Lots of alternator/charging system discussion lately. Every situaion is unique. Remember alternator speed is directly related to its amperage output.Advertised amperage rating is almost always max output under idea temperature conditions. Most auto parts stores with test benchs usually have single speed units that cant test thru-out the full operating range,merely identifies its ability to determine if it is able to produce some kind of charging rate. And the bench is only as good as it's operator.Alternators that are producing low amperage, during testing, could be driven too slowly,have a bad diode, or mislabled/rated. With a bad doide,you would see an amp ripple when monitoring with an induction type probing clamp.With a failed diode,AC current would be migrating back into your 12v electrical system. Thats a BIG problem for any ECUof any component-think high dollar ignition units and others.What I'm saying is to use the services of a reputable alternator shop that has the ability to check charging rates at multiple speeds. Very important for accuracy. A typical 90amp full funtioning alternator will only put out 55amps at low speed.It would be worth the time to do a second check, it could have been misdiagnosed. Bill