A shorted diode/ diodes causes the draw, if on removal of the battery wire from the alternator the draw on the battery dissappears the alternator needs to be rebuilt or replaced.

 

Save your receipts for your lifetime warranty! Do not lose that receipt!! I am worried that "lifetime warranty" translated into real words, means "we(NAPA, Autozone, Pep Boys, O'reillys, etc. etc.) have pumped out an inexpensive rebuilt alternator that, in the long run, won't cost the company any money even if the customer has to break down and be stuck 7 times in 7 years and replace the alternator 6 more times."

That is exactly what happened to me after I bought a Pep Boys lifetime warranty alternator. The average PepBoys alternator lasted me one year, before a bearing failed, a brush wore out, or rectifier opened up.

I finally wised up, 22 months ago, and had a reputable electric shop install a rebuilt Ford alternator. They rebuilt it themselves, they use only heavy duty parts, and put it together properly. So far, this alternator is holding the record for operating according to spec, for the longest amount of time.

A quick note about rectifiers. A bad rectifier(short circuit) will allow a higher ripple voltage to come out of the alternator. This results in a nasty humming that can usually be heard on your AM radio as interference that increases its pitch(frequency) as the engine RPM increases and consequently the alternator stator spins faster. The purpose of the rectifier is to smooth the Alternating current generated by the internal windings and rotation of same inside the alternator and transforms (shapes) the current into "Direct current" There is no such thing as perfect direct current, all DC possesses some sort of variation whether it is due to poor filtering, improper rectification or external noise sources(such as high voltage ignition systems). On a properly operating system, we can't hear such noise on the radio, but sometimes when the rectifier fails the humming is clearly evident on both AM and FM channels as interference on every channel while the truck is running. If the noise goes away while the engine is off, and radio on, then the alternator is probably cooked.

 

The noise would be coming in on the DC power line to the radio, so if you have a tape or cd player, the noise will be evident when playing them too. This is a good way to differentiate between radio interference and DC supply ripple.