Most folks have never heard of a transient current flow. Transient current flow can cause thousands in damage to a vehicles. Normally there are very few outward signs until it is too late. Fortunately it can also be detected and prevented, with a few simple steps.
When there is an electrical load in our vehicle, current flows from the battery to the load. A positive current leaves the battery and travels through wires to the accessory. The same amount of current must also return to negative side through a ground. The ground may be provided by a wire or sometimes the vehicle body. The body is electrically attached to the negative terminal.
The current flow leaving and returning to the battery has to be equal
If the cables, grounds and terminals work as designed, current flow is not a problem. When cranking the vehicle, we turn the key and electricity flows from the battery to the starter motor, through a solenoid. We need the solenoid, because the key switch cannot conduct the high amperage needed to operate the starter. The solenoid acts like a heavy-duty relay. It allows the light-duty key switch to control the high-amperage starter motor.
After flowing through the starter, the current returns to the battery through the engine block. They connect the engine to the electrical system with a heavy cable, attached to the negative battery terminal. This completes the circuit. The starter uses 275 amps to turn and the same current returns to the battery. Current flow on both cables equals 275 amps. The current returning to the battery equals that leaving it.
Problems occur when corrosion and loose connections cause high resistance. The ground cable may only be able to flow 250 amps. If the starter pulls 275 amps, twenty-five amps will find another path to the negative terminal. This is known as transient current flow; electricity taking a path, other than that designed, to return to the battery. The end result can be considerable damage to seemingly unrelated components.
Because the engine attaches to the body using rubber mounts, it is only electrically connected through the ground cable(s). If the cable does not conduct well enough, another path will be found. Instead, the additional current flows through the flywheel, transmission, drive axle, knuckles and back through the tie rods. Here is finds a body ground in the steering gear.
Transient current can be measured by placing an amp meter between the starter case and the battery ground
Transmission and suspension components are not designed to flow electricity. As the current flows through them, metal can be transferred from on part to another. This is similar to the electroplating process. In time the parts are destroyed and there may be a major failure.
Keeping all battery terminals clean, tight and with proper connections can help prevent transient current flow.
Battery terminal corrosion is a symptom of another problem
Simply cleaning the corrosion away, will only give temporary results. Terminals corrode because acid and gasses leak around their base. This is a defect in a battery and should be covered under warranty. Replacement of the battery and repair of the terminals will solve the problem.
Inspecting the battery and terminals regularly and replacing batteries that leak may save a transmission or suspension rebuild.