rmgill said:
Why not get a longer cable and attach it to the slave plug so you can run it to the batteries? Or you could get a shorter cable and run that down to the frame to get a better connection to the bolt connecting to the ground lead you attached to the cab sheet metal. That'd give you an additional grounding path over the one that runs from the engine to the cab sheet metal and then back to the frame.
Ideally, each device should have its own dedicated (ground strap) ground return.
The frame is then connected to the batteries.
The starter motor/engine is connected to the batteries.
The cab is connected to the batteries.
The instrument panel has its own ground return to the batteries and the panel itself is insulated from the cab.
The best place to connect an external battery (slave cable) is to the vehicle batteries, since it then serves all devices connected to them.
Again, the 757 is wired this way......
Many add-on devices, like the VIC-1, fuel burning heater, headlights, BO head light etc have grounds that rely on the wiring, not the case, which makes it possible to adhere to the above convention.