Lexus Ls 400

[kountryboytexas]

Hello I have a 92 ls400 that will not stay hold a charge on the battery. I replaced Alternator and battery, still same problem. It goes dead when I'm driving. Battery is completely dead. Voltage drops until car goes dead. Please help!!!!

[eatingupblacktop]

Load test both your battery and alt. Also check grounds. Even though you replaced it, sounds like your alternator.

[jcrome04]

Do you have an after market system or anything that would draw extra from the alt?

[SKperformance]

bad grounds or terminals also check your main fuses

[bigste]

Check your power steering reservoir is not dripping onto the alternator. It's a classic cause of this problem.

[kountryboytexas]

I will take the alternator out and take it to Autozone and let them check it. I hope it's the Alt. Thanks

[kountryboytexas]

Took alt. to autozone and it checked out fine. Also I am not getting an alt light(Batt) in the instrument cluster.

[eatingupblacktop]

How is your battery and have you checked out your cables?

[SKperformance]

Alternators may test fine when wound up to a high rpm but at idle 700rpm does it continue with the proper amperage and voltage.

[eatingupblacktop]

He's losing voltage while he's driving. There's got to be a disconnect somewhere.

[SRK]

If there is no charge light on the dash showing up it probably means that the field excitation circuit is not complete. The bench tester would have ensured that the field got current, so the wiring on the car is now suspect.

These alternators, like many, can produce high current even at idle rpm's, which is good considering the electrical load these cars can place on them.

[jcrome04]

Why is there no volt guage on these cars! There was one on my 89 Corolla GT-S!!!

[kountryboytexas]

You think I need to check Alternator again. I am not getting any voltage coming from regulator signal wire to junction box. I removed blue/white signal wire from junction box and started the car and no voltage was present. What else can I do?

[VMF]

there is a cirquit responsible for switching from battery to alternator and for charging. it looks like it have gone bad. first thing i would do is to test relay in the engine compartment that is marked as "charing" or "alternator"

[kountryboytexas]

How is your battery and have you checked out your cables?

Yes the battery is new and the cables are good.

[kountryboytexas]

Alternators may test fine when wound up to a high rpm but at idle 700rpm does it continue with the proper amperage and voltage.

I took it to autozone and they checked it out. It was good

[kountryboytexas]

If there is no charge light on the dash showing up it probably means that the field excitation circuit is not complete. The bench tester would have ensured that the field got current, so the wiring on the car is now suspect.

These alternators, like many, can produce high current even at idle rpm's, which is good considering the electrical load these cars can place on them.

Is there is anything I can check for the field excitation circuit?

[kountryboytexas]

there is a cirquit responsible for switching from battery to alternator and for charging. it looks like it have gone bad. first thing i would do is to test relay in the engine compartment that is marked as "charing" or "alternator"

I checked alt fuse it was good. Where else can I check

[eatingupblacktop]

Have you tested the battery's ground? With the battery in the car and connected, take a meter and measure the resistance across the negative terminal to a clean metal part of the body. If you have no continuity then your ground is bad. Next you can check the specific gravity. At 80F Should be approx 1.27 fully charged and 1.24 half charged.

Have you tested the battery under load? For example, measure the voltage across the battery terminals with nothing on in the car....should measure 13 - 14 v. Now turn on the headlights, if you see the reading drop below 12v, your battery is suspect.

To check your alternator, start the car, run it at around 1200 rpm, measure the voltage across the battery, should be 13 - 14v. Keeping the revs at 1200 turn on as many electrical accessories as possible (seat heaters, rear window defogger, blower at max).....voltage should stay at 13 - 14v (increase engine speed slightly if necessay to keep the voltage up). Ask autozone what the amp reading was when they tested the alt.....should be about 20 amps. If it is low then it won't be able to charge up the battery if you have several high draw accessories on while driving.

Another test for the alt in the car is as follows........start the car.....now disconnect the battery making sure you don't touch anything with the terminal while the car is running. If it stalls, your alt is suspect.

If both your battery & alt check out but the battery stills dies then you have a short draining juice from the alt and preventing the battery from charging. I would check continuity backwards to the alt, starting at the battery. Hope this helps.

[NewGuy19]

Another test for the alt in the car is as follows........start the car.....now disconnect the battery making sure you don't touch anything with the terminal while the car is running. If it stalls, your alt is suspect.

I did this to check my alternator earlier this year and found my alternator to be bad. I then brought it in to a Toyota dealer to have one of their Lexus mechanics change it out for me after I saw what a pain in the a$$ it was to get to it. I told the mechanic I disconnected the battery and he got all up set saying you never want to do that with a Lexus. And even if your alternator was not bad it would be very soon after you did that. I've used this method with other makes of cars with no problems, so I'm not sure if this has any grain of truth, but he does have far more knowledge than I do. Just thought I'd pass on his words of advice.

[kountryboytexas]

Have you tested the battery's ground? With the battery in the car and connected, take a meter and measure the resistance across the negative terminal to a clean metal part of the body. If you have no continuity then your ground is bad. Next you can check the specific gravity. At 80F Should be approx 1.27 fully charged and 1.24 half charged.

Have you tested the battery under load? For example, measure the voltage across the battery terminals with nothing on in the car....should measure 13 - 14 v. Now turn on the headlights, if you see the reading drop below 12v, your battery is suspect.

To check your alternator, start the car, run it at around 1200 rpm, measure the voltage across the battery, should be 13 - 14v. Keeping the revs at 1200 turn on as many electrical accessories as possible (seat heaters, rear window defogger, blower at max).....voltage should stay at 13 - 14v (increase engine speed slightly if necessay to keep the voltage up). Ask autozone what the amp reading was when they tested the alt.....should be about 20 amps. If it is low then it won't be able to charge up the battery if you have several high draw accessories on while driving.

Another test for the alt in the car is as follows........start the car.....now disconnect the battery making sure you don't touch anything with the terminal while the car is running. If it stalls, your alt is suspect.

If both your battery & alt check out but the battery stills dies then you have a short draining juice from the alt and preventing the battery from charging. I would check continuity backwards to the alt, starting at the battery. Hope this helps.

Thanks for the info I will try that.

[eatingupblacktop]

Another check you can do that I forgot to mention........fully charge the battery, turn off all accessories including remote locks. Let it sit overnight, then check the battery again. If it needs any charging and the voltage is down, then repeat the procedure, this time disconnecting the battery overnight. If the battery checks out ok, you have a drain/short in the system and will have to check relays and wiring to find the problem.