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Pete, Technician
Category: Lexus
Satisfied Customers: 28748
Experience:  Over 17 years diagnosing and repairing automobiles various makes and models with some training and certifications.
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I have a 2000 ls400, my alternator went out, so i had a shop

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hello, i have a 2000 ls400, my alternator went out, so i had a shop rebuild my oem to higher output 200a. I installed new one, worked perfectly for 1 week, then would not charge, i removed alternator, had bench tested, tested good, reinstalled, voltage meter in place,, started, worked for 20 seconds and with clicks in hood fuse box, battery slowly drained to 10.2 volts while idling. After that, it would not show the brief charge, I uninstalled alternator, checking oem wiring, reinstalled alternator, got 20 seconds of charging again, then battery drain to 10.2v.
I brought back to shop that rebuilt it, i explianed voltage regultor will woprk on bench test , because its been disconnected, brought car in. alternator never showed a charge, they even bypassed my car with the 3 wire harness and plugged into their bench test machine, it didnt work. so they built me a new alternator. I just installed and it doesnt work, did not even get the 20 seconds of charge, im losing my mind, i replaced 120a fusable link fuse, the 40a fusable link fuse. the only thing i can think of is a relay gone bad or a circuit breaker that im unaware of.
JA: Does the battery look like it's leaking? And does it have a strong smell -- like sulfur?
Customer: new battery
JA: What is the model of your '00 Lexus?
Customer: LS400
JA: Are you fixing your vehicle yourself? What have you tried so far?
Customer: its in the 1st message i sent
JA: Anything else you want the mechanic to know before I connect you?
Customer: this doesn't seem possible, this whole thing, i cant wrap my head around this

My name is ***** ***** I am a professional here at Just Answer. I noticed that your question had not received a response and thought I would see if you still need help with this.I apologize for the delay and I hope I can still assist you with this here.

Inspect the wiring leading to the alternator for any issues.If any issues are found with the wiring this will have to be repaired to resolve this fault here.If all wiring is checked and found to be o.k. let me know so we can continue.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service,your 5 star rating is appreciated.You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.

Thanks Pete

Customer: replied 4 months ago.
already done, i replaced the pigtail and shop confirmed connection is good

Do you have a test light or multi meter to test with at all?Is it in the vehicle now and acting up?

Customer: replied 4 months ago.
i have to charge my battery, it took a new battery down to 10% idling

O.k.,when it is charged up locate the black/red wire and the white/light blue wire at the alternator and with the engine running see if these two wires have 12 volts DC on them with the ground lead of the meter connected to the alternator chassis for a ground signal.Then test the blue power feed wire from the alternator and see if it has around 12 to 14 volts with the engine running and let me know.Let me know what you come up with here when your ready.

Thanks Pete

Customer: replied 4 months ago.
test those 2 wires with the plug disconnected from the alt?
Customer: replied 4 months ago.
or poke at rear of pigtail in each wire feed to get a connection?

You can test the two wires with it plugged in back probing them.

Customer: replied 4 months ago.
It's been a frickin monsoon around here. I haven't had an opportunity to get some dry weather to take some readings. I apologize. I'll let you know asap. I'm hoping soon. Thank you

No problem

Let me know when you are ready.

Thanks Pete

Customer: replied 3 months ago.
Sorry it's been so long Pete. Was in hospital with tonsillitis. This May is one for the books.
So I'm testing wires now.
White/blue stripe = engine off 12.39 volts, engine on 11.8 volts.
Black /red stripe = engine off 000, engine on 11.51volts down to 11.48 volts and held there for 3 minutes.
The blue power feed wire must be the pink one on mine = 10.47 volts down to 10.35 volts and held for 3 minutes.
I took the alternator and had it bench tested today and it showed strong 14.2-14.4 all the way to 180amps. If that matters at all
. Sorry again that it took so long.

Are you sure there is no damage to the power wire that feeds from the alternator to the battery at all?

Customer: replied 3 months ago.
I don't believe so. It actually goes from alternator to fusible links. Then to battery. I was gonna check positive voltage drops in a bit. The negative is max at..2volts.
. I've been reading a lot about the fuel pump effecting charging? My fuel pump relay makes all sorts of clicks and at idle low battery fuel pump, or something back there makes a rather loud hum.
Customer: replied 3 months ago.
Also was gonna check fuel pressure level but unsure of what it should be at. . Do you know how many psi?

Your fuel pressure should be in the range of 44 to 50 psi.Usually the fuel pump or relay or bad wiring to the pump or relay wont cause this fault here.

The only thing I can think of since the alternator checks out o.k. and the wiring connection and signals seem o.k. is a fault with one of the cables to or from the alternator or battery.

Customer: replied 3 months ago.
the volt readings checked good yesterday? They all seemed a bit low. The one was only at 10v

Let me re check this real quick.

O.k.,the blue large power cable from the alt to the battery was the one I was referring to I apologize for that.

The pink wire is the alternator control wire from the ECM.Located this info in a different diagram.This is making sense here now.

Since you have voltage but its low ( and it will be low since the alternator is not charging ) the component causing the fault here is the ECM itself or wiring from the ECM to the alternator.This is the pink wire feed.I dont show what this signal should be from the ECM but this has to be the signal that is missing telling the alternator to charge.Below is a diagram of this.

So the issue has to be a broken,corroded or damaged wiring leading from the ecm to the alternator or if this wiring is checked and found to be o.k. then the ecm itself is at fault needing replaced.

Let me know if you need anything else and feel free to rate my help here or above as this is how I receive credit for my time.Your 5 star rating is appreciated

Thanks Pete

Customer: replied 3 months ago.
f**k. That's everything I didn't want to hear. I've been reading some write ups about resoldering capacitors in the ecm. A new one is big bucks. And a used one will fail soon anyways. So I need to open. Up the ecm and check for oozing capacitors. Sound about right.?

It does sound right but unfortunately I do not have a diagram of the internals of the ECM.Only an electrical repair center can gain access to the internal diagrams of it.But if you have any oozing capacitors then they would need replaced unfortunately.I have seen ECM failure in the past cause this fault here.

Let me know if you need anything else and feel free to rate my help here or above as this is how I receive credit for my time.Your 5 star rating is appreciated

Thanks Pete

Customer: replied 3 months ago.
You're the man, brother. Never doubted you for a minute. Thanks for your time. I will rate ya. I might have 1 follow up question soon

No problem.Feel free to rate my help here or above as this is how I receive credit for my time.A 5 star rating is appreciated.You can always come back anytime after as needed to the same question here.

Thanks Pete

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