If you have a multimeter it’s not hard to test the charging system. You’re just looking at voltages at various points and conditions, for the most part. Checking current does require you to have a dc clamp meter or attachment or some kind of load tester, but knowing actual current/amps is mostly academic anyway. You can tell whether the system works just by voltages.
1. What is battery voltage when car is off, preferably after sitting overnight? If it’s over 12.60v it’s very likely your charging system works ‘well enough’, or at least did the last time the engine was running!
2. Start the vehicle and observe the voltage. IF voltage rises AT ALL past 12.6, THEN the charging system does ‘something’. If it jumps into the 13s right away it does ‘a fair bit’, if it jumps into the 14s it does ‘a lot’. This is vague but it does hold true that a weak charging system cannot push voltage as high as a strong one, though this symptom usually is only noticeable under greater load.
3. Turn on every electrical device in the car except the radio, horn, brake lights. The biggest loads you can turn on in a bare bones car like an old Rav or Corolla is the blower motor (preferably with the ac on) , followed by headlights/taillights and rear defrost, followed by wipers and hazards. Pretty much any car having all of this ‘standard’ equipment turned on will be asking 55-60amps from the alternator. Turn all of it on, then check voltage. If its above 13.0v at idle, your charging system is JUST FINE (at that moment, anyway). If its NOT above 13.0v, then you have to distinguish between a voltage drop in the wiring, and a bad alternator.
To do a voltage drop test on the charging circuit, leave all the electrical loads on and put your positive meter lead on the charging post where your ‘big wire’ attaches to the rear of the alternator. put your black lead on the battery positive terminal. Whatever voltage is displayed is the difference in voltage from one end of the wire to the other, aka the voltage ‘consumed’ by the wire’s resistance (which is really being converted into heat). This number should be under 0.5v at full load although thats a bit of a line in the sand. That was the positive side of the circuit. To test the negative side, put your red meter lead on the battery negative terminal and your black meter lead onto the metal housing of the alternator. The alternator grounds through its housing so anywhere on the metal housing is fine. This number should be below 0.2v or so. The threshold is lower for the ground side because it shares the ground wire for the starter, which is MUCH larger than would be necessary for only the alternator, thus it SHOULD drop less voltage. Remember that voltage drop is proportional to current, so it should be tested under the highest continuous current possible, ie ‘with everything turned on’.
The main idea is that the charging circuit shouldnt drop more than about 0.7v total. Considering that the alternator will not usually put out more than 14.X volts and a fully charged battery is 12.6v, there is less than 2 volts of ‘wiggle room’ and if the circuit drops over 1 volt it can pretty quickly make the alternator appear not to work when it actually does.
Report back if you do any of these tests.
