i thought this might help after reading all the other posts about alternator faults
(this is not vauxhall specific)
this is how i test every alternator job i do, i have a tool called an amp clamp which is brilliant and a must for any mechanic
available on ebay for £26.30, item number 370482639139
1) check battery voltage, 12.2v is 50% charged and the minimum voltage to be running tests
2) start vehicle, battery voltage should not drop below 9 volts and using the amp clamp check the amp draw when cranking, a basic rule is a petrol starter draws 150 amps and a diesel is 300 amps
3) with engine running check battery voltage 13.5v - 14.5v is good but be warned some new vehicles will charge up to 15 volts and check amps
4) load the alternator by switching on heated rear screen, blowers to max speed and headlights
5) check battery voltage, will drop slightly above 13.5v is good, compare to amp reading eg 14volts 30 amps is a good reading 12.8volts and -20amps is alternator failure and discharging your battery
6) volt drop engine earth by putting multimeter on battery negative (black lead) and to engine block (red lead) 0 volts is good if you get a reading above 0.5volts could be loose or broken engine earth strap
7) volt drop battery positive to alternator b+ terminal, again 0volts is good and above could be loose or broken cable or connection
8) check you have battery voltage to ind wire on alternator by back probing multiplug whilst connected
i hope this helps solve any problems
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