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How To Test Your Alternator Or Dynamo
 
 
The quickest and simplest way to test the efficiency of your dynamo or alternator is with the use of a multimeter. The cheapest and most simple multimeter is adequate for this.
Run the engine running at 1000 rpm (just above tickover for those cars without a rev counter)
Make sure all electrical accessories are turned off (lights, heater etc)
Check the voltage output across the terminals
The multimeter should read between 13.5 and 15 volts
If the output is less than 13.5 volts then  the battery is being undercharged - not enough current is being produced to charge the battery efficiently
If the output is more than 15 volts then the dynamo or alternator is overcharging (and may also produce voltage "spikes") - this puts excess strain on electrical components leading to premature wear and possible intermittant faults.
Check the drivebelt tension (a rule of thumb is 1 inch of play in the longest section, but check your manual). A loose belt may cause slippage leading to a low output. Conversely an overtight belt strains the alternator or dynamo bearings leading to premature spindle failure.