Alternator Voltage Drops after installing new battery and alternator - Lexus IS Forum
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post #1 of 8 (permalink) Old 11-18-2013, 12:41 PM Thread Starter
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Alternator Voltage Drops after installing new battery and alternator

Good afternoon folks! First time posting on the site, but I've always been on the site getting info & DIYs for my 2001 IS300 Auto. My car went into limp mode this past Thursday and when I checked the alternator and battery, the alternator was completely shot, so I replaced the alt and battery over the weekend. Did everything just like the DIY. Wasn't hard at all to remove and replace. As i began to drive the car, I noticed that the voltage kept dropping and the car seems to stall a little bit, but the stalling doesnt happen too often. Furthermore, there seems to be a whining noise coming from my power steering pulley (it wiggles laterally slightly at idle, but straightens out when I step on the gas). The voltage drops whenever it wants if you will. Sometimes it drops and sometimes its perfect. I guess what I'm trying to figure out is why the voltage drops when I replaced both the battery and the alternator. What could be the cause of the voltage drop and whining noise coming from the power steering pulley? (it never made that noise until I replaced the alternator) Ive searched the site for info but I cant quite seems to find an answer. Any help would be extremely helpful. Hopefully together we'll figure it out. Thanks in advance.
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post #2 of 8 (permalink) Old 11-18-2013, 12:53 PM
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If the voltage is dropping then the alt is not producing charging voltage or the new battery is very weak. What tests have been done? How do you know the old alt went bad? Is your 120A ALT fuse in good shape. Seems like you need to do some testing, this link will help.

https://my.is/forums/f221/diy-chargin...attery-426000/

As far as the power steering noise, if there is play in the pulley that is no good. I would first verify you have proper level and make sure the fluid is not emulsified(filled with air bubbles).

'02 IS300 MSM LSD
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post #3 of 8 (permalink) Old 11-18-2013, 01:41 PM Thread Starter
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Jasonm4: The only tests ive done is the ones offerred at Advance autoparts. There was no voltage on coming from the alternator. I really dont have an amp or ohm meter to check it out myself, so I went to advance. My question is why would the voltage be at 14 (based on our charge gauge) during one drive, I park , and then when I start driving again, the voltage goes down to about 12.5? The car just threw a CEL. I will check it out now when I get out of work and see if that leads me to an answer.
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post #4 of 8 (permalink) Old 11-18-2013, 02:00 PM
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It still kinda sounds like the alt isn't doing anything and the battery is running the car. Vice versa, if the battery is bad voltage will usually increase with rpms. The trouble is, you really can't go by the clusters analog meter because it's not very accurate. I wonder if your original alt was really bad or not, I hope you had that alt bench tested before you gave them your core.
What I would be checking for is all your main connections, especially at the battery and block. Make sure your 120A Alt fuse is good as well as the 7.5 ALT-S fuse.

'02 IS300 MSM LSD
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post #5 of 8 (permalink) Old 11-18-2013, 02:00 PM
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It still kinda sounds like the alt isn't doing anything and the battery is running the car. Vice versa, if the battery is bad voltage will usually increase with rpms. The trouble is, you really can't go by the clusters analog meter because it's not very accurate. I wonder if your original alt was really bad or not, I hope you had that alt bench tested before you gave them your core.
What I would be checking for is all your main connections, especially at the battery and block. Make sure your 120A Alt fuse is good as well as the 7.5 ALT-S fuse.

'02 IS300 MSM LSD
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post #6 of 8 (permalink) Old 11-18-2013, 02:20 PM
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I would check your new alternator, it would'nt be the first time someone has bought a new alternator that is bad out of the box. Check the output at the alternator and at your battery. If the alt is good but low at the battery then you have a wiring problem or a possible burnt fuse. Make sure you check your grounds as well. good luck

02 IS300 GGP M/T
A few bolt ons that make her sit low, look and sound nice, and go faster
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post #7 of 8 (permalink) Old 11-18-2013, 03:24 PM
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If your battery light never came on then your old alt probably never went bad. That's why you have to bench test it. If the voltage drops while running then I would not drive the car until this is sorted as I believe you are only drawing off the battery and soon that will be dead. In all honesty, you need a mulitmeter to perform certain test, even the $5.99 at Harbor Freight is fine but you can't realistically solve the problem by visual inspections.

'02 IS300 MSM LSD
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post #8 of 8 (permalink) Old 11-19-2013, 05:43 AM
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Based on your op, you are running off battery power and soon that battery will die too because the alt isn't producing power. If it was and you had a battery problem, then the voltage meter would go up with rpms.

Again, you don't have to follow all the steps in the link I posted but you do need to do some basic tests with a meter. Checking voltage output and continuity of grounds and all power B+ wiring. If you don't have the tools then would take it somewhere. I'd take your old alt too because I doubt it's bad if they didn't bench test it(which they are supposed to do unless told otherwise).

'02 IS300 MSM LSD
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