98 international 9800 troubleshooting

Yes, an alternator supplies 3 phases of AC then rectifies and filters out the negative alternations via capacitor and diode bridge. Giving you lumpy DC.

I modified one to charge a battery, stick weld and run brushed AC power tools.

 

Make sure the solenoid is bolted down with a good solid ground to the plate.
You can also load test the trigger wire and the ground on the plate.
It is possible for a wire to carry enough voltage to light a test light but not carry enough amperage to activate a solenoid.

 

Did you check voltage to the center wire with it connected and the engine running? May have to run RPM up to 1500 or so for it to inguage.