Posted by Ben on March 08, 03 at 08:00:00:
In Reply to: Re: Alternator Diodes posted by Jeff in CO on March 08, 03 at 05:47:48:
Yes, very common for folks to jump start another vehicle this way. Did it regularly till noticed alternators would to south shortly thereafter. Also noticed that tow truck's didn't use their vehcile battery, but stand alone battery to jump start. Still in college, but worked in small motor controls company and did lots of work on power stuff. Made the connection one day after blowing up a 1,000 amp hockey puck SCR (thyrister), which had 1/2 cycle current capacity of "several" thousand amps.
Starter motor's inrush is in the hundreds of amps draw. Why better batteries are rated near 1,000 amps cold cranking.
That inrush will want it's current (amps) from highest voltage source, the alternator of the "good" battery vehicle. Alternator diodes are rated in the neighborhood of 30-40-50-60 amps each and have current spike ratings just above 100 amps, maybe couple hundred if really good diodes.
Such high current will destroy any diode. Some die on the spot, but most will die later all the while going short, open, short, open. Flaky and will show up in weird electricals.
Procedure is to "charge" the dead battery with the jump vehicle...before trying to start the dead batteried vehicle.
Documented at #1 Son's Suburban/GM Truck info/help site, hot link below. It's in one of the topics sectons, think battery or electrical.
Also common GM side post battery positive connection going bad will also cause the alternator to go bad. Same thing happening but at the batt terminal. Load dumping is the name/cause. Also documented at #1 Son's place in below hot link.