When testing an alternator on a car, it's important to first check to see if a battery has enough voltage. Use a test light on a car's alternator with help from an auto mechanic in this free video on car repair and diagnostics.
Expert: Tom Brintzenhofe
Bio: Tom Brintzenhofe has been a mechanic for the past 17 years.
Filmmaker: Reel Media LLC
I have a fan on the intercooler of my car which takes a good amount of volts runs about 13.5. runs fine on during the day but at night when i have the fan and the lights on the voltage drops to about 12.5 and fluctuates to about 13volts back n forth. I have a meter installed in the car and while im driving the alternator doesnt seem to regenerate more volts but only maintains or drops about 0.5volts. Main question is, is it normal that it does that or should i get a stronger alternator?
well it makes cents because a 12 volt battery puts out 12 volts of power then the alternator puts in 13 volts or 13 and a half volts so it gives it more then it needs.
If it doesn't then it means the alternator is fucked
If you're talking about 400 watts total, and assuming you have a standard 12 V battery, you'd be talking about a 33 amp draw on your battery. Considering that the cranking amps of a battery are in the hundreds, and the operating amp of a battery are much lower, I would imagine that it would be fine.
There has been no mention of the exciter feed from the dash light i.e. a blown bulb/ interrupted power supply . Or voltage drop test between the neg battery terminal and the alternator chassis.
Some diesel engines use one of the alternator terminals for the engine rev counter .
This can be a factor to be aware of .
The battery acts as a buffer and stabilizes any voltage spikes that may cause damage to the vehicles electronics so disconnecting the batt is not a good choice. Do a voltage output and curren output test, a voltage drop test, a full field test if the output test fails. Make sure the batt is fully charged.
This guy speaks the truth whether you believe it or not. When connecting your DVOM from the battery to the Alt. it doesn't matter which one is connected where color test lead wise so long its from the batter Neg. terminal to the Alt. Hot lead ( not the connector ). You should get 13.5-14.8V (DCV) anything under or over that the Alt. is defective.