BMW Ground Strap Theory & Replacement
If your vehicle is slow to crank and starts hard, these are usually two signs that a vehicle is in need of a tune-up or a new battery. While either of these diagnosis may be true, these symptoms may also indicate poor grounding. Here's how to diagnose and replace the ground cables in almost any BMW.
What is a Ground Strap?
Vehicles utilize ground cables that link the alternator and starter to the rest of the vehicle's electrical system. This is done by attaching the engine to the chassis using electrical cable.
BMW uses a braided-style ground straps/cable over solid ground straps to allow movement between the engine and chassis, and to eliminate the transfer of engine noise to the cabin of the vehicle. Think about it, the only way an engine sitting on rubber mounts can be earthed (grounded) is through ground straps or cables. This is the only way that the electrical components can function.
How and Why do Ground Straps Fail?
Ground straps fail in two main ways:
In the image below, the ground cable has corroded and turned green. At this point, the cable has hardened and has lost its flexibility and ability to conduct electricity. Engine movement wears these straps over time, causing them to crack. Once the cable has corroded and cracked, the vehicle will typically develop a long/slow crank and eventually refuse to crank if the problem is not addressed.
Also possible but unlikely is that it can fail without warning. If this happens, even using jumper cables won't start your vehicle as the engine needs to be grounded to the chassis first. Severely worn engine mounts are also a contributing factor to ground cable wear as engine movement greater than allowed will stress the ground cables greater than they're designed to be.
Locations of the Ground Straps on your BMW
The locations of the ground cables will vary depending on your BMW's year and model. You should consult a repair manual or other reputable source to locate your exact model's ground cables, or do some exploring under the hood. More than likely, you'll find one somewhere around the perimeter of the engine next to the frame of the vehicle.
In the case of the BMW E30 in the photo below, it has a grounding cable attached to the bare metal of the hood. This isn't a typical location for grounding cables anymore as it increases the potential stress on the cable.
On more modern vehicles like the BMW E90 (Visit our BMW E9X Owner's Hub), one ground cable connects the driver's side engine mount bracket to the body. These engine ground cables are secured with 13mm nuts. Replacement is as easy as removing the nuts and installing a new cable.
We hope this quick guide was helping in diagnosing your start-up woes. Be sure to check the condition of the ground cables whenever you're under the hood to catch any problem before you're left stranded!