To carry power, ground and signal between every electrical component on the car, survive the thermal, chemical and vibration environment, and remain serviceable.
Multiple function-split harnesses rather than one loom: ECU harness, dash (C185, now DDU11) harness, power and controls harness, battery harness, fuel rail harness, plus sensor and controls sub-harnesses. Each is built from concentrically twisted tinned copper with Tefzel (ETFE) insulation, M22759/18 and /32, in gauges from 8 to 22 AWG selected by sustained current. Raychem DR-25 heat shrink covers the whole harness for abrasion and fluid resistance across -75 to 150 C at a 2:1 shrink ratio. Deutsch DTM connectors terminate at sensors and breakouts, DT connectors at high-current items such as pumps and fans, with weather-sealing boots where fluid ingress is a risk. Wire colour is fixed team-wide: black ground, red +12V, yellow CAN high, green CAN low, red/white 5V, black/white 5V and signal ground, other colours signal. Two-wire shielded carries the CAN network, three-wire shielded carries wheel speeds.
Design the harness from the connector inward. The process is: place the main connector to the datalogger or ECU, place the sensor connectors, connect them, then form bundles and choose split points. Splices are grouped in one area so they can be worked on. Buy quality where the failure mode is known. Deutsch DT/DTM was chosen over Molex MX150 because it is what the motorsport industry uses, and because a 50% discount made the price difference tolerable. The assembly is harder the first time and much faster after. 1. Collect the sensor list and the datalogger assignment for each sensor. 2. Draw the harness in RapidHarness: main connectors, sensor connectors, connections, then bundles and split points. 3. Order wire early. BMRSWired lead time has run 2 weeks to 3 months and is the single highest-impact long lead item. 4. Cut and crimp terminals, assemble connectors, build service loops. 5. Twist the harness, cover with DR-25 heat shrink, test fit on the frame, then install and recover transition boots and connector boots.
- Reduce connector failures. - Reduce weight. CR22 came in at 399 wires, 1362 ft, 5.916 lb across three harnesses. - Improve serviceability: group splices, use boots, make the harness documentable. - Improve documentation so a future member can trace a circuit without the original designer. - Split critical function wiring from data acquisition wiring (CR26E direction).
- Connector failures on CR20 and CR21, addressed by the move to Deutsch. - Extra weight from running separate 12V and ground harnesses on earlier cars; CR22 combined the PDM15 and busbar harness. - Undersized power wiring. The 16 AWG REC+ wire on CR22 heated up and stopped carrying charging current over an endurance. - BMRSWired lead time. Wiring cannot start until the wire arrives, and old wire is not reused because of reliability risk. From the team's Car Wiring Lessons Learned document, the recurring failures worth designing against: - Wires melting where they are routed near the exhaust without heat protection. - Grounds corroding or being damaged at the frame. If a fault is weird and unexplainable, measure resistance between the component ground and the battery main ground; above about 5 ohms the ground is the problem. Sand the paint off every frame grounding spot. - The engine assumed to be grounded through its mounting bolts, when paint or corrosion prevents it. The recorded fix is a frame tab with AWG #6 running to an easily accessible bolt on the side of the engine, so engine grounding can be monitored. - Weather Pack connectors are not all indexed the same. A whole spare harness was wired with connectors that then did not fit the sensor connectors. - Connectors placed where they cannot be rotated to get on and off, and connectors on designed parts left inaccessible. The MAP/IAT combination sensor was pointed at a frame tube member and was impossible to connect. - Female pins go in male connectors and male pins go in female connectors. It works the other way but is not intended to. - Check ignition coil resistance before trying to run the car; different part numbers differ. Denso 129700-4150 is 1.6-2.6 ohms, Denso 129700-4541 is 0.4-0.7 ohms, and the harness expects 0.4-0.7 ohms. - Put a 3 ohm, 5 W resistor on the normally-on part of the master switch so excess voltage bleeds safely to ground rather than damaging the rectifier/regulator.
RapidHarness for harness design and documentation. MoTeC i2 Pro for validating that signals arrive correctly. Standard crimp tooling for Deutsch closed-barrel and stamped terminations.