Low Voltage at Idle

Mike Holt

Member
Replaced my alternator a couple of weeks ago.
Tonight, we took the car to dinner and I noticed at idle, the meter was only reading ~11.5 volts. As soon as I bumped the throttle, I’d get ~13 volts.

Ground?
Bad Battery?
No Moon?

I’m planning on putting it in the air tomorrow anyway, to make sure everything is good for next weekend.
 

Danbo

FFR6198
Contributing Member
Mike, were you running your fan and lights?
What type of alternator are you using?
 

Mike Holt

Member
Dan,
Standard Mustang replacement from NAPA.
Had fan and lights on. When I used turn signal, you could see meter fluctuate. But, even prior to using lights, I noticed it drop to 12v at idle.
 

phil mead

Contributing Member
Member
Definitely "no moon". :) Hey does the new alternator still make the noise? If nothing else was changed I would suspect bad new part. NAPA will replace it.
 

Mike Holt

Member
Phil,
No noise. I also used my stethoscope to listen for the “whistle” I mentioned. Not the alt nor the water pump.

When we were at Meding’s, I mentioned to Rob that my check engine light was on but the car was running great.
The next day, I pulled codes and it said “right bank O2 sensor reading lean”. I cleared codes. Last night was the first night driving since then. The car idled a little lower than normal. But, I put it down to the puter relearning. Then, on the way home I noticed the voltmeter issue.

Related?
 

Rob Burton

Contributing Member
Member
Mike, Most if not all of those NAPA replacement alternators are rebuilt units. You may have gotten one that was not rebuilt well...
 

Mike Holt

Member
Mike, Most if not all of those NAPA replacement alternators are rebuilt units. You may have gotten one that was not rebuilt well...
I’m going to throw it on the lift today and double check the grounds. If they’re OK, I’ll probably pull the alternator and take it back.
I should probably pull the battery as well and have it tested. I can’t remember when I last replaced it. Could be better than 3 yrs ago.
 

Scott Harrison

"Growing old is mandatory, growing up is optional"
Staff member
Contributing Member
Member
Standard Mustang alternator is not a one-wire.
It's possible the exciter circuit is faulty and not providing sufficient voltage until the revs come up.
 

Scott Harrison

"Growing old is mandatory, growing up is optional"
Staff member
Contributing Member
Member
From the WEB

Loose or corroded connections on the back of the alternator can increase resistance and restrict the current flowing through the circuit. So can broken or frayed wires inside a connector or the alternator wiring harness. The connectors and wires may appear to be OK visually, but unless you actually test them, you have no way of knowing if they’re making good electrical contact and are clean, tight and undamaged.
If the wires and connectors are not checked, you may replace the alternator only to discover the new unit you just installed is “no good.” Now you get to replace it again and, on some vehicles where the alternator is buried under a lot of other stuff, that can result in a lot of lost time and labor.
You can do this test by using your voltmeter to perform “voltage drop” checks across the connections when the engine is running. A voltage drop test is done by setting the voltmeter on the 2-volt scale, then touching the positive and negative test leads on opposite sides of a connection. If there is resistance in the connection, some of the voltage will try to bypass the resistance by flowing through the voltmeter. If you see a reading of more than 0.2 volts, it means trouble. Ideally, the voltage drop across any connection should be zero, or less than 0.1 volts.
Check for voltage drops at the positive and negative battery cable connections, the alternator BAT+ power connection and the engine ground strap(s). Poor ground connections are an often-overlooked cause of low charging output and alternator failure. Voltage drops on the positive side of the charging circuit can cause undercharging. Voltage drops on the negative side can cause overcharging.
When a new alternator is installed, check the battery voltage and use a battery charger to bring the battery up to full charge before you return the vehicle to your customer. Also, start the engine and use your DVOM to check the charging output of the alternator. Don’t assume everything is working OK just because you bolted in a new alternator.
 

Danbo

FFR6198
Contributing Member
Mike I have a power master one alternator 90amp, with no electric draw it’s at 14.2, with the fans on it drops for me. I’d say your good.
 

Mike Holt

Member
Fixed. Couple of culprits. Bad battery. Wouldn’t hold a load. Also insufficient grounds (unverified, but I only had a ground from starter prior to this. And of course negative battery cable to block). I added an additional ground from the block to the frame. Volt meter holds a steady 13.5 on the gauge whether at idle or speed.
 
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