Can do volt drop testing with car running, along the cables to see if you're losing power or ground.

You need to see as close to 0v as possible when going from the battery to the pump on the power cable, then same along the ground.

If your meter shows voltage when testing that will be where it's losing voltage (volt drop)

If for arguments sake you test the pump and its getting 12v when running, and you have 14v across the battery, you're losing 2v somewhere, if you volt drop test the ground and it shows 2 volts..hey presto..

More on volt drop testing here. http://www.fluke.com/fluke/uses/comu...sevoltdrop.htm