'91 Toyota Celica GTR (ST183) charging diagnosis

Question

I've been working on getting a Celica up and running after a couple years of laying mostly idle, it's been dry-stored for most of it.

The biggest issue I have right now is that the system is not charging the battery at all, I've done some basic testing with a multimeter and to my (limited!) knowledge I think the problem is within the alternator assembly. The vehicle is specifically a 1991 Toyota Celica GTR 4WS with a 3SGE engine.

At idle, I have battery voltage, around 12.6V but dropping gradually when running. The battery is brand new as it needed a new one (note that it did have a bad battery in it for a long time which didn't hold a charge) From the Battery positive post on the alternator to the Negative terminal on the battery, I'm reading the same voltage as the battery.

I think I've done voltage drop tests correctly on the alternator wiring (positive to positive / negative to ground using the voltmeter?) and the values were very small, only 0.01-0.02V at most.

I don't notice any strange noises or smell from the alternator, I just want to get some opinions on what it could be. I've posted pictures at the link below on what it looks like and what's I've done, any help is appreciated! What I think may have happened is that because it was using a very bad battery for a long time - we might start it up every few weeks, that the alternator brushings (not sure if that's the correct term) may have worn down by trying to charge it too much

I have some basic knowledge of circuitry and electrical systems so feel free to comment and help a guy out!

Link to Imgur album of the alternator -> https://i.sstatic.net/lCrse.jpg

Answer 1

With the engine running your charging voltage should be 13.5 - 14.5 volts. If you are reading 12.6 or less, the battery is running the car until it's depleted.

Alternators have an armature (a coils of wires) surrounding a permanent magnet rotor to generate alternating current. The Alternator also has diodes to convert this to direct current.

The rotor also has a coil that is energized when power is needed from the alternator.

If the alternator is turning but not generating power, the diodes could be bad, or the regulator is not able to send power to energize the field in the rotor's windings (bruses bad in the alternator or the regulator has failed).

Your car has a separate voltage regulator. https://www.rockauto.com/en/catalog/toyota,1991,celica,2.0l+l4+turbocharged,1274138,electrical,voltage+regulator,4884

You can remove your alternator to have it tested - if it tests good, replace the voltage regulator. Always remove the positive battery cable when you disconnect the alternator.

You may be able to perform a full field test. Here is text from freeasestudyguides.com.

Voltage Regulator: Full Field Test Test the alternators regulator with a full field test when the alternator has low output. This test will rule out the regulator by bypassing it and energizing the rotor with unregulated voltage.

A Type-A circuit has the voltage regulator on the ground side of the field coil. A Type-B circuit has the voltage regulator located on the power side of the field coil. In many late model vehicles the regulator is controlled by the PCM. They’re controlled by a duty cycle and best diagnosed with a scan tool. Always check with the manufacturer for the steps required to field test a circuit.

Some alternators have a tab on the back that when grounded to the case full-fields the alternator for maximum voltage. If the alternator produces low voltage, and when full-fielded suddenly produces maximum voltage, the regulator is at fault. If it still produces low voltage when full-fielded, the alternator itself is at fault.