Dash charging light on

[shafferl]

It used to come on after driving for a while but would be off again if the the car sat and cooled for a while before starting it again. If you started it shortly after shutting it off, the light would come on immediately. Now it seems it is on all the time. I had my battery tested and the guy at the shop said the CCA was low and was the reason the charging light on the dash was on all the time. (after they tightened the alternator belt) He said the charging system checked out OK. However, the battery currently in it spins the engine very quickly and it starts fine. Also, the warning light and my headlights pulse when idling. Any ideas what to check for? Voltage regulator? Thanks.

[kodioneill]

Need year model info. But a good guess is the alternator is bad/ worn out bearing,brushes etc...

[shafferl]

It's a 1984 911.

[shafferl]

I believe I have the alternator rebuilt about 20K miles ago......

[wwest]

Rebuilt often leaves out replacement of the slip rings themselves....

[Joe Bob]

Rather than guess, put a volt meter on it.

[rick-l]

Originally Posted by shafferl

I had my battery tested and the guy at the shop said the CCA was low and was the reason the charging light on the dash was on all the time.

You might want to go somewhere else if you are not going to DIY

EDIT: my vote goes to the regulator

[NYNick]

Throw another battery in it and see what happens.
Have you checked the tightness of the belt? Should have very little leeway.

[rick-l]

Originally Posted by wwest

Rebuilt often leaves out replacement of the slip rings themselves....

When you get a rebuilt one remove the regulator and look at the grooves worn in the slip rings. if it is more than 1/2 the thickness of the ring I would send it back.

The other thing to do (if you are lucky like me) is remove the windings from the terminals and check all 17 diodes. I think the second set of 6 diodes (the second winding) in these alternators could all be bad and it would not light the warning light.

[wwest]

17 diodes for 3 phases...

6 high power/current diodes last I knew.

[rick-l]

Originally Posted by wwest

17 diodes for 3 phases...

6 high power/current diodes last I knew.

My 88 has 17. One heat sink module has eight, the other six. The schematic shows this alternator as having two three phase windings. I assume one is offset electrically 60 degrees from the other to reduce ripple. Two of the power diodes establish ground in the Y connected windings. Then there are three diodes in the diode triple to develop the field current.

[eolson]

Any update on this? I just had my alternator rebuilt and it now charges the battery fine but the alternator/battery light is on with no key in the ignition. This was not a problem before removing the alternator, just the charging issue. I'm wondering if I'm now in the bad diode club, perhaps one was damaged during the rebuild. Anyone have a link on how how to discover the bad diode?

[JoeFic]

I went through this for years with the local mechanic, every other or third year it was this or that. I installed a little digital volt meter inside the car and quickly figured out all I needed was a new voltage regulator. It's been fine for years since then.

[rick-l]

Originally Posted by eolson

Any update on this? I just had my alternator rebuilt and it now charges the battery fine but the alternator/battery light is on with no key in the ignition. This was not a problem before removing the alternator, just the charging issue. I'm wondering if I'm now in the bad diode club, perhaps one was damaged during the rebuild. Anyone have a link on how how to discover the bad diode?

Here is a cut and paste from a previous post. What kind of meter do you have? Pretty likely there is a shorted diode.

You can take the alternator out and unscrew the terminals and disconnect the windings windings to check the diodes with an ohm meter with a diode function. Like this:

Then remove the diode arrays. One array has 8 diodes in it and the other 6. The upper heat sink goes to ground and the lower one to B+. Check the diodes between the terminal (other side)and heat sinks. This is only for you own amusement and to verify your diagnosis because I doubt you can source those parts.

The reddish brown thing in the top picture is the diode triple which powers the regulator, turns off the alternator light and supplies current to the field windings.

[shafferl]

It's a 1984 911 Carrera.

[rick-l]

Is this a legitimate merchant and has anyone used them? Alternator & Starter Parts Wholesale - RECTIFIER-VA

Is it one of the parts?