main earth?

It may help a little if you can describe briefly and concisely the actual problem you have.

XXX
September 2016

The main earth on a vehicle is the connection between the battery negative terminal and the vehicle chassis - a large black cable.

Clean up the connections at each end of the cable and make them bright - including the -ve post on the battery and the connection to the chassis. Fit and tighten both connections and for completeness smear them with regular Vaseline grease to ward off corrosion

Good Luck...
September 2016