Low Alternator Output

Swafflife

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Apr 16, 2017
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I'm running a 2003 5.0 Gxi-C and I recently installed a Blue Sea Add-a-battery where I have a dedicated starting battery (Optima blue top) and two house batteries (Optima blue top 31 series) for the house loads. After install and testing, I noticed that the LED on the ACR was not solid green showing that the relay was combining and charging while the engine was running. After some discussion with tech support, I ran through a troubleshooting worksheet sent to me from Blue Sea and read the same voltages engine stopped and engine running. It looks like my Alt isn't putting out the necessary voltage to trigger the ACR to combine and charge the batteries while cranked. I haven't taken a voltage reading at the alternator yet, but at the starting battery while cranked, I'm only getting about 12.5 volts. The Blue Sea tech says the ACR won't even activate until there is around 13.5-14 volts detected from the charging source. My dash voltmeter typically reads around 12.5 while running which is obviously low. Does this sound like a bad alternator or possibly a bad voltage regulator? All assistance is much appreciated.
 

Bondo

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Ayuh,.... Welcome Aboard,.... Check the voltage while runnin' at the alternator,....

If it ain't 13.6v or greater, repair/ replace the alternator,....
 

Swafflife

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Thanks Bondo...that's my next step. I should also mention, I was seeing the same voltage on the dash prior to installing the Blue Sea add-a-battery.
 

km1125

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The ACR shouldn't be active during cranking at all. Are you saying these measurements are during cranking or during running. Definitely 12.5 while running is low and means the alternator isn't putting any charge into the battery.
 

Swafflife

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I tested 12.5v at the alternator while idling...guess I'm replacing it :facepalm: any suggestions? Ebay?
 

Jsulla

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I tested 12.5v at the alternator while idling...guess I'm replacing it :facepalm: any suggestions? Ebay?
Most alternators don't produce a lot of voltage at idle. Bring it off idle a little bit and check again
 

dingbat

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Check voltage at alternator output. Could be as simple as a bad cable / connection
 

Swafflife

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No Title

I took a reading with the multi meter leads at the positive and negative posts on the alternator and got 12.5v. Would this measure the alternator output?
 

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bruceb58

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Yes it would.

You need to make sure the field wire is being energized. If it isn't, the alternator won't output
 

loco

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a) There can be an activator/exciter circuit in there. From memory, it either needs a resistor on it, or it goes to a warning light on the dash. The alternator won't produce charge unless it's been activated. Lots of circuit diagrams on google image.
b) Test the charge at the actual alternator, as voltage can drop through rusty or loose cables
c) I took my alternator out when the voltage readout was a bit low at my dash, and gave it to an auto electrician, who had a bench tester that would spin it up and put it under load. I actually had a corroded cable connector, so a good cleanup brought dash volatage back up to ~14V

Be cautious with alternators though - a faulty alternator will get you several miles offshore before your engine packs up and leaves you stranded if you're not keeping an eye on it.
I have three separate readings of voltages to keep a close eye on it (digital voltmeter, volvo dash analogue gauge, and fishfinder) when doing a long cruise.
 

Swafflife

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No...the exciter circuit is just a wire that gets energized by the ignition.

So, in order to test the field wire I should unplug it and put the positive lead of the multimeter on the field wire connection?

Thanks for all the help. Electrical is no where near my wheelhouse....I know just enough to tear stuff up!
 

bruceb58

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That would work. Better to do it with a load on it though so if its possible to measure it in circuit, it would be better. You could also unplug it and use a test light as well.
 

Swafflife

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That would work. Better to do it with a load on it though so if its possible to measure it in circuit, it would be better. You could also unplug it and use a test light as well.

Sorry, but please explain how I measure it in circuit.
 

bruceb58

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Might be tough if you can't jumper the plug to the alternator socket
 

Swafflife

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Spent about 30 minutes installing a new alternator today and I'm reading 14+ volts now :rockon: and my ACR is working perfectly! Thanks for all of the feedback.
 
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