For proper testing of the charging system you need to get an ammeter involved but for voltage testing the battery should be fully charged and alternator output current below 10 amps. At idle voltage across the battery should be between 13.5 to 15.1. For current testing without an ammeter run the engine at 2500 rpm with loads applied. If the voltage doesn't drop more than 1 from what was measured at idle the system is working. If the battery was fully charged prior to doing this test you should discharge it slightly by disconnecting fuel and spark and cranking the engine for several seconds.
Some info for the technically inclined: The stock alternator is of the compact type. Compact alternators generally spin faster than conventional types so the rotor and fan have been consolidated and placed internal to the case for improved cooling. The compact type also offers improved serviceability ie; the regulator and brush holder are easily accessed without disassembling the case.
The alternator uses neutral point diodes in addition to the 6 rectification diodes. They're used to add the neutral point voltage to the alternator's output, thus improving it by appx 15% from midway on up to the rated speed of about 5000 rpm. The alternator's sense terminal is connected at the battery positive, unlike on many cars where sensing is done out in the dash somewhere
Toyota's Type M monolithic IC regulator is used. It's the highest performance regulator of the three types Toyota employed in those days. As with all regulators it's job is to limit voltage output as rpm changes to the standard voltage (often called the regulated voltage). It does this by varying field current using PWM. This makes the charging system the constant voltage type typically found on cars of the era.
Being an integrated circuit the Type M can regulate tightly, typically within .2 volts. There is nearly zero hysteresis with this regulator. Also, unlike many older cars, the charge warning lamp is not part of the field control circuit and a failure of it's filament will not effect charging system operation.
The Type M regulator monitors several things and will illuminate the charge warning lamp when any of the following three conditions exist:
1) An open in the field winding occurs. Of course, that would include brush failure as well.
2) An open in the sense terminal occurs. Should the sense terminal become open alternator output is automatically limited to between 13.3 and 16.3 volts.
3) Voltage at the sense terminal falls below 13 volts even though it's still connected.
Number 2 is an important point to remember when troubleshooting this system because you can have a charge warning with a functioning alternator/regulator and charging still occurring even though regulation has been lost. Bottom line: Keep the sense connection (it's in the fuse box) in good condition.
Should the alternator's output terminal B or the positive battery terminal become disconnected output will be limited to 20 volts max. Better than a non-IC type regulator but still something to keep in mind the next time anyone suggests doing a load dump (disconnecting a battery terminal with the engine running) to check the alternator. That said load dumps are bad for other reasons.
Another quirk: If your car has a 7.5 amp fuse labeled "Charge" it's function as related to the charging system is only to supply power to the charge warning lamp. Failure or removal of this fuse will not effect charging system operation; you could yank that baby out and charging will continue. Course, the charge warning lamp won't illuminate should a real problem occur.
Lastly, the alternator field can be flashed through the small hole labeled "F" on the rear of the case by inserting a thin probe and grounding F to the case. It can be used to determine whether the alternator or regulator is defective. Since with a functioning alternator this will result in maximum field excitation I don't recommend doing it unless you fully understand what the consequences could be. -JetJock mkiiitech.com
