Alternator tests fine, only makes 12.4v on car???

JC6715

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Oct 24, 2005
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Been struggling with some charging issues. Newer battery (2 months old), newish alternator, voltage regulator (electronic) and solenoid (all about a year old or so).

With a fully charged battery everything is beautiful, starts and runs like a dream.

After taking for a 15 minute drive I'll shut her down and won't have enough juice to even turn the starter over.

Checked voltage at alternator when running and it's reading about 12.4-12.6... So I figure it's the alternator, take it to the local parts house and have it tested. They say it is fine.

I hooked up my new Auto Meter gas gauge and noticed it is jumping around a bit... This is leading me to believe a new voltage regulator is in order?

Another weird thing i noticed is when I flip ignition switch "on" to activate gas gauge it goes to where it needs to be. After a drive where the power has been drained and won't restart, when I switch to "start" the gas gauge loses power. Could this mean I have a short somewhere?

Please help, thought I knew what I was doing, but am starting to second guess myself...
 
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SoCalCruising

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When I bought my '66, I was warned that the battery would discharge if I left the battery cable connected. Sure enough, the battery would drain down to nothing. I destroyed two batteries before finding the culprit. For my case, the original alternator had been replaced with a one-wire. Where the alt. harness had been modified, it was shorting. I went to a 3G alt. (probably not important) and in the process replaced the alt. harness (which is when I identified the short). That solved my problems with battery drain. Again, the alternator replacement had no bearing on the fix. It was the bad alt. harness, in my case. If your battery drains when the car is sitting (ignition off), I would definitely look for a short. If not, and your alt. is functioning properly on the bench, then look for installation glitches. Once, my volt. reg. plug was loose and not making contact. In turn, the alternator never received the needed 12V signal to start putting out. Another time, the ground was not good on the alt. (I used a new, painted spacer to the block and the grond was inadequate). I added a separate ground to the alt. case and all was well. So, there you have my experiences. Hope that helps you think through this.

If you have a wiring diagram and know what the outputs should be, then you can check to see that you alternator is seeing the correct voltages, etc. in Ignition ON and OFF states. You will need a voltmeter, but you seem to have one from your post.

-just saw Brian's post. That's a good point, too.
 

Hardtop68

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12.6 V is too low... I am used to see 13.5 V... 12.6 V is battery voltage... if alt is OK check regulator... I had the same problem with an AUTOLITE electromechanical regulator, it was sending only 11.8 V thus not charging the battery... I replaced it with a new electronic one...
 

JC6715

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Oct 24, 2005
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It sounds like a weak ground to me. Is the block grounded to the chassis?
Yes, and I've checked the connections to ensure they are clean and tight.


SoCal - I will take another look at my alternator wiring. I could definitely see that being a problem source. I need to just rewire the entire car as it looks like ALL of the PO's did crappy jobs of adding or fixing problems...

There is no way to test a voltage regulator is there? It's just a replace and if it fixes the problem, the reg was the problem? Trying to diagnose before I start replacing stuff...

Durned Electrical Gremlins.
 

SoCalCruising

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Well, on my 3G alt., it has to have a 12V "signal" for it turn ON. I know where the 12V input to the alt. is, so I checked to see if Ignition ON supplied the 12V via the reg. It didn't when the connector was loose, so I was pointed to the reg. In my case, the plug was just loose, but it acted the same way a faulty reg. would act. If you have a service manual, you can look at the wiring diagrams. If it's not original, then I don't know ... fortunately, regulators are cheap. You should also check to see that ignition ON supplies the reg. with 12V.
 

JC6715

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Oct 24, 2005
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Hillsboro, Oregon
Well, on my 3G alt., it has to have a 12V "signal" for it turn ON. I know where the 12V input to the alt. is, so I checked to see if Ignition ON supplied the 12V via the reg. It didn't when the connector was loose, so I was pointed to the reg. In my case, the plug was just loose, but it acted the same way a faulty reg. would act. If you have a service manual, you can look at the wiring diagrams. If it's not original, then I don't know ... fortunately, regulators are cheap. You should also check to see that ignition ON supplies the reg. with 12V.

Exactly what I was just thinking. Trying to work while you've got Mustang on the brain is tough!

Good advice, I will do that right after work today and hope for the best.... Thanks.

Edit: And now that i think more about when I was trouble-shooting, I was only getting the 12.4 from the battery post on the alternator, and nothing from any of the other alternator posts. Thought, "Mmmm, that's weird," but that's it. HA HA! Narrowing it down...