Mass braid for BMW Serie 3 - E30

Why the return current flows through the bodywork

In a vehicle, the current does not return to the battery via a dedicated wire: it returns via the bodywork, which acts as a common conductor. However, the components must be connected to it, and this is the role of ground straps.

Why a braided cable rather than a rigid one? Because the engine and gearbox rest on flexible mounts and move by several millimetres. A rigid cable would fail due to fatigue, and a simple metal-to-metal contact is not sufficient: the mounts are made of rubber, and are therefore insulating. The braided cable, made up of hundreds of fine strands, accommodates this movement without breaking, whilst providing a large cross-sectional area. It is often supplemented by a shorter braided cable between the engine block and the bodywork, particularly in set-ups where the bonnet or wing houses an electrical component.

Faults caused by a faulty earth

A faulty earth connection does not cut off the current; it adds resistance. The symptom therefore depends on which component is drawing the most current at that moment, which explains why faults may appear seemingly unrelated.

• The starter motor turns slowly or clicks, even though the battery is charged and the terminals are clean.
• The headlights dim when the engine is idling or whilst another electrical load is in use.
• The instruments display erratic readings, with needles wavering or gauges drifting.
• The current seeks an alternative path: it flows through a throttle cable, a hose or a flexible pipe, which heats up and becomes damaged. This is a common fault that is rarely suspected.
• Radio interference occurs, and the contactor or relay overheats abnormally.

The test is carried out using a multimeter, by measuring the voltage drop between the component and the negative terminal of the battery whilst the circuit is active, for example during starting. Any significant reading indicates a short to earth that needs to be rectified.

Cross-section, length and terminals

• The cross-section, in square millimetres, must be at least equal to that of the positive cable supplying the same circuit. An undersized starter ground cable overheats and causes a drop in voltage at precisely the moment it is most needed.
• The length should be measured when the cable is slack, following its actual route. If it is too short, the braided cable will be subjected to tensile stress with every movement of the engine and will break; if it is too long, it will rub or come into contact with a hot component.
• Terminations: eye-to-eye for a connection between two bolted points, or eye and terminal for connecting directly to the negative terminal of the battery.
• The diameter of the eyelets must match the fasteners in place.

Opt for bare braided cable rather than sheathed cable for engine connections: it can be checked at a glance, whereas a sheath hides oxidation that progresses unseen.

Fitting a durable ground connection

A new braided cable bolted to painted sheet metal conducts almost no current. The contact is just as important as the component itself.

• Strip both sidesdown to bare metal across the entire surface of the eyelet, removing paint, primer and oxidation.
• Tighten firmly, using a fan-shaped washer that grips the metal and maintains contact despite vibrations.
• Then protect the established contact with contact grease or wax to prevent moisture from returning. Never apply protection before tightening.
• Do not remove any existing earth terminals on the pretext of adding a better one: they do not duplicate each other; each serves a different circuit.
• Check them all during an electrical overhaul, including those in the boot, the dashboard and the tailgate.