is not the issue right?

Maybe the Autozone battery check wasn't super thorough either. He had me turn the car off and on while running 1 test, so I'd assume it tested load.

What do you guys think, and are there any tests I can do to narrow it down further? Thanks!

Also, probably unimportant, but there's 2 voltages in the app: control module voltage and OBD voltage. The control module voltage seems to match what my multimeter is reading, while the obd voltage is ~1 volt lower consistently. I've been using the value that matches up with my multimeter, but what's up with the other one?

 

I'm betting on a bad battery based on the "overnight" voltage. Should not drop that fast. Added something sucking the life out of the battery? Bad battery will often load test good but be dead 2 days later.



-- Chuck

 

Sorry I should have clarified. I measured the battery on 2 separate days right after driving. The first day it was 12.6 and 2 days after it was 12.2. I wouldn't be surprised if I measure it tomorrow and it's at 12.4. The starter has sounded like it's struggled to start a lot more on random days than others since I've bought the car (and well before I noticed the voltage drops). Also, the voltage drops usually happen in the middle of a drive, not right in the beginning (which is what I would expect to happen if there was a battery drain/battery that couldn't hold charge).

It seems like the battery isn't being charged when it should be, and the car begins relying on the alternator alone when the battery voltage gets low. And since the voltage drops go away on their own, it seems like the battery is eventually getting charged. Would a bad battery be able to act up for a little bit but then act completely normal?

 

The car always relies on the alternator. All the battery does is start the engine. Quibblers will post other things but once the engine is running you can remove the battery.

-- Chuck

 

Should the alternator be able to handle the changes in engine load perfectly, or does the battery help mitigate those? I don't know much, but it does seem like it would be a lot to ask the alternator to handle a constant 14.6 volts when engine load and rpm can vary drastically within a second.
The spike downs I see are less than 1/2 a second in duration, so if I had no battery installed should the alternator be able to handle them and I would see no difference?

 

Compare your battery voltage in the morning after driving the day before (should be 12.6v - 12.7v). If not 12.6 either the battery or alternator is bad. Put the battery on a good battery charger overnight and do the same measurements after the battery has rested 12 hours.. If the battery doesn't measure good after an overnight rest on either the changer or alternator it's probably bad. If it measures good on the charger the alternator is bad. Etc.

105amps at 13.5v is the number I've seen for the S2000 alternator. What amperage draws are you seeing? Even with every accessory and light On the amps will be below that level. 10,000 watt audio system though...

-- Chuck

 

Just to confirm, is the voltage regulator part of the alternator? On a website I see the "alternator assembly" and "alternator core", where assembly is significantly more money, but both actually seem to refer to the same part in the schematic. I will check amperage later on, but I doubt my stereo is pulling more than 50 watts. I have a radar detector wired into the car, but it turns off when the car is off and I can't see it drawing anything significant.

https://www.hondapartsnow.com/genuin...0-pzx-003.html
https://www.hondapartsnow.com/genuin...pzx-505rm.html

 

Voltage regulator for my 2006 is part No. 31150-PZX-003. It's mounts on the alternator but is a separate part. Google the part number for sources of yours.

There's a section in the Service Manual on troubleshooting the alternator and charging system.

-- Chuck

 

I've driven the car with battery completely removed. Its not good for the charging system so I don't recommend it, but point is it drove fine and battery wasn't required to buffer those times when alternator was not spinning high rpm.

I didn't have a meter on there to note voltage, but car drove perfectly and no odd idling or anything.

You mentioned all those tests passing, but bear in mind those are meaningless since you have an intermittent issue. You'd have to run those tests at precise moment whatever is causing issue happens, which is of course impossible.

This is why intermittent issues are so difficult to troubleshoot.

 

How old is your battery and what brand and type or model?