Pontiac Repair: not cranking over, cold crank, negative cable

Question
i have a 78 grand prix with a v8 305 5.0 and my batterie want out wich then the alternator went out so i changed both the new batterie seemed to not have enough power cause it only turned it on once then would not even jump start, so i exchanged it and the same thing happend so i got a stronger batterie i went from a 660 cold crank to a 870 cold crank and it turnd on a few times and ran great but now the same problum has returned. im not running any accesories and cant figure it out what can i do? could it have any thing to do with the auto choke or the starter? it cranks once then feels like the power drained but the batterie still reads 12.60?

Answer
Hi Mike:

My guess would be wither a starter motor that had gon bad and has an excessive draw or a bad battery cable that has too much resistance in it.. check the battery volatge with a digital volt gauge. then chec the volatge at the starter when you crank it it should be the same if it's not the cable is bad. If you have long enough leads on the meter you can do what is valled a volage drop test and check for high resitance in each cable. put the meter on dv volatge scale and put one leak on at the battery and the other at the starter motor and crank the car the meter should read more then .025 Volts if more then that the cable is bad. do the same with the negative cable that could be bad as well. If the drop volatge is good less then .025 I would suspect a bad starter motor. good luck..
Oh P.S. make sure the negative cable has the little lead on it going to the body as well at the engine block.