I just recently purchased my new to me/ used 96 Blazer. Here is what it does. If you are just driving down the road the volt gauge reads 14 Volts. Now floor it and when it downshifts the volt meter goes to 9!!! I'm considering swapping my alternator from my 94 Sonoma to see if it changes anything. BTW it has a new belt, idler pulley, and tensioner pulley. Any ideas??

Thanks

 

does the belt on it look like its ever been changed??? After awhile they stretch out, and can cause flutter and its starts to slip on the pulleys... also have you checked the battery charge at all?? do a test while its running to check for the charge and then do a load test on the battery...