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Discussion Starter · #1 ·
I started my gt up yesterday, and let it run for little while. I notice that it runs little rough thinking it needs to be timed. But I noticed that the battery gauge was dropping a little when it was running but when I hit the gas it go back up does that sound like a bad alternator?? I brought it in to be tested and said it was ok, but putting two and two together I think its bad
 

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Try checking your connections, make sure there clean and tight. If that's okay, have your battery load tested if the needle on the gauge drops quick your battery is bad. It sounds like something minor, you could have a bad ground. Most part stores, will ceck your battery by load testing it. That will show, if it has internaal damage or just an old battery, Mike. SCT Tuner.:bigthumbsup
 

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Discussion Starter · #3 ·
battery is brand new and timing is little off would that be a problem?
 

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Do you have underdrive pulleys? If so, that can be normal.

Otherwise it could be a bad alternator. Normally a bad battery will not drop when the engine is running because the electrical system is entirely powered by the alternator. The exception would be a bad alternator system or perhaps underdrive pulleys slowing the alternator too much.

Tom
 

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Discussion Starter · #5 ·
Tom

Everything is stock but brought the alternator into the auto store they tested it and said it was ok unless the bushings are going bad
 

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If i read right, you let it idle.
If thats the case, your car was just useing more power then the alternator was putting out at idle.
Mine does the same thing, i have to go up to almost 1200 before it charges.
I plan a bigger alt soon as i get to it.
 

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Discussion Starter · #7 ·
how many amps you think should buy?
 

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Depends on what you need.
Big Amps? Big Electric Fans? Lots of Lights?

The original i believe is 75 amps, they make a 110.
Most of the 3G ones i have seen are 130.
For Real energy hogs, i have seen up to 240 amp.
Look around for, or google for an alternator OverDrive pully.
It could help a little, and cheaper then a whole alternator.
I saw one on Ebay, i was concidering getting, think it was around 12 bucks.
 

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Just get a 3g from a 94-95 mustang. The pulley on the 3g from an SN95 car is real close to most underdrive pulleys.
 

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Tom

Everything is stock but brought the alternator into the auto store they tested it and said it was ok unless the bushings are going bad
If you are using standard drive pulleys there is no reason to have a substantial battery voltage drop at idle in a properly working system. Here is why.

A freshly fully charged battery with NO external charge or load sits at 2.2 volts per cell when the charging is first removed. There are six cells for 13.2 volts. After a short period of time it will come down to 12.6 or so all on its own and stay there. The normal fully charged voltage is 2.1 volts per cell, and six cells, so it is 12.6 volts. That's why radio tubes for old 12V car radios were all 12.6 volt tubes. :bigthumbsup

If you have a good fully charged battery, it will measure 12.6 volts after sitting without being charged or loaded for a while.

My car with an Advance Auto standard replacement alternator stays at 14.60v at 800 RPM idle. If I turn on the high beams (which I have modified to run both lows and highs at the same time) the battery is at 14.50 volts at idle.

At high RPM it goes to 14.60 volts....no more. With the lights and stuff on, it drops to 14.55 volts when the engine is at 2000 RPM.

With headlights on high beam and other accesories running, it drops to 14.50 volts at idle.

IMPORTANT VOLTAGES TO REMEMBER!!!

Gassing voltage that you do not want to ever exceed is about 14.50 volts. I do not know of any lead-acid battery that is designed to take over 14.50 volts for extended periods.

Charging takes place at 14.2 to 14.5 volts.

The lowest it will go with a fully charged battery under lights and accessory load is about 12.3 volts.

If you are seeing a big swing on the volt meter it is not normal, unless perhaps you have underdrive pulleys. An oversize alternator pulley or undersized crank pully will at best save a small fraction of a horsepower on an alternator. It is NOT worth it. Anyone who says it is doesn't understand how an alternator works.

If you slow the alternator down, it CAN ruin the low speed charging but when the motor revs it will draw almost EXACTLY the same horse power! This is because the voltage regulator controls the power demand of the pully. If you slow the pulley down, the regulator inreases the field current and the alternator demands more torque from the belt system. It is a closed loop unless you make it so slow it can not supply the demand the regulator calls for.

Underdrive alternator pulleys are a consumer hoax.

You can tell more by measuring your battery voltage in the car than any parts store can tell with a tester.

I designed battery testers and voltage regulators in the 80's, and we did hundreds of tests on alternators at our plant. We made all the test gear for virtually every manufacturer and most retail stores.

You could have a bad connection someplace (not likely at the battery posts for what you see, but elsewhere is more likely like perhaps negative post to vehicle chassis). You could have a bad alternator.

I didn't buy a bigger alternator for my car because the maximum demand I have is only 28 amperes. That's high beams, A/C, windshield wipers, ignition, power supply for computer, and slightly larger than normal stereo. If you feel better buy a big one (it won't hurt), but a properly working normal size is more than enough even with cooling fans as long as you run a normal pulley.

Tom
 

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