C15 bxs won't start and making a weird clicking noise.

Did you shut it off or did it die on it's own. The weird clicking is suppose to be there. Check your battery connections and clean them up good. Nice and shiney

 

Help us help you. Does it crank, but not start? Or does it not even crank? Why did you change the batteries? Make sure that all the connections are good and that you put them all back where they came off.

 

The clicking you are hearing is probably the intake actuator solenoid, that is normal for an Acert. Check the ECM fuse(s), they usually in the battery box, but not always. Some trucks have 1, some have 2, it depends on what kind of truck. MOst trucks will have ECM power and ground right to the batteries with inline fuses. Make sure that you have a good connection. There is also a fuse (about 5 amp) for the key switch power to the ECM. I'd have to guess that it is OK because the clicking is telling me that the ECM is powering up. It may not be fully powering up though, they have 2 or 3 powers and grounds to the ECM.

I'd focus my efforts around the battery connections first, because that's what you did last. Also check all the connections on the starter, you may have wiggled some wires at the starter while changing batteries.