Here on TMF we see lots of Charging System / Flat Battery posts so I have put together a Basic Test On the Alternator & Its Wiring.
You will need a Multimeter to carry out the tests.
Basic Alternator Tests on the Lucas 16/17 ACR Alternator used as a Example, The later Minis have a different alternator model and method of connecting the loom connections to the alternator, but the method of testing remains the same.
1. Check the fan belt is not worn, the thinner it gets, the lower it will run in the pulley and the less grip there will be to drive the alternator.
2. Ensure that the fan belt is correctly tensioned. ( 1/2-inch/12mm max movement of the belt along the longest side)
3. Ensure that a good, healthy, fully-charged battery is fitted. Failure to do this and you will not get accurate results, and make sure you have a good earth connection on the vehicle in the various earth points.
4. Remove the plug from the back of the alternator using your Multimeter, set on DC volts, connect the black probe to a good earth and test each terminal in the plug in turn with your test probe.
5. You should see battery volts on each of the larger connections of 12v+. You may only have
one large terminal in the alternator plug in this case you should only have power on the one large terminal.
6. You should only have voltage on the smaller terminal when the ignition is switched on and if you don’t then check by replacing the plug in the alternator and with the ignition in the ON position the Ignition light should be on and glowing Red. If you do not get the ignition light on then remove the plug from the alternator and connect a male lucar into the SMALL 6.3MM Lucar Only in the alternator plug and touch the end of this wire to earth on the engine, The Ignition Light Should Now Come On.
If the ignition light does not come on then you will very likely have a wiring fault or the ignition light bulb blown. Investigate and repair before continuing on with the tests.
WARNING: Do not connect the large terminal/s to earth in the alternator plug as each is connected directly to the battery and a short circuit could cause a wiring loom fire in the engine compartment.
7. Once you have carried out the above basic checks and confirmed there is 12v+ on the large terminal/s in the alternator plug, and with the ignition switched to the ON position and engine running you should see 12v+ on the small lucar in the alternator plug, you have now confirmed the alternator wiring is correct.
8. Now we will check the charge rate at the battery, remember we have confirmed the alternators output wire is connected to the battery.
9. Place a Multimeter set to 20v DC scale and put the Black probe on the – terminal of the battery and the Red probe on the + terminal of the battery, now start the car and with all the accessories turned off rev the engine to 2500rpm, your looking for a battery voltage of between 13.8 to 14.3v
10. If you have the required results of 13.8 to 14.2v then you have confirmed the alternator is charging.
Readings Indicating A Fault
1.Battery voltage stays at 12.5v approx. (No Charge)
2.Ignition Light Just glowing but volts rises to above 12.5v but not to 13.8 to 14,3v (Diode Pack Fault)
3.Voltage rises beyond 15 v (Voltage Regulator Fault)
These results will require either a strip and investigation of the parts required or a replacement alternator.
This is purely written as a Basic Guide to help someone diagnose where the fault may lie, and there are other items within the alternator which could cause different reading, but assuming the skills of the average DIY Mechanic those repairs/diagnosis go beyond what this guide was written for.
Edited by KernowCooper, 10 June 2013 - 11:57 PM.