Function of Three Wires Connected to Back of Alternator?

EDIT: Based on information supplied by TnShooter, I verified normal operation of the wiring harness and took the next step of replacing the alternator. All is working correctly now.

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1995 Tacoma 2WD, 2.4L, Automatic

I’m having a charging issue. Not complete failure, but it can’t keep up with the lights.

When idling, battery does not charge. I assume the electrical load is greater than the alternator output.

I seem to be able to drive indefinitely in the daytime with no lights (200 miles), but when I stop and check voltage it’s only keeping it at about 12.5 volts. So the alternator is barely keeping up with usage. If I turn on my lights, the system voltage will gradually drop.

1. Terminals are clean.
2. Battery charges fine from an external charger.
3. No detectable current flowing out of the battery with everything turned off.

Before I declare the alternator bad, I’d like to be certain that the signals/voltages at the 3-pin connector on the back of the alternator are correct.

Anyone knowledgeable on how I could measure this?

 

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You should have a white, red, and yellow wire.

White - This is the sense wire. It “monitors” the current charge of the battery. It should have 12v at all times.

Red - This wire turns the alternator On. It should read 12v with key ON.

Yellow - This wire is grounded by the alternator to turn the Battery/Charging Warning Light on.

You need to have the alternator tested.

 

Thanks! This is exactly the information I was looking for.

But I’m curious, why does the alternator need to be told to turn “on”. Especially if it is told this whenever the key is on.

If it’s never off when the motor is running, why signal it? Just let it produce whenever it’s being spun.

 

Simplified, if the alternator (regulator) isn’t turn off, it would drain the battery.

 

Got it. What you’re essentially saying is that the regulator isn’t smart enough to stop the field current without a signal.

Thanks again for the fast replies!

 

The charge light does light when the ignition is in run position engine not running??

With a full electrical load is the charge light being lite??

Are you measuring the output of the alternator at the B terminal on the alternator?? Then compare with the battery voltage.

What happens with a full load Lights , Blower Motor on High , Hazard lights and wipers ?? This is the critical number you need to know.

Being this is a 95 Have the battery cables or any of the major charging harness been upgraded or changed??

I am thinking either line loss voltage drop or the alternator voltage regulator is set really low.

Depending on your answers I lean toward line loss.

Good Luck.

 

Think about beefing up your ground....

I did a neg post strap to head.....and dropped off another off it to the block..

Toyota depends on body grounds......thats crazy....even when everything is new and perfect.......