what exactly does this do?

 

They are transfer pumps that supply the injection pump. There are various styles and sizes.

 

Yep, Redliner's correct. The FASS replaces the stock transfer pump (aka lift pump) which is poorly implemented by Dodge and is prone to failure. Failure of the transfer pump also takes out the much more expensive injection pump soon after.

Mike

 

If your thinking of doing any kind of fuel upgrade (eg. Chip install) please consider installing one of these pumps on before hand weather it be a FASS or Air Dog. This one upgrade will save you much unwanted hassel down the road.

Rob