I have a 99 ES300. I replaced the alternator not because it was completely dead, but because after 200k miles, voltage would drop to below 12 if you turned on all accessories. Under normal conditions with just the AC on or something, it kept the battery charged but my son commutes to college in the car so I wanted to be proactive. I say all this to rule out in advance that it is NOT the battery that is the problem.

I got a rebuilt alternator from NAPA and replaced it myself this evening. Everything went ok except the harness that plugs into the alternator was a bear to get out. The plastic clip broke, but the body of the harness is fine. If you have ever replaced one of these you know how you have to really work to get the harness out of the way to remove the alternator. I am wondering if I damaged a wire in the loom or something in this process.

I cranked the car after getting the new one in and put a multimeter on the battery. It was in discharge at 11.8 volts with no accessories on. I checked voltage at the alternator output post and same thing, 11.80.

So what I do know is the charge wire itself is not damaged because with the car off, I do get voltage at the alternator. I believe this alternator may have an excite wire in the regulator that starts it charging? Its almost like that part is not working.

I assume the alternator is not defective as it has a test sheet with it. I am hoping I just missed some small detail that someone can help me with. I hope it's not a broken wire in the harness.

any ideas?

 

Prior to the alternator change.

How did you test the old alternator for voltage output with engine on?

Was the battery charge light on?

Was battery still able to start the engine easily?

With headlights on, did the light output change with engine RPM?
Dimmer at idle and brighter at higher engine RPM?

Did you ever run the engine with the alt output wire to the battery disconnected? The larger wire going the alt.

Breaking off the catch happens. The plug needs to be firmly plugged in and retained. For plug attachment, use a plastic zip tie to secure the alt plug to the socket.

What color wires are you checking for voltage at and are you checking at the unplugged harness plug?

Check each color wire at the unplugged harness plug engine off.

White should be battery voltage.

Yellow runs through ign switch. Should be 0V with switch off. This is the battery charge light wire.

Red/Blue runs through ign switch. 0V switch off, battery voltage switch on.

The larger Black/Green wire supplies the alt output current to the battery. Should be battery voltage.

Check ALT-S, AM1, AM2, IGN and GAUGE fuses.

Use a volt meter to check the voltage at the battery with engine at 2000 RPM. Should be approx 14-15 volts.

Engine off and car sitting for time, full charge battery voltage is 12.6V.

 

I thoroughly tested the old alternator with a progression of accessories on. It did ok at idle with no accessories but as accessories were added it fell off pretty quickly and soon was in discharge state.

On the 'new' alternator, I tested the white and red/blue and they had the proper voltage when car on and off so I figured it had to be a bad alternator. I pulled it off and had it tested and sure enough it was dead as a door nail. It was a NAPA unit which are usually pretty good, but I will be picking up an OEM one from Lexus tomorrow, I dont want to do this again.

Thanks for the help.

 

It happens these "rebuilt" alternators are no good out of the box.

For future reference to readers having the skills to do so. Repairing or rebuilding the OEM alt is often preferable to a "rebuilt" aftermarket. Parts are available.