Charge indicator light on.

[Kevin1]

The charge indicator/battery light is coming on when the car is running. Voltage at the alternator with the car off read 12.5 (battery voltage). With the engine running it read 14.8, which seems like it is charging but the indicator light stays on.

I am scratching my head over this. Any ideas?

[AriK]

Verify how much the 14.8 V drops when all the accessories are on.

[spider2081]

Verify how much the 14.8 V drops when all the accessories are on.

+1

Measure the AC voltage on the alternator output post if you have a quality meter. Another person posting here found the Harbor Freight digital meter does not measure the AC correctly. I confirmed it with one of their meters. The AC voltage should measure less than 1 volt AC preferably less than 0.5 Volts AC. If a Diode fails in the alternator then AC voltage will be present and alternator needs repair.
Check the engine ground and that the alternator is mounting hard ware is all tight. The ground is necessary for the alternator and voltage regulator to operate correctly.
Check the belt condition and tightness
If those are good I would substitute the voltage regulator for a known good one. If you have a Bosch alternator this can be done with the alternator on the car and the battery disconnected.

[Kevin1]

14.8 volts remained 14.8 with headlights, heater fan, wipers, and seat heaters on.
AC voltage measures 70.5 mv.
All measured at idle speed, belt is in good condition and tensioned. Fluke meter.

I think I will cut to the chase and swap in a known good stock Bosch unit. There is a 90 amp alternator in there now that was put in when the seat heaters were added.

[spider2081]

I am not familiar with the correct way to connect 90 amp alternator into a Spider.
The original Bosch alternator had a light blue wire with a black trace that connected the Bosch alternator field to the Alt out light.
The light gets battery voltage applied to one side when the ignition switch is in the "start or run" positions. the other side of the light connects to the alternator's rotor (field). When the alternator is not working the light sees a ground through the rotors windings. When the alternator is working the rotor produces enough voltage to put the warning light out. When the alternator is working the alternator out light has a voltage source on both sides of the bulb so it does not light.
You could try and disconnect the light blue/black wire from the 90 amp alternator and see that the light goes out. If it does then start the car and see if the 90 amp alternator is working with the same tests you performed previously.

[Kevin1]

Double checking myself , I realized the tests should have had the battery disconnected. Without the battery the AC voltage measures 1.1, DC voltage is 14.9.

If I am reading this right 1.1 volts means a bad diode. Is that correct?

[spider2081]

The AC measurement is made with the battery connected. I have never made the measurement with the battery disconnected but I would think 1.1 volts would be acceptable. The battery is actually the primary filter forth AC ripple that is left after the diodes convert the AC to DC.
THe greater the load on the alternator the higher the AC component of the output will be, So I like to measure the AC at the alternators output post at about 3ooo RPM with everything on in the car. The cars battery needs to be fully charged to be a good filter. An example of this is: If when you start your car you hear alternator wine in the radio audio but it goes away after driving for a while. When you first start the car the battery is less than fully charges and so the wine is ripple on the DC voltage. As the battery charges it filters the ripple to lower levels eventually eliminating it.
I would disconnect the lt blue/black wire at the alternator. The light should go out. If it does not then t he wire is shorted to ground some place. If it does start the car and see if the alternator is still putting out 14 volts.

[Kevin1]

To wrap this up, a new alternator was what it needed. Thanks for the help figuring it out.

[spider2081]

Great