They have expensive tools and equipment to test alternators but you can get by with a simple digital volt meter. With the volt meter on DC voltage attach each lead to the corresponding terminals on the battery. With the engine running and all the electrical loads on (such as the head lights, radio, defrost, wipers, radio etc). the voltage reading should be between 13 and 14 volts. If you actually only have 9 volts then the alternator isn't charging and you'll have to look in to that more. The truck wouldn't be able to start or run very well with only 9 volts so i'd be willing to bet the gauge is bad or possibly a wiring fault.
A quick and easy way ...start your vehicle ....disconnect battery ...turn on all accessories ....if motor shuts down ...then you will know for sure ...it's not recommended to do this ...but ...the best way is to take it out or go to your local garage and test it with a proper volt meter ,some will even do it for free ....
Take a portable voltmeter and attach it to the battery with the positive(red) lead on the + side of the battery and the negative (black)lead to the - side of the battery. have someone start the truck and observe the readings. it should have the output of about 14 volts running at about 2000 rpms. if you don't have that voltage then check the large red wire for voltage before replacing the alternator. the large red wire should have 12 volts all the time whether the key is on or off. good luck.
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