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Well my 145 amp aftermarket alternator failed after a little more than 2 years. I replaced it with a rebuilt Denso 90 amp alternator which is working just fine.

Symptoms were charging at random voltages (from 18v to 12.1v irregardless of rpms) and a high parasitic draw when the engine was off. Obviously the voltage regulator in it when nuts (technical term). I will rebuild it later after I get a chance to get a new voltage regulator for it.
 

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Well my 145 amp aftermarket alternator failed after a little more than 2 years. I replaced it with a rebuilt Denso 90 amp alternator which is working just fine.

Symptoms were charging at random voltages (from 18v to 12.1v irregardless of rpms) and a high parasitic draw when the engine was off. Obviously the voltage regulator in it when nuts (technical term). I will rebuild it later after I get a chance to get a new voltage regulator for it.
18V !? poor battery but it can take it for some time (less than a month) without noticeable loss of service life. Hope the replacement lasts as it should, longer than 7 years.
 

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18V !? poor battery but it can take it for some time (less than a month) without noticeable loss of service life. Hope the replacement lasts as it should, longer than 7 years.
I narrowed down the problem on day two so the battery didn't get too much abuse. I was worried about the electronics too because the battery light came on, all the gauges maxed out, and some other warning lights came on. So I stopped and tested to see what was happening and it was at 18v, I had to get it home (about 2 miles from where the problem started). The next morning the battery was dead, so I disconnected the battery and recharged it. Then I checked for a parasitic draw and found that it had a 2 amp draw coming from the alternator and the charge from the alternator would jump from 12.1v to 18v and back again. I checked the wiring to make sure I didn't have a problem but each leg was good. I parked ¡Ay, Caramba! for a couple of days until the alternator arrived. After installing the rebuilt alternator everything was good; no parasitic draw, 14.3v output from the alternator, and the battery was good still showing 950 CCA. All is well with ¡Ay, Caramba! again.

I got it off of eBay.com. I was worried about shipping because I've had problems with ebay vendors taking over a week to even getting around to processing my orders, but it came in really fast, shipped from California to south Florida and it arrived on day 3, thank you ace_alternators.
 

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I narrowed down the problem on day two so the battery didn't get too much abuse. I was worried about the electronics too because the battery light came on, all the gauges maxed out, and some other warning lights came on. So I stopped and tested to see what was happening and it was at 18v, I had to get it home (about 2 miles from where the problem started). The next morning the battery was dead, so I disconnected the battery and recharged it. Then I checked for a parasitic draw and found that it had a 2 amp draw coming from the alternator and the charge from the alternator would jump from 12.1v to 18v and back again. I checked the wiring to make sure I didn't have a problem but each leg was good. I parked ¡Ay, Caramba! for a couple of days until the alternator arrived. After installing the rebuilt alternator everything was good; no parasitic draw, 14.3v output from the alternator, and the battery was good still showing 950 CCA. All is well with ¡Ay, Caramba! again.

sounds like the problem found its match :) - congrats on textbook troubleshooting and repair! It seems not only reg part failed but one or more rectifiers failed as well leading to high parasitic draw. They're both parts of the same assembly so I expect the new reg to fix all those issues at once.

I wouldn't worry about electronics too much- usually its sensitive parts are behind local voltage regulators and those can withstand 20-30V easily. Depending on the type they have thermal protection (in case temp of the regulator chip increases too much due to increased input voltage) and often Zener shortening its input (and consequently blowing related fuse up the path) if input voltage goes really high.
 

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Eodgator, thanks for the heads up. Can you post any identifying info you find on the old voltage regulator? Also what is the manufacturer's name on the now-failed 145 amp alternator?

My 1998 Rav4 has an aftermarket alternator. The charge warning light came on this past December. The battery voltage had been declining to the point of starting being difficult. In other words, the battery was not charging properly. I found the alternator brushes way worn, at about half their original length. So I am figuring the alternator is around 100k to 200k miles old. I replaced the brush holder set and the Rav has been running fine since. The voltage regulator is made by Rand and so far seems fine. My local U-pull salvage yard will sell me a used Denso voltage regulator for a little under $20. Per toyodiy.com, a bunch of 2000 and earlier Toyota models use the same voltage regulator as the Rav4. For the interested reader, the voltage regulator's OEM part number is 27700-74030. So far I cannot find a Denso part number.
 
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