1G Amp warning light electrical problem

[SrKegler]

Not sure if I really have a problem. Was driving home last night and headlights got dim, car started sputtering, warning light came on for the alternator. Luckily I was at the house and was able to get it into the driveway. Car is running now after I replaced the alternator.

Question I have is the warning light didn't come on until the battery was almost completely discharged. Shouldn't the light come on whenever the alternator isn't putting out power? If so, how do I troubleshoot it.

 

[talontd92tsi]

That all depends on the nature that the alternator fails in. Generally, the light won't come on if the alternator is not "exciting", but sometimes the alternator is putting out the requisite voltage (i.e. alternator is "excited"), but the alternator cannot provide enough amperage to sustain proper voltage to keep the car running. The battery will drain slower, but will still drain, and it may not trigger the voltage light until very low voltages.

 

[rough]

Did you see any other signs that the alternator was going out? Like gauges fluctuating, stereo going in and out?
I don't think these systems will throw the light unless it sees a drop in voltage below it's threshold, and I think it measures that from the battery/ecu. If you had some way to monitor the alternator output , and wire that up to the dash light, and it works, please let us know

 

[SrKegler]

I thought that was what the idiot light was supposed to do, monitor output from the alternator. Had no warning at all. Was about 1/4 mile from the house when the lights went dim. That's also when the idiot light came on. I may have to wire in a volt gauge.

 

[talontd92tsi]

-Speaking from 1g experience here, 2g's are a little different as the ECU actually sees something from the alternator-

These things aren't really that "smart". They have a threshold of what is considered "working" and "not working" that's about it.

Also, an alternator can still output 14 volts and be garbage. Amperage is what really matters. There is a huge difference between 14 volts at 1 amp (sometimes they will not register any measurable amperage) and 14 volts at 65 amps (stock alt size IIRC). This is the difference.

 

[rough]

I thought that was what the idiot light was supposed to do, monitor output from the alternator. Had no warning at all. Was about 1/4 mile from the house when the lights went dim. That's also when the idiot light came on. I may have to wire in a volt gauge.

I thought so to, but it appears we thought wrong! Our cars require the overall voltage to drop, if the battery still has a charge above the threshold, that light won't come on