So on my 98 Lexus ls400 I starting to hear a humming noise coming from the engine. I have looked/searched and I saw a post saying it was the alternator going out. I decided to get my alternator tested just to see and turn out that my voltage is good getting 13.5v but the diodes failed the test. So does this mean I have to replace the whole alternator? Is it better to replace the whole alternator? Plus my car idle low like 300-400 and I can feel the car vibrate cause of low idling but when I turn parking light on it goes normal above 500 rpm. I do not have power steering leak, I have replaced that awhile back when I did my timing belt stuff.

 

How did you check the diode pack with the alternator still on the car?
You have to remove then strip down the alternator to get to the diode pack then measure each one of the 6 individually.

 

I went to orielly and they use that tester thing they use for battery, starter and alternator. Ran it twice and both times it says the diodes test fail

 

Had the same problem with my 99LS. O'reilly's told me my diodes were bad as well. Turned out my bass amp had a faulty power supply. I had the alternator replaced and then tested afterwards and it was just fine. Wasted $200 but now I have a spare alternator.

 

Mine really did took a ****, a day later the battery light came on so I order one and put it on and problem solve lol

 

u ordered one from no-reillys?

 

Nope, order a denso reman one off eBay. Saved mine and will probably get it rebuilt with higher amp so will have it for spare lol

 

In my 95 LS I replaced only the diodes and the regulator (mine was faulty as well), leaving the alternator as such intact. Both parts were the same as used in the original alternator (probably denso, but really don't remember) and it cost me about 35-40$. Since then, the alternator works like a charm and had no problems with it. If you have any technical skills the job shouldn't be hard for you.

 

Only $35-40? That's strange to me becuse the Denso's regulator alone costs more than $100.
I have seen many aftermarket regulators were failing in 2-3 years.

 

I can't check it without removing, but I am pretty sure it was around that price. Or maybe they didn't give me the Denso. I will see in 2 years then.

 

Yamae is right, a NEW one is over 100, but a used one would be less, and I would rather have a used one than ANY aftermarket..