Download Mercedes W123 Battery Ground Cable Replacement – Step 1 — Disconnect the battery Start by disconnecting both battery terminals, beginning with the negative (-) side, which travels just a few inches to a bolt attaching the other end to the car’s body. Never remove your positive (+) cable first or you risk injury from electric shock. Step 2 — Remove the old ground cable Loosen and remove this 13mm bolt that holds the cable to the metal of the car’s chassis. Now remove the old ground cable. Step 3 — Install the new cable You can see here that the new ground cable is substantially thicker. It is a 2 gage cable. The ends are also crimped better, protecting the inner wire from corrosion. This cable is also a bit longer than the old one, giving some more wiggle room. Step 4 Install the new ground cable, using the new captive bolt, and nut, that came with it.