Alternator Not Charging

[GGX2]

Greeting folks, looking for some ideas on why my alternator won't charge.
I know that if the battery light is burned out that it won't charge, but my battery light acts pretty weird... it doesn't come on when the key is turned on, but it does come on once the engine is running. The battery light should be on with the ignition on, correct? So what could cause it to not come on then, but work once the car is running? Any help is appreciated.

-George

[GGX2]

It appears my alarm bypass was to blame for the light not being on with the ignition turned on, I popped my alarm brain back in and the light came on... it's still not charging though, what would be the most likely cause? Voltage regulator?

-George

[flash968]

frequently

how do you know it isn't charging?

[GGX2]

The voltage never goes up when it's running, it just slowly drains down. It'll be 12.5v or so when off, then 11.9v and dropping once it's running. My 944 doesn't have a volt gauge, so this is tested at the battery with a Fluke multimeter.

-George

[flash968]

if it's measured at the battery, that is no test of the alternator, and you could just have a dead cell in the battery, or be low on water (only use deionized or distilled water - never tap)

you need to measure it at the alternator to tell if it's outputting correct voltage. even then, it may still be bad if it isn't putting out the right amperage as well. i've seen alternators with bad bearings put out full voltage, but only 5 amps.

[mytrplseven]

I'm dealing with a similar problem. My alternator won't charge unless I rev the motor up to above around 2k then it works fine. My alternator light never comes on and I'm just now wondering if that's part of it. I had the instrument cluster out to fix the plastic light issue and replace the odometer gear but didn't realize that the alternator light might be part of the issue. I guess I'll look into it soon when I replace the entire dash.

[VINMAN]

George, I had the same issue. Just had to replace the brush/regulator unit on the alt. The brushes do wear out in these.

[GGX2]

Yeah, I'll probably replace the regulator, they are pretty cheap to replace anyway.

-George

[GGX2]

I figured I should update this with my recent findings. I ordered a new regulator, but that did little to fix my problem, I eventually narrowed it down to the alarm connections even though they were bypassed.
Long story short, after many hours of testing I ran a whole new wire from pin 11 on the back of the gauge all the way up to the alternator, it now charges as it should.

-George

[AKCJ]

Originally Posted by flash968

if it's measured at the battery, that is no test of the alternator, and you could just have a dead cell in the battery, or be low on water (only use deionized or distilled water - never tap)

you need to measure it at the alternator to tell if it's outputting correct voltage. even then, it may still be bad if it isn't putting out the right amperage as well. i've seen alternators with bad bearings put out full voltage, but only 5 amps.

Flash - I thought that if you measured the battery voltage with engine off, then measured it with engine running, you could tell it the alternator was putting out a voltage higher than the battery. Isn't that how the battery gets charged? I don't get why you have to measure alternator test voltage at the alternator and not the battery.

[HondaDustR]

Originally Posted by AKCJ

Flash - I thought that if you measured the battery voltage with engine off, then measured it with engine running, you could tell it the alternator was putting out a voltage higher than the battery. Isn't that how the battery gets charged? I don't get why you have to measure alternator test voltage at the alternator and not the battery.

The only reason you would do that is to determine if the battery cables are no good. You can also measure the voltage drop of the cables directly by just putting one probe on the battery post and the other on the respective target (ground for negative cable, alternator output for positive). Zero volts is unrealistically ideal, more than a couple tenths of a volt is bad.

Just for fun, I had the field winding fail in mine once, and coincidentally, a trace inside the instrument gauge to the charge lamp fail at the same time. Had to solder a jumper to repair, in addition to getting a new alt. What are the chances? Easy quick way to check the charge lamp circuit is to ground the ring terminal...the light should light brightly.