Why does an earth connected to a painted chassis not work?
Paint, electrophoretic coating, rust and dirt are insulating; however hard you tighten the bolt, conduction runs through the few microscopic contact points where the paint has been crushed. These points heat up at high current and oxidise, and the resistance grows. The result is a voltage drop exceeding 0.5 V under load and an intermittent fault.
An earth connection is a metal-to-metal contact problem. On a painted surface the real contact area is a very small part of the visible area, and that small area heats up as it carries the current. The heated copper-steel interface oxidises faster, the resistance rises and it heats up more. This self-feeding loop is the most common reason why an installation that works on the first day fails a few months later.
The correct order of work:
- Remove the paint and rust over an area slightly wider than the lug diameter, with a wire brush or abrasive paper, until the metal shines.
- Fit a crimped lug; on a vibrating installation use crimping rather than soldering.
- Use a serrated or toothed washer; the washer breaks through the remaining thin oxide layer and delays loosening.
- After tightening, cover the exposed metal with protective grease or paint to insulate it against moisture and salt.
- Retighten the connection after the first days of use and measure the voltage drop under load.
On aluminium or galvanised surfaces the contact of dissimilar metals produces galvanic corrosion; use the same metal pair where possible and seal the connection against moisture.
The full topic: Installation, redundancy and field continuity