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Old 02-24-2013, 02:39 PM   #1 (permalink)
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Default Alternator diagnosis/advice

I am wondering if my alternator could be bad? AutoZone said it failed but when I checked it myself I was getting around 14.05-14.15 volts while the cars running. I know the battery is good, it reads 12.75. The issue I am having is my Kenwood stereo keeps going into protect mode. It's not the stereo because I switched that out, and I changed all the speakers. The weird thing is after re-setting the stereo it goes into protect after a few minutes of driving and mostly when hitting the brakes. If I shut the car off and run the stereo with nothing on it can run for hours with no issues. Any advice would be appreciated.
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Old 02-24-2013, 03:33 PM   #2 (permalink)
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What is protect mode and what triggers it? A voltage dip? What's the threshold voltage?

Your charging voltage seems good, but if it's fluctuating too much it's bad. Do you notice your headlight flickering or dimming when you slow down (with the stereo off obviously)? While driving around the voltage should not fluctuate more than 0.2 volts. I suggest you monitor the voltage while a helper drives you around.
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Old 02-24-2013, 03:56 PM   #3 (permalink)
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Protect mode is something installed on most aftermarket stereos to prevent damage. It can be triggered by a bad speaker, wire connection or not enough Battery power. I have a gauge I can plug-in to Cigarette lighter to check the output, with everything turned on including AC rear, defroster, lights etc. it's getting about 13.5 volts while driving. The headlights do Flickr whenever I turn the stereo on And I have noticed Some power serges every once in a while.

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Old 02-24-2013, 04:00 PM   #4 (permalink)
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13.5 volts is getting down there if that's the reading with the stereo off, so the alternator might be bad after all (it should be able to handle headlights and rear defrost at idle without going below about 13.5 volts). Again you are looking for voltage stability more than anything while you are driving around.
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Old 02-24-2013, 04:05 PM   #5 (permalink)
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It reads about 13.5 with the the stereo on and everything else on. The head unit will only work for around five minutes of driving. I don't have subs just aftermarket speakers.
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Old 02-24-2013, 05:21 PM   #6 (permalink)
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You need to test the alternator and see how much A/C voltage it is putting out. Anything more than .5V A/C you have some diodes going bad and that could be a cause.
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Old 02-25-2013, 03:20 PM   #7 (permalink)
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Thanks cjjames23.....I tested the AC voltage I was getting around .0500 - 0600 and 14.05 DC both while idleing. Just as a test I checked my roommates Ford Ranger. It read .001-002 AC and 14.5 DC , so it looks like I need an alternator. Does anyone know of a good alternative besides the Motorcraft, its $266.00... I will definitely stay away from AutoZone alternators, they are junk.

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Old 02-25-2013, 03:46 PM   #8 (permalink)
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.05v ac is well within CJ's suggested tolerance of 0.5, and 14.05v is no indicator of a faulty alt to me.
An alt can certainly have intermittent problems. I had one all out fail on me in the summer heat, but when I took it in to verify its failure so I could get a warranty replacement -- it tested fine.

The symptoms sound like a bad alt, but the testing you did doesn't confirm that.

As far as a replacement -- from all I've read anything but a new motorcraft is going to be hit or miss. And new MC's look to be around $400.
Rebuilds vary quite a bit in how extensively the failed parts were replaced.

I just buy the cheapest one I can get with a lifetime warranty, which for the 130A alt my car uses means Advance's Worldwide brand reman at around $100 after discount code.
I run a high power sound system and I seem to burn up my 130A alt every couple years.
I'd rather spend $100 once and a couple hrs every 30-40k replacing it than $400.
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Old 02-25-2013, 03:56 PM   #9 (permalink)
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liquid_force the Ford dealer quoted me at 266$ for the Motorcraft if I bring in the core. If all the others are hit or miss I will definitely spend the extra money on the motorcraft. I have no idea if my alternator is bad or not, I'm deftly not an expert when it comes to the electrical workings of a car. What makes me think it is the alternator is the stereo can run for hours with the car off, as soon as I start driving it goes into protect mode.

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Old 02-25-2013, 05:58 PM   #10 (permalink)
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Yeah, are you sure you got those decimal places right on your AC ripple voltage readings? If you're not sure take a picture of your meter showing the reading and the range setting for me.
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