The Most Common Alternator Problem
The most common alternator problem on a Model A Ford is not the alternator at all but the fan belt that drives the alternator. The Nu-Rex MaxR alternator pulley (included with any Nu-Rex alternator) and the replacement type Nu-Rex Hi-Speed pulleys have a special charcteristic that makes an improper fitting fan belt or a worn out fan belt very obvious and easy to detect. It is called a pully wear indicator or "wear strip".
A worn out or improper fitting fan belt can make your Model A Ford appear to have an alternator issue simply because the charging rate of the alternator becomes non uniform. It may charge for a while then slip and as the demand increases for energy to recharge the battery the load on the alternator increases which in turn makes the belt slippage increase causing the battery to discharge. This becomes noticable when the ammeter needle starts to show discharge or begins to fluctuate. Tightening the fan belt which commonly takes care of belt slippage will not improve the problem if the belt is worn out or has an improper fit to the pulley. The Nu-Rex Pulleys will tell you with a simple visual inspection if a slipping problem exists.
A properly fitting fan belt will ride on the cantered sides of the pulley and never touch the bottom flat surface of the pulley. In fact, a properly fitting fan belt will ride on the sides of the pulley and leave 1/4" to 3/8" gap below the belt and never touch the flat bottom of the pulley. The problem arrises when the belt does ride on the bottom surface as well and then begins to slip and tightening the belt will not fix it.
This slippage can be easily detected because when the belt contact with the bottom of the pulley happens and belt slippage begins , it wears off the black plating that all Nu-Rex alternator pulleys have leaving a silver or chrome looking streak on the flat bottom area of the pulley that is obvious to see. The bottom flat surface of the pulley should ALWAYS be black in color if the belt fit is proper. So if you have an unusual charging rate or a low battery condition with an alternator you may want to check the pulley first to make sure all is proper with the fan belt.
The solution to the problem is to replace the fan belt with a proper belt purchased from a Model A parts source or here at nurex.com and be very careful to check to make sure the new belt fits with the 1/4" to 3/8" gap below the belt. Also, if a new belt is needed you probably have a discharged battery as well. This would be a good time to clean the battery contacts and fully charge the battery with a battery charger before using the car.
After the repair it is may also be a good time to replace the black "wear strip" indicator on the pulley using a black magic marker so if the problem occurs again your indicator will make the problem obvious again.