HOW TO TEST YOUR ALTERNATOR OR DYNAMO
The alternator or dynamo is at the heart of a vehicle’s charging system, providing power to the various electrical components while the engine is running and also making sure the battery is always charged.
The quickest and easiest way to check if your alternator is under/overcharging is to measure the voltage across the battery terminals. DO NOT DISCONNECT ANY OF THE BATTERY LEADS WITH THE ENGINE RUNNING AS DAMAGE TO ELECTRICAL COMPONENTS MAY RESULT! Using a digital multimeter and with the vehicle engine running at tickover with no electrical accessories switched on, check the voltage output across the battery terminals.
Anything between 13.5 volts to 14.5 volts tells you the alternator/dynamo is charging around the correct parameters. Anything lower than 13.5 volts shows it is not putting out enough power to charge the battery efficiently and electronic items on the vehicle may behave erratically.
If there are more than 14.5 volts produced from the alternator/dynamo or voltage regulator (separate on positive earth vehicles) then it is overcharging, sometimes producing higher voltage “spikes” which can produce too much voltage for sensitive electronic components, causing them to fail or create intermittent electrical faults. Also check on the condition and tension of the alternator/dynamo drive belt as if it is too loose or in poor condition the belt may slip causing the alternator to have a low output. Alternatively if the belt is too tight it will damage the alternator/dynamo bearings.
This is a basic charging test for alternator/dynamo output.