this is what my gauge cluster is doing

[stevo]

ok turn the car on get a variable of volts from 12.7-13something, give it some rev and i get 14.56 no matter how high i rev it, it doesnt pass 14.56 but once i let go of the gas it drops back to 12.7-13something volts

Okay, it at least sounds like you can eliminate a bad alt from the equation. Sorry, I can't be more help further than that.

Stevo

[bonnie&clyde]

awww hell, anyone else???? i also grounded the case of the alt to the neg in the battery aswell, thanks for trying

[Blue88Coupe]

check out post number seven in this thread:

http://www.allfordmustangs.com/forums/2005-2010-mustang-gt-tech/57410-underdrive-pulleys.html

They're saying that from 1200-2800 the alternator should only be putting out 12 volts.

[Blue88Coupe]

Your car is idling at 500 rpm with an under drive pulley on the crank. I'd imagine that you're seeing the affects of the alternator turning way too slow at idle.

[jdgregory84]

My car did that exact same thing when one of the o2 sensors went out as far as the fluctuating idle goes. I'm sure it affected the voltometer to an extent, but I can't remember if it did it that much. I don't see how a charging problem would affect the idle. Those typically seem to be a it'll either run or it won't type deal. However, it seems that the very low RPM's could casue the fluctuation in the output of the alternator.

[davbrucas]

It's probably a combination of several things...like the UD pullies, battery, alternator..and accessories running at the time. All alternators tend to put out less at low RPM. Some are worse than others, and a very common mistake people make when they have a low-RPM charge problem is to buy an alternator with a BIGGER peak amperage rating. In general, if you have the same basic alternator in several different peak amperage ratings, the SMALLER peak amperage rating version will generally have more output at idle than the higher peak-rated version. Say you have an 80 amp (peak) and a 40 amp (peak) version of a particular alternator. The 80 amp version may only put out 10 amps at idle and 80 at 2000 RPM, but the 40 amp version may put out 20 at idle and 40 at 2000 RPM. This is because the higher rated version tends to have fewer turns of heavier guage wire in the windings. Of course its not true if the two alternators in question are actually a different design, say a Nippondenso vs. a Delco vs. a Bosch. The worst things in the world are the "super" alternators sold by car stereo companies. many of them go for huge 'peak' numbers, and are terrible at idle. With the UD pullies, this issue could be worsened. You say you have a new 200amp alternator. What brand? This may be your issue. Good luck driving at night on a really cold day in stop and go traffic!! How new is your battery? Alternators regulate voltage only, not current. At a set voltage, the current is determined by the state of charge of the battery.

[bonnie&clyde]

yeah i think after that 2800 is when it jumped up to 14volts, at idle wich is about 7-800 its only 12.78 or something like that but no more than 13.7, i just cant figure out why the hell its jumping like that

[bonnie&clyde]

My car did that exact same thing when one of the o2 sensors went out as far as the fluctuating idle goes. I'm sure it affected the voltometer to an extent, but I can't remember if it did it that much. I don't see how a charging problem would affect the idle. Those typically seem to be a it'll either run or it won't type deal. However, it seems that the very low RPM's could casue the fluctuation in the output of the alternator.

but if when i get it tuned they just turn off the 02 sensors it should not matter right??

[davbrucas]

Youre gonna trash your battery. Jumping around due to the UD pulley and variations in idle due to cam.

[bonnie&clyde]

Your car is idling at 500 rpm with an under drive pulley on the crank. I'd imagine that you're seeing the affects of the alternator turning way too slow at idle.

thats what im thinking too, so is it just always going to be like this?

[bonnie&clyde]

It's probably a combination of several things...like the UD pullies, battery, alternator..and accessories running at the time. All alternators tend to put out less at low RPM. Some are worse than others, and a very common mistake people make when they have a low-RPM charge problem is to buy an alternator with a BIGGER peak amperage rating. In general, if you have the same basic alternator in several different peak amperage ratings, the SMALLER peak amperage rating version will generally have more output at idle than the higher peak-rated version. Say you have an 80 amp (peak) and a 40 amp (peak) version of a particular alternator. The 80 amp version may only put out 10 amps at idle and 80 at 2000 RPM, but the 40 amp version may put out 20 at idle and 40 at 2000 RPM. This is because the higher rated version tends to have fewer turns of heavier guage wire in the windings. Of course its not true if the two alternators in question are actually a different design, say a Nippondenso vs. a Delco vs. a Bosch. The worst things in the world are the "super" alternators sold by car stereo companies. many of them go for huge 'peak' numbers, and are terrible at idle. With the UD pullies, this issue could be worsened. You say you have a new 200amp alternator. What brand? This may be your issue. Good luck driving at night on a really cold day in stop and go traffic!! How new is your battery? Alternators regulate voltage only, not current. At a set voltage, the current is determined by the state of charge of the battery.

i got my alternator from alterstart here in dallas, its a load boss alternator, the battery is brand new and has only been on this car since the build

[bonnie&clyde]

Youre gonna trash your battery. Jumping around due to the UD pulley and variations in idle due to cam.

but then why does it start real quick every time i go to crank it on, i would imagine that if it was draining my battery i would have more and more trouble starting it every time i try to start it right? but im not, it just cranks right up every time

[bonnie&clyde]

so davbrucas, your saying i should just go back to the stock alternator?

[Blue88Coupe]

Youre gonna trash your battery. Jumping around due to the UD pulley and variations in idle due to cam.

The car is idling just fine at 500-600 rpm. Bonnie&Clyde you're hitting the gas to show that the volts are jumping with rpm, its not doing that on its on correct?

[jdgregory84]

Youre gonna trash your battery. Jumping around due to the UD pulley and variations in idle due to cam.

The cam shouldn't make it idle like that. It should idle at the normal 8-900 rpms.

OP, getting it tuned and bypassing the o2 sensors might fix it IF that's the problem. I'm just stating from personal experience, my car had a very similar symptom when my o2 sensor went out. As long as the o2 sensors are good, there shouldn't be a need to bypass them. I had a H/C/I car with full exhaust, no cats, and didn't bypass the o2 sensors and it ran like a charm.