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Sounds like a dieing alternator to me.

Grab a meter (even a cheapy will do) and put it on the battery post's.

Should be close to 12v

Have someone fire it up and it should be close to 13.5v

Then slowly start turning things on, it should never drop below 12.5 with the car running 12v at the least.

if below 12v on bat car off have bat checked.

if it drops with running below 13v, pull the alt and take to local part's store they will test for free usauly.

 

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You can always do the shadetree alternator test..... fire the car up and then pull the positive cable off the battery. If the car shuts off alternator is bad. Just be careful... the red wire bites... not that I'd know from experience

 

Easy to do and aways right.

Also, about that battery test, with every thing on, Heater, windsheild wipers, radio, light ect.... Place the test leads on the appropriate terminals on the battery. With the car stopped NOT RUNNING you should still get very close to 12 volts.

If it is way off you have a dead plate in your battery. And it is not recharging as should be.

 

I took it to the shop and he said he thinks the regulator isnt kicking in

 

That could certainly be the culprit. Although I'm not sure why the lights would dim while you drive and not at idle. Usually charging problems work the other way around.

 

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Has the car ever cut off while running or not started? If not you probably have a bad connection somewhere..... start at the battery and go from there. Don't neglect your ground wires either!

Also, how did your mechanic come up with belt slipping? Was it squealing or was the belt shredded?

 

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IDENTICAL PROBLEM as I am having. I am on my third alt in 3 days just to make sure. All the alts are good and i tried 3 differant optimas. same problem. I tried all new battery cables too but ni dice. I am at a loss. Autozone said it was only putting out 40 some amps. I even put a good alt from the formula and it put out 100 something in the bird and when in camaro put out 25-40. We dont know whats wrong.

 

Well I got a new alternator today and it seems okay but we will see tonight when i go to work....