Does this mean there is a problem with the original accessory or circuit (which uses the lower Fuse Slot A)? Or could the problem still be with the new accessory or wiring that I added (upper Fuse Slot B)?

The original/host fuse is 5A, the new accessory fuse is 10A. It's the 5A one that keeps blowing (immediately when I turn on the power/ignition). The 10A fuse is fine. Does this mean the cause is a draw greather than 5A, but less than 10A? (Or does the Add-a-Circuit always cease working once the lower host fuse blows?)

The weird thing is that this setup has worked fine since June. All of a sudden the 5A fuse blew today and replacing the fuse doesn't fix it so I need to know where to start troubleshooting. Thanks in advance!

 

I forgot to mention: When the 5A (original) fuse blows, the new accessory (on the intact 10A fuse) stops working.

 

It's definitely the new accessory. I reversed the polarity/orientation of the add-a- circuit to the correct way, and the host fuse survives and the new accessory fuse blows. All the connections appear fine so I will need to trace the whole wire. Could it be an internal failure of the accessory too?

 

I rewired everything (but re-used the accessory's pigtail wires and spade connectors), and even after taking power straight off the battery and using a different ground, it blew the in-line fuse every time as soon as contact was made. Does this point to the accessory itself as the problem (internal short, etc.)?

 

Yes, it means there is a short circuit in the "mystery" accessory after the fuse.

 

Thanks! It's definitely the new accessory. I reversed the polarity/orientation of the add-a- circuit to the correct way, and the host fuse survives and the new accessory fuse blows. I rewired everything (but re-used the accessory's pigtail wires and spade connectors), and even after taking power straight off the battery and using a different ground, it blew the in-line fuse every time as soon as contact was made.

BTW, the accessory draws way less than 10A. Because I had the add-a-fuse in the wrong orientation, the 5A fuse was covering both the host circuit and the new accessory. I will replace the new accessory fuse (in the upper Slot B) with a 5A or lower rated fuse.