A chassis ground is needed in conjunction with the ground to the engine because although the engine is bolted to the frame, the engine mounts insulate the engine from the chassis with rubber mounts for vibration reduc. To understand the reason for several ground/common wires from the battery, a brief basic overview of how the car battery system works is in order. Why are car batteries ground. Some cars are produces with the battery located in the trunk. Other people decide that the weight distribution of a the heavy battery in the back of the car rather than the front along with t. When making a ground connection there is a lot of room for error and a poor connection results in a high resistance that when high enough will restrict the current flow from the batt. A multimeter is a handy tool to have and if you own one, you can test between engine block and frame to determine if you have an adequate ground. You need to determine the resistance (o.