General fuse question (build up on fuse blades)

[cubangt]

Helping a friend with his truck, ac stops blowing from time to time.. no rhyme or reason.

We started with the simple, check fuses and relays..but of course when we go to test, everything is working..

 

But fuses and relays all seem to be working and not blown..

 

the only thing i found was that the fuse blades, had alot of brown/black build up on them.. I clean them up so that they were all shiney again and reinstalled, but ive never seen that before.

 

Does anyone know what would cause the fuses to have brown/black build up on them?

 

I told him to go ahead an pick up a set of fuses and we can replace for a few bucks.. just to know that everything there is clean and new.. but concerned about the build up.