Battery light comes on (very faint) - Causes?

[Enricor99]

Hi,
Recently I noticed that my battery light comes on - very faint. I was thinking that it could be the alternator so I started the car and shortly after disconnected the ground from the battery - car kept running making me think that the alternator was still good i.e. it was providing power to the engine. Could there be other issues causing the battery light to come on??

P.S I recently changed the iginition coil

 

[BigJimZ16]

Diode Trio in the alternator is bad. Easy to change, parts are available at any autoparts store. Might as well throw in a new set of brushes while you're at it.

 

[LLC5]

Hi,
Recently I noticed that my battery light comes on - very faint. I was thinking that it could be the alternator so I started the car and shortly after disconnected the ground from the battery - car kept running making me think that the alternator was still good i.e. it was providing power to the engine. Could there be other issues causing the battery light to come on??

P.S I recently changed the iginition coil

Your original problem was just a diode trio, but disconnecting your negative cable with the engine running could have damaged the alternator. Hopefully it is still good and just a trio will be needed, and as stated previously brushes would be a good idea at the same time. Good luck with it.

 

[Peer81]

I'm wondering how this could have damaged the alternator?
I had the same problem when I had the volume with the amps up to high and full headlights. But that was because I asked more amps then the alternator could give.

Greetings Peter

 

[Tim81]

Peter,

It could also be caused by dirty Batt. connectors or bad ground from the Batt Neg. cable to frame...Tim

 

[Buccaneer]

Just my two cents here, but like mentioned above...
Some people will tell you that you can check your alternator by disconnecting it from the battery to see if the alternator can produce enough current to keep the engine running. BAD IDEA!! Disconnecting the battery will subject the voltage regulator (computer and audio equipment...) to significant voltage spikes which may cause an otherwise good alternator to fail. Even if there were no damaging spikes, this test really would not indicate whether or not the alternator was good because the engine will easily run with a weak or failing alternator.
Hope this helps a little.

 

[LLC5]

I'm wondering how this could have damaged the alternator?
I had the same problem when I had the volume with the amps up to high and full headlights. But that was because I asked more amps then the alternator could give.

Greetings Peter

Because the alternator has no voltage reference and will go max output, possibly damaging diodes and the voltage regulator, and any accessory that is on in the vehicle.

 

[Enricor99]

I checked the battery and alternator with a multimeter and everything seems find - battery ciomes in at 12.3 and when car is on 14.57. I am thinking now that the car idles too low and that is why the battery light comes on?

 

[Peer81]

The problem witht the alternator isn't the voltage output but the amps output. Your alternator should put out around 60 to 70 amps but if it only puts out 30 amps then you also have the same problem. But you can't check the amps yourself but an alternatorshop can

Greetings Peter

 

[BigJimZ16]

The diode trio is a small circuit that verifies the output of each of the three phases of the alternator after each phase is rectified. The anode of each of the three diodes is tied to the output buss of a rectifer leg. The cathodes are joined together and ultimately go directly to that red light we're talking about. The three signals are summed together this way.
What has happened is that one of your diodes has probaby openned and not passing the signal from one phase along to say "hey, I'm OK" and you see the red light.
What you are not seeing is that the charging of the alternator is OK, just that red light is on erroniously.
Take the alternator off the car, take the back off, and change the diode trio. Changing the brushes is a smart thing to do at this time. You'll probably be out $30.00 and an hour of your time. Or you can change out the entire alternator in 30 mminutes for an extra $100.00.


Just say'in...