Before replacing your alternator or dynamo its worth checking that it is at fault (rather than the battery or a wiring problem) as obviously a replacement won't solve the lack of electrical power problem if the charging unit is actually in good order.
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.
To check voltage:Run the engine 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 intermittent faults.
To check ampsRun the engine at maximum safe speed up to 5500 rpm - most alternators develop full output at around 5500 rpm
The multimeter should read 5 -10% below the alternator's nominal output (this is because they are rated free so a 60A alternator should produce 53-55A)
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.
New alternator
New dynamo
