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Death of an alternator

My 82 Subaru GL 1800 killed 2 Alternators.

Could a 4 year old battery that starts every morning with 12.5 volts be guilty of murder when the cooling fans start up.

Charging voltage starts out at 13.75. After about an hour of driving it drops slowly then gives up the ghost, or could it be something else?

Some more info might help.
Are the alternators known for a fact to be dead?
Does the red battery light on the dashboard work?
Condition of alt. belt?

Sorry I can’t be more specific there are a number of things that can cause alternators to act up on this cars.
Dragging water pump leading to belt slippage is one.
Erratic bulb light operation on the dashboard battery warning lamp.
Corroded connection on the fusible link. This is located near the battery positive post and should be a short length of heavy wire. The link may not be burnt but corroded connections on the end of the link are common.

Thank you for your suggestions. Both alternators tested good at parts store, Dash has a guage (10-12-14).I didn’t notice a light. Belt cond. good. I will inspect water pump for dragging. Before second alternator install, I checked link at battery post clamp. It broke off. I cleaned wire and installed new clamp. I will check plug end of link. You’ve been very helpful. thanks again. Tim

If I remember correctly, this model should have a small red battery warning light on the dashboard also. (Top center of the instrument cluster with the brake light indicator, etc.)

If the bulb is burned out the alternator will not charge at all. Battery current passes through this bulb when the key is in the RUN position and this excites the fields in the alternator.
A rough rule of thumb is to turn the key on and touch the tip of a screwdriver to the alternator pulley. If you feel a magnetic attraction by the pulley then the alternator should be good. If you feel no magnetism this usually points to a dead alternator or a burned out bulb. Hope that helps.

The alternators tested good at the parts store before or after the problem? If the voltage regulator is external to the alternator, has it been checked/replaced? You say the gauge gradually drops… when it “gives up the ghost”, how low is it? Is the car running at this point? The gauge should be over 12 volts at all times while you’re driving.

I’m not able to verify charge light on dash cluster. I’ll check later this week. No magnitism on alternator. It is charging right now with 13.8 volts. I disconnected neg clamp, it stayed running. Chargeing voltage increased to 16.3. It still charges. Fuse link good. Maybe I’ll try a new battery after I check the dash light. Your help is much appreciated.Tim

The alts tested good before install. Alt has internal regulator. THEN it gives up the ghost means it’s no longer charging and running on battery power only. It starts out with 13.8 volts charging then gradually drops down to 12.03 running cooling fans and using brake lights. Next payday after checking dash cluster light I’ll try a new battery. I’ll post results later. Your response is much appreciated.