Alternator Output

[palfer]

Wondering if my alternator isn't up to snuff in my 97 Impreza.  I've read, with the car running an no accessories on I should get about 14.4 volts when testing at the battery.  I get 13.63 volts, about 13.8 when I test at the B+ terminal. 

 

Is that differential significant enough to replace the alternator? 

[Fairtax4me]

That's fine if the engine is at operating temp.

14.0-14.5 is typical when the engine is just started because the alternator is producing more current to try to re-charge the battery.

When the engine warms two things happen.

 

One, the battery has been charging for 15-20 minutes already and the energy lost during starting has mostly been replenished, so the alternator doesn't have to charge at a high voltage. So you'll see the voltage drop from 14+ to around 13.5. Typically if you see charging voltage around 14.5 and the engine is warm, the battery is going bad and is overworking the alternator.

 

 

Two, as the alternator heats up, the charging voltage lowers due to increasing resistance in the windings of the alternator.

[palfer]

Thanks..  I changed the brushes just because it's 20 years old and didn't want my son getting stuck while at school.  4 miles away from the house it broke down.  I took it apart again and re-soldered new brushes and have it running in the garage. 

 

All seems fine...  If I was smart I would've checked the voltage output before I started messing with it.

[Fairtax4me]

Were the old brushes worn?

Did you lathe the comm?

[palfer]

They were but probably had about 30% length still left. 

 

Lightly sanded the comm to get the surface build up off. 

[golucky66]

While checking the voltage that it can put out can be useful. That is not the main way to check an alternator. The main way is to see how much current it can put out. That is done by turning everything on and using an amp clamp around either the positive or negative wire and measuring it that way (also bump the rpms to like 1500). My impreza 96' charges at 14.2 V and puts out 75 amps at max.