ford 的spec 只是福特，而且是它的上限值。再看看通行的介绍。

Alternator Performance Test. Connect voltmeter to alternator terminals and ammeter to alternator�s positive output cable, per Figure 13. Make sure ammeter is at least 6 inches (15 cm) away from alternator to eliminate the possibility of faulty readings. Make sure voltage is present at alternator�s output terminals. Start engine and run it at 1500 RPM�s (operating RPM). Check that all vehicle loads are turned off and reading on ammeter is less than 20 amps. If ammeter reading is greater than 20 amps double check that all vehicle loads are turned off and that batteries are fully charged. Record voltage on voltmeter. Reading should be between 13.8 & 14.4 v for a 12 volt system, 27.8 & 28.4 v for 24 volt systems. If the voltage is not within these ranges then try adjusting the regulator if available. If the regulator can not be adjusted alternator is defective.

摘自prestolite electronic 的 training manual.

再说，你还得承认15.7v 是高过正常值的，包括福特。