im having a real hard time tryin to find out what alternater i need. i just bought a 85 camaro z28 vin F. i dont know wat amps i need, the car wont charge with a 78amp? can someone help me out.

 

What do you mean?

It won't charge at all?
It won't charge at an idle?
The battery is always dead?

A typical older car only needs a 30 amp alternator. More modern cars still using a 10SI alternator can get away with a 63 amp alternator. 1996 and up cars now use a CS130 alternator.

 

yup the battery always dies. its charging at 12.4. mayb it the wire harness its in pretty bad shape. not sure excatly the best way 2 replace it?

 

Have your battery tested. It might not be holding a charge. If in doubt, try a different battery. Batteries don't last forever. Typical life expectancy of a battery is 5 years. If it survives to 10 years, it's doing really good.

If the battery tests ok, have the alternator tested. If it passes then you have a wiring problem.

 

12.4 volts is what a half-dead 12 volt batt would be reading.

With the eng running, and all the accessories turned off, you should have over 13 volts if the alt is working at all. Typically the alt puts out 13.5-15 volts if working properly.

What's the voltage at the post on the back of the alt with the eng running??

 

Typically I like to see 13.8v charging. Having a battery with a dead short in it will make it look like a bad alternator. A battery with a dead short also won't hold a charge. A simple load test will say if the battery is good or not. Most garages or parts store can easily test it.