The first time I tried to trace a wiring fault on my 720S using the factory schematics, I felt like I was reading hieroglyphics. The symbols looked nothing like BMW or Mercedes diagrams I was used to from working on other cars.
After doing it enough times, it clicked. This guide is what I wish someone had told me before I spent three hours chasing a ghost voltage because I misread a connector pinout.
The 720S has roughly 70 electronic control modules talking to each other over CAN bus lines. There are four main buses:
Knowing which bus a component lives on saves time. A Powertrain CAN problem affects different systems than a Body CAN problem. If your infotainment is acting up, you are looking at Body CAN, not the engine.
Every wiring page in the SIS has a number like "01-42-A." The first digits are the system code (01 = engine management, 09 = chassis electronics, 22 = A/C, 87 = infotainment). The second set is the specific circuit. The letter is the revision — always use the latest revision.
If a circuit continues to another page, there is an arrow with the destination page number. Follow it. Missing cross-references is the most common reason people think a diagram is incomplete.
McLaren uses two-letter codes:
Many wires have a stripe. "BN/WH" means brown with a white stripe. Match both the base color AND the stripe when probing — there are often multiple brown wires in the same connector, differentiated only by the stripe.
Every connector has an X-number (like X1234). On the diagram, it appears as a rectangle with pin numbers inside. Pin 1 is always top-left. Pins number left-to-right, then top-to-bottom. A slash through a pin means that slot is empty.
The SIS includes separate connector location diagrams showing you exactly where each X-numbered connector lives in the car. Check these before digging — they save hours of hunting.
Grounds are marked with a downward triangle and a G-code (like G101). The ground location diagram shows you the physical bolt for each code.
Bad grounds cause symptoms that mimic almost every other electrical fault: dim lights, intermittent warnings, module communication errors. Test them:
Main fuse box: Left side of engine bay. Covers engine management, lighting, A/C compressor, fuel pumps.
Interior fuse panel: Driver's side dashboard. Covers interior lights, infotainment, seat controls.
Relays are identified by three-letter codes (R123). The diagram tells you which circuit each relay controls.
Here is my actual workflow when something electrical goes wrong:
Disconnect the battery before unplugging any module. Wait three minutes after disconnect — the airbag capacitors need time to discharge. Never short pins on airbag connectors. Use MDS, not a cheap OBD2 scanner, for diagnostics.
All wiring diagrams are in the SIS browser under Wiring Schematics. Also see: DTC decoding, common electrical issues.