What voltage should the alternator be putting out?

[CobraSix]

Lately I've been having a problem where the battery doesn't seem to stay charged. Originally thought this was a dead cell in the battery, but I replaced that. now the new battery (only like 2 months old) doesn't seem to have enough charge to start the car if it sits more the 24 hours. I ran a couple of tests:

Voltage across battery terminal: 11.98V
Voltage across battery termincal while starting: 6V
Voltage across alternator while running: 12.46V
Voltage across battery terminal while running: 12.46V

The alternator is less then a year old, the voltage regulator is like 8 months old, the wiring is less then 1 year old, and the battery is 2 months old. I'm stumped. I can't get enough juice while starting. is my alternator putting out enough Voltage? I always though it needed 14V or so. Thanks.

Slade

 

[Falcon62]

Slade - Does the output change if you bring the engine up off idle - say 1500 - 2000rpm?

Otherwise, engine running, you should see ~13.8 - 14.3vdc across the battery. Engine off, battery voltage should be ~13.8vdc. Sounds to me like you have a charging problem and it's either your alternator or regulator (I'd bet on the alternator). Pull the alternator and have it tested.

Of course, this assumes all connections are clean and tight and alternator belt tension is correct.

 

[CobraSix]

Phil,

No change in voltage at 2000 RPM. I tested that one, forgot to mention it. I tested another alternator, it was definitly bad, only putting out 11.98V at idle, 13V at speed. So my connections are all pretty tight. Everything looks good.

Slade

 

[Anonymous]

Slade;
The symptoms you describe can occur 2 ways, presuming wiring and belts are OK:

1. Voltage regulator. If a mechanical type, it can be adjusted with a small wrench and screwdriver, under the cover. Smaller gap=higher volts. If a solid-state type, there is usually no adjustment and replacing with a different unit is the best way to test: see if a significant difference occurs with a different regulator. I HAVE seen these bad "out of the box" when some other guy misconnected it, then returned it, and it got sold again because the Parts Counter Guru didn't know from straight up about it..
2. Bad diode (or diode connection) in an otherwise-good alternator. Typically, these units are poorly tested on rebuilt alternators: they get tested with an ohmmeter and if more than a 10:1 difference shows up in the forward-reverse polarity test, they 'pass'. However, the forward-reverse values should be >1000 to be considered good on small alternators. Also, the forward resistance should be tested for all 6 diodes inside: if they vary from one another more than 5%, the whole pack is suspect, as it will generate much less current than it should.

Hope this helps.

Merry Christmas!

 

[CobraSix]

I got a new alternator from Autozone. It was free because they couldn't test my old one that was under warranty. I'll plug that in tomorrow, if that doesn't do it, then I'll go after the voltage regulator again.

Is there something that may be causing these problems, or just sheer dumb luck, since my entire charging system is practically new.

Slade

 

[MustangSix]

You should definitely be over 14V at a fast idle. One thing that should help if you don't mind the non-original part, is to use a solid state regulator from a later model. It bolts right in, has the same connections, but is only about 3/4" tall. The voltage is more stable and you are not at the mercy of the points and temperature variations in the old style regulator.

 

[CobraSix]

I'll have to go out and check, but I think that is what I have now. I noticed when I put my new one on, that it didn't ahve the adjustments my old one did. Does that mean it's solidstate?

Slade

 

[MustangSix]

The solid state ones have no adjustments. Grab one for an 80 Fairmont. It should look like this:

Price is about $10 at any autoparts store.

 

[CobraSix]

That's what I got then. I'm going to swap alternators today, if that doesn't do it, I'll do the regulator since it's warranteed as well. That's the only reason I go to Autozone, nation wide warrantees on parts. I've only had to 'buy' two alternators from them ever. The other 6 have been replaced under warrantees. I always pay the little extra to get the lifetime warranteed parts, like waterpumps, regulators, etc. I figure I'm going to have this mustang for a while, I might as well make the parts free to replace.

Slade

 

[CobraSix]

New alternator didn't help.

New Regulator, still same problem.

I guess after christmas I'll just drive up to autozone and keep replacing stuff until it works and I can drive away. This is getting frustrating.

Slade

 

[Lazy JW]

I like to see at least 14 volts at fast idle. You can get by with a little less if your vehicle doesn't have many accessories and you make relatively long trips. I had a GMC pickup that put out almost 16 volts, it was hard on batteries until I put in an Optima battery. That battery is 8 years old and the man I sold the pickup to is still running it.
Joe

 

[65coupei6]

Also remember you need a good ground to have a electrical system work good. Use a 4 gauge wire from the battery to the block. You can get it at an auto parts store or any audio store. Also run another ground from the neg. terminal to the radiator support. Use at least 10 guage for the second ground.

 

[BIGREDRASA]

I had a problem with mine when I got it. It turns out the alternator was not properly grounded. The alternator does not ground through the mount. You must run a wire from "Grnd" to the block. That solved my problem. 8)

 

[Falcon62]

Slade - There has to be something simple being overlooked. The charging system on your 65 is about as uncomplicated as they come, and it sounds as though you've covered everything that would matter. Double check all your wiring to and from the regulator.

 

[CobraSix]

I feel stupid now.

My old alternator was bad. It was only putting out the voltage at the top. Plus, it had a bearing going bad.

After all this, I took a break becaused I knew I was getting frustrated.

My problem was when I re-hooked up everything. It came to me after reading Phil's post, and looking at my alternator. I remember having three wires with caps attached to three posts on the alternator with cap holders. Well, when I looked at my alternator, I had one wire hooked to a post without a cap adaptor that had a cap. traced the wires to the regulator, found out I had the FIELD and the GROUND wires hooked up backwards. As soon as I jumped it this time...instantly got 13.5Vs at idle.

Thanks.

Slade

 

[Falcon62]

8)