All your getting is the difference in voltage drop over the different circuits. Do yourself a favour and buy one of these
http://www.uktools.com/sealey-ppx-au...4v-p-5079.html
They are brilliant, put the two leads to the battery and there are two small lights which detect what you have , green which is for the earth and a red for the live, further more its got a rocker switch so if you need a live press it one way and an earth press it the other. You can then run round all the circuits testing everthing ,bulbs ,sender units, fuseboards the lot for live and earths.
Bob