87 buick no spark

87 Buick century cranks great but has no fire. I have changed the coil pack,ignition module,crank sensor cam sensor, and the pcm computer. This is a 3.8L it had a yellow spark until I changed the ignition module and coil pack, I have low voltage at terminals b+c on both the cam and crank sensors. I sure could use some help the vehicle has 49,120 miles on it.