Open the tailgate.
Start the engine and run at low idle. With a voltmeter, check the voltage between the B+ terminal and ground at the starter [Figure 50-30-2].
The voltage must be higher than 13.5 volts but lower than 14.7 volts at 21°C (70°F) (Alternator Temperature).
If the voltage is higher than 14.7 volts, proceed to the high voltage test.
If the voltage is lower than 13.5 volts, run the engine at high idle and recheck voltage. If voltage is still below 13.5 volts, proceed to the low voltage test.