--- title: "Sports car dashboard coupling problem" subject: "Instrumentation II" chapter: 6 key: "semester_5/instrumentation_ii:sports-car-coupling" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/sports-car-coupling site: BCT Engineering --- # Sports car dashboard coupling problem Instrumentation II · Chapter 6: Grounding And Shielding - Asked in: 2073 Chaitra, 6 marks - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/sports-car-coupling - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-6 ## Question A new model sports car had a disconcerting problem: occasionally the dashboard lights would all illuminate simultaneously. Two service calls later, replacement of a wire harness for the spark plugs solved the problem. Explain what the coupling mechanism was and how it can be reduced? ## Short answer The spark plug wire's high-voltage fast-rise pulses coupled capacitively (and inductively) into the dashboard wiring, triggering the high-impedance lamp inputs. The problem was solved by replacing the harness with resistor-type spark plugs and shielded cables, rerouting away from dashboard, and adding filtering to inputs. ## Answer The spark-plug wire carries a very high voltage pulse (10-30 kV) with a very fast rise, so dV/dt is huge. The faulty/old harness was close to the dashboard wiring. The coupling was mainly **capacitive (electric-field) coupling**, together with some **inductive** and **radiated** contribution: the spark pulse is also a large di/dt event, and the spark gap radiates a broad-band EM burst. The coupled pulse switched on the dashboard lamps' control inputs (high-impedance logic), turning them all on together. Noise current: I = C·dV/dt. Even with a tiny stray capacitance, a spike of kilovolts in nanoseconds gives enough current to trigger a high-impedance input. #### Reduction - Use **resistor-type (suppression) spark plugs/plug wires**; they limit di/dt and radiation. - Use **shielded, high-quality ignition cables**, with the shield grounded. - **Route the ignition harness away** from the dashboard and signal wiring; cross at 90 degrees if needed. - Put an **electrostatic shield** or grounded metal between them; twist and shield the dashboard wires. - Reduce the impedance of dashboard inputs (pull-down resistor/filter capacitor) and add RC filters/TVS to inputs. - Bypass the harness with ferrite beads and ground the engine, chassis and body properly. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What was the main coupling mechanism between the spark-plug harness and the dashboard wiring? A. Conductive coupling through a shared battery earth only B. Capacitive (electric-field) coupling due to the huge dV/dt of the spark pulse C. Pure thermal coupling from engine heat D. Optical coupling from the spark light Answer: B. A 10-30 kV pulse with a very fast rise passes noise current through stray capacitance into the dashboard wiring. (Key point: Main coupling mechanism) 2. Why can even a tiny stray capacitance upset the dashboard? A. I = C dV/dt becomes large for a kilovolt spike in nanoseconds, enough to trigger high-impedance inputs B. Tiny capacitance stores a large charge that leaks for hours C. The spark pulse has high current but no voltage change D. Dashboard inputs are low impedance so they pick up everything Answer: A. dV/dt is enormous, so the displacement current is sufficient to switch high-impedance logic inputs. (Key point: Noise current) 3. Which measure reduces noise at the source in the ignition system? A. Using thinner ignition wires B. Increasing the spark voltage C. Removing the chassis ground D. Resistor-type suppression plugs/wires that limit di/dt and radiation Answer: D. Suppression resistors slow and limit the pulse current, cutting radiated and coupled noise. (Key point: Source side remedy) 4. Which set of actions breaks the coupling path or hardens the receiver? A. Run the harness parallel and close to the dashboard wires B. Use unshielded cables with higher input impedance C. Route harness away, grounded shielding, lower input impedance, RC filters/TVS D. Leave the engine and chassis ungrounded Answer: C. Spacing, grounded shields, low-impedance inputs and filtering all reduce capacitive pickup. (Key point: Path and receiver remedies) ## Related in Instrumentation II - [Connecting two DTE RS-232 devices](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adte-dte-problem) — Instrumentation II, chapter 3, asked 2× - [Case study: feasibility, drawbacks, convincing](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Acs-2072kartik-feasibility) — Instrumentation II, chapter 10 - [ATM in hotel card reader failure](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Aatm-hotel-problem) — Instrumentation II, chapter 6 ## Related in other subjects - [Wire-frame model and polygon table](https://www.bctengineer.com/question.md?key=semester_5%2Fcomputer_graphics%3Awireframe-model-polygon-table) — Computer Graphics, chapter 6 - [Resection, two point problem, intersection](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aresection-two-point-problem-ab-p) — Surveying II, chapter 5 - [Two point problem; advantages, disadvantages](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-point-problem-adv-disadv) — Surveying I, chapter 6 - [ESL and replacement strategy for printing machines](https://www.bctengineer.com/question.md?key=semester_6%2Fengineering_economics%3Aprinting-machine-old-new-aec-esl) — Engineering Economics, chapter 5