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Discussion Starter · #1 ·
Today my car was idling at like 12 volts and I thought it was supposed to be 14.4 or something like that so I went to get my battery and alternator tested at advance auto. They wouldn't bring the tester outside because it was slightly raining so I took both of them out and they both tested good. I put them back in and the voltage is at 13.8 now at idle. Is this the correct voltage?
 

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high? far from it. normal voltage running on any car should be anywhere from 14-15. if you are seeing 13.8 your about a volt low, your alternator voltage regulator is going out, you need an alternator. if your seeing 12 volts, your alternator is gone and your running off the battery.
 

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sitting at idle will be a little lower than when the rpms are up. idle voltage at 13 is about right, especially if he has underdrives.
 

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Discussion Starter · #5 ·
high? far from it. normal voltage running on any car should be anywhere from 14-15. if you are seeing 13.8 your about a volt low, your alternator voltage regulator is going out, you need an alternator. if your seeing 12 volts, your alternator is gone and your running off the battery.
No underdrives since I'm supercharged. Why would the alternator test good if it's bad? Also the voltage does increase slightly as the rpms go up maybe .5-1 volt? It was still under 14 though. Most of the time 12.5 at idle and 13 at 2500rpm. When I first turn the car on it's 14-14.2 and drops from there.
 

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Here's the voltage info from the shop manual...



The "A" circuit is the battery voltage sensing--used by the voltage regulator to control output;

The "I" circuit is from the ignition system and used to turn on the regulator;

The "B+" circuit is the alternator output;

As you can see there are no decimal/fractional values shown--this is because they are irrelevant in the overall scheme of things, keeping a wet lead-acid battery charged does not require that level of precision.

The 13.8V value hardhitter is seeing is probably because his battery is no longer perfect and is starting to develop some internal shorts across the plates. Most car batteries get this way after 3+ years, they'll still test OK on a typical battery tester but it's not new anymore.
 
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Discussion Starter · #7 ·
It's an optima battery, I thought these were supposed to last longer? I've only had it for 2.5 years.
 

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It's an optima battery, I thought these were supposed to last longer? I've only had it for 2.5 years.
There's nothing wrong with it, it's just not new anymore. You probably had a bad ground at the alternator (the regulator needs a good ground reference to properly charge the battery) or battery, and when you yanked 'em out to have them tested and then replaced 'em things got better.

If the regulator was undercharging the battery for a while (6+ months) then the battery could develop internal shorts from being undervolt.

Now for my opinion, flame me if you want--Optima batteries are overrrated.

At one time, when they were first introduced 20+ years ago, they were superior to anything on the market. However right now in 2008 they are no better than any other manufacturer's top-of-the-line battery--in fact (like other top-of-line batteries) they are more sensitive to improper charging than most "el-cheapos".

The el-cheapos will only last a couple years, but they take undercharging abuse better than good batteries.
 

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So new battery should fix it?
There's nothing broken right now--right? If the battery is charging, and you are getting 13.8V or so at idle then you're OK, within Ford's specs.

Are you still getting a DTC?
 

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Discussion Starter · #11 · (Edited)
There's nothing broken right now--right? If the battery is charging, and you are getting 13.8V or so at idle then you're OK, within Ford's specs.

Are you still getting a DTC?
I was never getting a trouble code. Well I thought the voltage was okay but its still goes down to 12.5-13 volts after the car has warmed up. Also after the car sits for 3+ days it won't start.
 

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Discussion Starter · #13 ·
Also the reason I'm worried about it is because I'm going to put a sound system in my car and I want it to be at the correct voltage.
 

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The alternator is just grounded by the engine so how could it be bad? There's no ground wire.
As you have said, it is grounded through the top mounting bracket and the lower bolts, to the block--which is connected to the negative battery post by a cable. Corrosion at the points that the alternator connects to the block could cause the regulator (which is in the alternator) to have a bad ground reference--it would be higher than the negative terminal--which would cause undercharging of the battery.

It sounds as though sometime you don't drive the car for days at a time? Do you also take only short trips?

If so then the battery may never be fully charging and you may need to keep a trickle charger on it when it sits for 2 or 3 days.
 

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Discussion Starter · #15 ·
No I drive it about 50 miles a day on weekdays and I've only let it sit like that 3-4 times.
 

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Then what I'd do for now is keep an eye on it.

Just out of curiosity, what is the voltage just sitting there an hour or so after driving 50 miles? It should be around 12.6V, if not then the battery has some shorted plates.
 

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Discussion Starter · #17 ·
Well I only do 25 at a time with a break in between. I've never tested it after doing that. I drove a lot last night and right now the voltage is at 11.98.
 

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Well I only do 25 at a time with a break in between. I've never tested it after doing that. I drove a lot last night and right now the voltage is at 11.98.
That's low, sounds to me like the battery has some internal shorts that might not trigger a simple battery tester like the FLAPS use.
 

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Discussion Starter · #20 ·
The battery also gets really hot if I put it on a charger and it sounds like it's sizzling inside. That doesn't sound right does it?
 
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