[Bob Lincoln]

Certainly. The main ground cable goes to the engine block, to ground the starter and other electronics mounted on the engine. But other electronics, such as gauges, computer, stereo, etc, ground to the chassis (firewall, dash, body panels). The engine is electrically isolated from the frame at the engine mounts, and only has a casual electrical connection from wherever the transmission and axle touch the frame in any way. The resistance there can be several ohms. Therefore, there is also a ground wire from battery to frame/fenderwell/bulkhead. Without that ground connection, any ground return from the chassis/dash, etc. has several ohms of resistance between it and the engine ground. Therefore, those grounds are not at zero volts, but at some higher voltage as determined by the resistance. Therefore, you have devices running inside at somewhat less than 12V, and you have a ground loop, or current, flowing between the virtual grounds of the interior and the engine ground.

 

[Scrounge]

This makes sense, but what about older vehicles from the 1960s back that didn't have the extra ground wire? I recall "suppressor" spark plug wires were sold to keep static off the radio. Would a ground wire to the frame have performed that same function?

 

[Bob Lincoln]

All the vehicles that I ever saw, 1960s, 1970s, had that ground wire to the left fenderwell.

Noise suppression to the radio is a different issue. It is both airborne and conducted noise from the alternator, and a braided ground strap from engine to firewall, as well as the capacitor on the side of the ignition coil, take care of that issue.

 

[Scrounge]

The '60s Mopars I've owned didn't have the extra ground wire from the battery to the frame, they only had a single cable to the block. These included '65 Monaco, '66 Fury, '69 Satellite, and '69 Imperial. Granted, I bought them used, so the wire could have been included brand new, but was removed at some point. In the big blocks (Monaco and Imperial), the temperature gauge always read hot.

 

[Bob Lincoln]

I don't know what to tell you. Every car I've owned has had that ground, including my 66 Belvedere and 72 Dart. Others can chime in as to their experience. Regardless, now you see why it's necessary.