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  #1  
Old 05-29-2022, 10:54 PM
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Alternator A/C voltage question

I'm trying to diagnose a nagging alternator whine with a new stereo install. Checking if the alternator is causing issues or not.
I've been trying to solve this for months and can't figure it out
https://www.diymobileaudio.com/threa...3#post-6033045
I'm trying to figure out if I should be a new alternator.

I just wanted to compare the readings and see if mine are out if whack.

I set the multimeter to AC, positive to alternator stud, negative to the battery or ground.
I'm getting a reading of about 0.150V AC

What I'm trying to find out is if this reading is normal or if I need a new alternator.
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Old 05-30-2022, 03:43 PM
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Meter should be on DC voltage to start.with load on engine ,should be around 14v dc.

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Old 05-30-2022, 03:48 PM
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You can check DC voltage the 2 jump start locations under the hood.

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Old 05-30-2022, 05:45 PM
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Trying to measure alternator ripple with a multimeter is pretty much futile. Scope is much better for that but if your alternator is otherwise fine and previous stereo was working fine it's very unlikely the alternator is at fault.
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Old 05-30-2022, 07:54 PM
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Ahhh I see. Was told on audio forum that the AC is too high and I need to chance the alternator as it's causing the alternator whine...
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Old 05-30-2022, 09:28 PM
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Quote:
Originally Posted by Bmwe5320023.0 View Post
Ahhh I see. Was told on audio forum that the AC is too high and I need to chance the alternator as it's causing the alternator whine...
While in general terms, that may be the case, its a bit more complex than that!

The Alternator generates an AC voltage, which is rectified (turned into DC) and regulated (to around 14.4volts) by the regulator/rectifier. The output should be pure DC (no AC) but in the real world there is always a small AC component in the main output. It is this residual AC that the audio chaps are likely referring to but if this issue started when you changed the stereo, the alternator is not likely the problem - the problem is either within the new stereo or related to the wiring of the new stereo.

How is the new stereo earthed (the return path for the supply voltage)? A poor earth can reveal alternator whine.

Also, is the whine present on all sources (radio, etc.) or only on a source external to the stereo itself?
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Old 05-30-2022, 09:59 PM
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The interconnects (usually RCA cables ) can be a huge problem especially if you have long runs. Try using some different ones. I used to make my own with twisted pair, foil shielded cable. The shield should be grounded only only the source end of the cable. If the head unit has adjustable or switchable output level then going with the higher level and turning down the gain on the amp can be a big help in at least diagnosing the problem.
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Old 05-30-2022, 11:03 PM
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The amp is grounded on the ground stud in the trunk. Same stud where the battery negative terminal is grounded from factory. I tried 3 different RCA cables. If you have a suggestion which RCA cables to buy, please let me know.
Noise is present for Bluetooth, am and fm. These are thebonly sources of audio I have.
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Old 05-31-2022, 04:48 AM
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So you have an external amp connected to a head unit via RCA interconnects?

You most llely have a "ground loop" - a situation where multiple ground paths exist, with a potential difference (a small voltage difference) betwen the ground locations.

These can be tricky to fix - you could try lifting the ground (shields) from the RCA cable to the amp (at the amp end only) - so that only the centre connection (carrying the audio signal) is connected.
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Old 05-31-2022, 08:52 PM
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Yes, I have two amps, one for speakers and one for sub. 4 crossovers, 4 speakers, 4 tweeters.

Yes I figure I have a ground loop, but unable to find it. That's the tricky part, nobody seems to be able to figure out how to locate the ground loop.

Your suggestion lifting the ground portion of rca cables is interesting. I never heard if before. I need to bend them away? I guess this would destroy the rca cables for future use?

I've just ordered an alternator filter just in case.
Just to rule out any ripple effect from the alternator.
Kinda expensive but I don't have too many ideas. I just want to fix this problem
https://www.wmjmarine.com/150a.html
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