Alternator concern

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cerd

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I put this in general because I have a 1990 G30 chassis MH which uses the same 350 TBI used on almost everything with a SBC on the 90s, so I didn't think it was MH specific.

I recently replaced my alternator with a Reman from CarQuest. I got the "heavy duty" model which is 105A. Yesterday, I thought my voltage was a bit low on the gauge, although its hard to tell on those old ribbon cluster panels due to small circuit busses. So I measured it with everything turned off and it reads 13.4V. When I turn on all of the lights and accessories, it reads 13.1V.

I am not so familiar with older automotive tech. My 2002 Buick has an 85A alternator, but uses almost no power. My 2005 Dodge has a 160A alternator. Both of them measure 14.5-15V, so 13.1 seems low.

My front battery reads 12.4v with everything off and my smart battery charger reads 75% life on it. Is my front battery starting to die causing a low running voltage or did I get a bad reman alternator? Or, is 13.1V acceptable for an older vehicle?
 

Gary RV_Wizard

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Indications are the battery is on its last legs.  It should always show 12.6v at full charge - 12.4v is only about 75%.

Typical automotive alternator output is 13.6-14.4 volts depending on circumstances. However, a weak battery can make the output appear low. Has the battery been load tested?  Most auto parts stores can do that (free) if you bring it in.
 

John From Detroit

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One of the advantages of an alternator over the older generators is they still work at full idle. but ... what they don't mention is if you load 'em down they do not put out full voltage at full idle.  I assume you were measuring voltages at full idle on the 13.x readings.  Anything over 12.6 is good at idle. It should kick up if you go to Fast Idle (over say 900 RPM).
 

cerd

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My smart charger has a recondition feature that sometimes descales the plates. I have about a 50% success rate with it bringing back a battery that reads below 12.0. I left it on that feature under the hood since yesterday morning. When I checked it last night, it was still at 12.4, but sometimes it takes a full 24 hours to finish. I will check it tonight.

If its on the way out, I will probably use it for the weekend since we are leaving tomorrow morning and replace it when we get back. There's a battery shop nearby that sells blemished Johnson and Exide batteries for $40 plus core with a 1 year warranty. They are my go to guys for all automotive batteries.
 

cerd

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So last night, I tested both batteries. They both tested good. The engine battery is a little lower, testing at 12.5V, but still about 90% health. The house battery was at 98% health. I took the fuse out of the solenoid that connects the house battery to the charging system while the ignition is on and it was sitting happy at 13.8V. When I plugged the fuse back in, it dropped again to 13.5V.

At this point, I think I am going to have to drive it to the next town over and have Advance Auto run their full charging system tests on it. While I am there, I am going to see if they honor returns from their daughter company, Carquest. I do have a few auto parts stores in town, but they don't have the full on-vehicle charging system tester like Advanced does.
 

cerd

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12.5 not running. 13.5 when running and both batteries are connected and 13.8 when only the engine battery is connected.
 

John From Detroit

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Note if you are running those tests at FULL IDLE..

From the test results. the house batteries are sucking power.
at low RPM the alternator has limited output (AMPS) and if the voltage may drop because of that.

Try this.. if you can get a plug in voltmeter (RV stores have 'em so do auto parts) that plugs into your 12 volt Dash accessory outlet.

Start car. note voltage.. Rengine at oh say the equivalent of 40 MPH note voltage
Turn on headlight sitll at about 40 MPH engine speed note voltage

Drop back to idle and note voltage

You should learn from that sequence.
 

cerd

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John From Detroit said:
Note if you are running those tests at FULL IDLE..

From the test results. the house batteries are sucking power.
at low RPM the alternator has limited output (AMPS) and if the voltage may drop because of that.

Try this.. if you can get a plug in voltmeter (RV stores have 'em so do auto parts) that plugs into your 12 volt Dash accessory outlet.

Start car. note voltage.. Rengine at oh say the equivalent of 40 MPH note voltage
Turn on headlight sitll at about 40 MPH engine speed note voltage

Drop back to idle and note voltage

You should learn from that sequence.

Theoretically, the voltage shouldn't change much. On my newer alternators, the voltage stays within 0.2V due to the regulator, but the current does change. I do not have a proper current meter to measure the output current. Otherwise, I would do that. That is why I think making a trip to have a proper test would probably be my best bet.
 
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