Alternator or something else?

[adxss]

The other day my battery light came on and it has stayed on.

I assumed it was the alternator going, so I swapped it out with the identical spare from my parts car. The battery light is still on. The battery is a 1 month old Bosch.

I ran these tests from Bentley:

Voltage across battery terminals:
-car off: 11.7
-car running: 11.5

Voltage from alternator red terminal to ground, car off: 11.7

Voltage from alternator black terminal to ground, car running: 1.6

Any ideas or suggestions? Electrical is not my strong suit.

Thanks

[bimmerrob10]

odd i did a meter test on mine today and my volts with the car off was 12 on the battery

car running was 11.60... on the battery so my alt is beat yours looks low all around

i was told we should be between 13 and 14 volts when running
thats meter on the battery getting between 13 and 14

[adxss]

Yes, you are correct.

I assume my battery is a bit low because it has been discharging.

It is supposed to read 14 across battery when running.

It is also supposed to read 12 on black (ground side) of alternator when on.

So is your alternator shot? I am wondering what to look at next. I am puzzled because I swapped out the alternator. I haven't touched anything in the electrical system, so I'm not sure where to begin looking for a faulty ground.

[bimmerrob10]

yeah my alt is shot from what my readings tell me . its odd on short runs or start ups its fine... i drive a long distance all my lights dim and i lose major power and car eventually dies from lack of charging

i would check the major grounds battery and motor grounds first
and if u want, take your alt u just put in out and get it bench tested i dont know off hand who does that but
maybe your parts car alt was on the way out also or maybe just from sitting... just a guess on the parts car alt

just for shits and giggles... anyone u know have a brand new or close to new battery u can change out just to see if u get diff readings?

[jmo69]

The battery in my car is at least 4 years old and the other day I checked the voltage not running and it was over 12v, like about 12.35, I'd say your alternator is shot.

[fun2drive]

Your alternator should start off at battery voltage because that is its source after all and ramp up slowly to 14.5VDC then taper off once your battery stablizes at 12.6 VDC. The taper should be around 13.2 VDC and the process is not immediate and in fact if your battery will not hold a charge the alternator will continue to put that voltage out.

Thus your alternator is in need of a voltage regulator most likely which contains the brushes for the alternator which I would bet are toast.

It is very easy to change the voltage regulator and new ones run someplace between 25 bucks to 60 bucks.

Hope that helps some...

[fun2drive]

Your alternator should start off at battery voltage because that is its source after all and ramp up slowly to 14.5VDC then taper off once your battery stablizes at 12.6 VDC. The taper should be around 13.2 VDC and the process is not immediate and in fact if your battery will not hold a charge the alternator will continue to put that voltage out.

Thus your alternator is in need of a voltage regulator most likely which contains the brushes for the alternator which I would bet are toast.

It is very easy to change the voltage regulator and new ones run someplace between 25 bucks to 60 bucks.

Hope that helps some...

[KevinMullins]

Voltage from alternator red terminal to ground, car off: 11.7


What is your alternator reading red terminal to ground with car running?

[adxss]

Voltage from alternator red to ground with car running is 1.6 volts.

black side is constant around 11.7

[gc325is]

You may have a shorted cell in the new battery. Do you have a battery charger? See if it can charge it to 13.2V. You could take the spare alternator to a shop who will test it, some places are free. Worn brushes are the most common problem & easy to check once alt is removed.

[bimmerrob10]

i seen a sign today at advanaced auto saying free bench test on alts and batteries