Doug Lassiter

My wife's Civic wouldn't start. Bad battery. Battery was eight years old. Installed a new battery, and it all starts fine. But I say, hey, I'd better check the alternator to make sure it's charging. So I put my DVM across the battery, when the car is running, and it reads 12.5V. Hmmm. I turn off the car, and it still reads 12.5V. That says to me that the alternator has gone south. I'm looking for 14V when the car is on. So we take it to the shop, and they scan the electrical system and ... alternator is fine. The whole electrical system is fine. Whaaa? So my test was evidently false.

I've always tested the alternator by looking for a higher voltage across the battery when the car is on, than when it's off. If that's the case, the alternator is good. Is that not right??

BrotatoChip

You should see ~14V with engine running and ~12V with engine off, you are correct. There is a fuse between the alternator and battery, located in the under-hood fuse box.

ezone

Better read up on Hondas ELD controlled charging system.
The alternator is not always charging at your expected voltage.
The PCM can command the alternator to run at at high voltage or low voltage as it sees fit.

Doug Lassiter

So the Honda ELD system makes this standard alternator test not work? Interesting. How do you easily check the alternator, then?

Now, it occurs to me that this may have to do with the fact that I was doing the test with a brand new, fully charged battery. The RIGHT way to do it is to have a dead battery, get a jump start, and do the alternator test on a dead battery that is charging. That is, the charging system may *know* the charge status of the battery, and if the battery is full, it just doesn't put out any more voltage.

In this context, I asked the shop - so how did you test the alternator to find out that it works?? Their answer was that they have a complicated unit that they attach to the car's electrical system, and it does the whole shebang - load test, charging status, etc. etc. So it knows all.

I'm wondering what the lesson here is. Don't do a simple DVM alternator test on a fully charged battery?

D17VTECPOWER

iTrader: (1)

What ezone said. What year is your Civic? I presume you checked the voltage while it is running in park? Specifics vary by vehicle, but here is a general list of perimeters that need to be met for the PCM to switch the alternator to low charge mode:

Electrical load below a certain amperage
Alternator duty cycle below a certain percent
Coolant temp above 167 degrees F
Intake air temp above 67 degrees F
Engine speed below 3000 RPM
Vehicle speed below 45 MPH or idling in drive
Deceleration fuel cutout not active
AC not on