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Alternator doesn't charge. Voltage regulator? (LIKELY SOLVED)

NjDL

NjDL

Member
Joined
Oct 12, 2023
Hello again. I've been trying to figure out why the alternator on my 1989 244dl won't charge. I've read through manuals and forum posts and can't find an answer.

With a full battery the car starts and runs with a battery charge indicator light on but my radio and lights eventually die. Afterwards the battery is drained and the car won't start again. I assume either the alternator is still powering the fuel pumps, spark and injectors or I just haven't ran it long enough for them to die yet.

Voltage reads 12v at the battery posts and the same at the big red alternator wire. This voltage decreases as the battery runs out. The alternator ground to engine block checks out and the exciter wire reads 1.6v before starting. The alt>starter>battery wires have continuity and no drop in voltage but I am wondering if they are problematic since I recently washed that area under the engine block.

After all that it could be the voltage regulator, slipping belt, a short, a bad battery, or just a bad alternator? What symptoms could rule these out?

I appreciate the insight! Thanks!
 
I'd guess that the brushes on the voltage regulator have worn down too far. If the brushes and alternator are working correctly, you should see ~13.5 volts across the battery posts at idle. If it's just 12v, and the alternator light is on, then the alternator isn't charging. After charging the battery, it will start and drive OK until the battery voltage drops too much.

On amazon, you can find cheap cigarette lighter USB adapters with a voltage display. They're great for monitoring voltage if you don't trust your alternator.
 
Can't recall whether Volvo does this, but on some cars, the brushes complete the exciter circuit for the dashboard charge light. Hence, when the brushes wear out, the light cannot turn on to tell you there's a problem. A rather ironic situation.

ALL of your symptoms are classic "car is running on the battery only." Need a little more diagnosis to determine why.
 
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