It's a good idea to use a shampoo that doesn't leave anything behind when you have dreads. It is more important to use a shampoo that doesn't leave anything behind when you have dreads rather than when you just have normal hair because dreads hold a lot of stuff that usually falls out of normal hair when you brush it. The stuff will build up in the dreads and will cause the dreads to break down, loosen, and fall apart.

Some shampoos can be really bad for dreds because they leave residue behind on the hair, which can lubricate the hairs and cause the knots to fall apart. Residue is not a problem for normal hair because normal hair is straight and free flowing, whereas dreads and braids are a little harder to rinse well because they hold residues much easier because of the way it is tightly packed together. The residues can also cause itching and irritation on the scalp because they build up and the scalp cannot breath.

First you need to find out where the smell came from. If it's just a moldy smell it's probably because you don't dry your hair well after washing them. Or if you sleep when your dreads are wet. But you probably just need to wash your hair.

Washing your hair on a normal basis will keep your dreads from growing bacteria. But if you do have bacteria growing in your dreads soak them in rubbing alcohol and wash it out. This will kill all the bacteria.

Icky! Wash the dread locks really good, and soak the moldy part in rubbing alcohol. The reason mold begins to grow is because the dreads aren't getting dry, make sure you squeeze the water out of your dreads when you wash them, and towel dry them or use a hair dryer.