Porsche 911 CDI Diagnostic & Troubleshooting Guide (3‑Pin & 6‑Pin)

Overview

Porsche used two main versions of the Capacitor Discharge Ignition (CDI) system on air-cooled 911s:

• 3‑pin CDI (1969–1977): points-type distributor
• 6‑pin CDI (1978+ 911 SC): reluctor pickup distributor

This guide provides a structured, step-by-step process to diagnose no-spark, weak spark, and common CDI-related misfires.

Main components

• CDI box
• Ignition coil
• Distributor (points or reluctor pickup)
• CDI wiring harness / trigger wiring

Safety first

• Never leave the high-voltage coil lead unterminated (can permanently damage the coil).
• Always ensure the spark plug body is properly grounded during tests.
• Do not hold the wire or plug during cranking (serious shock risk).
• Disconnect power before removing the CDI unit.

Part 1 — 3‑Pin CDI Troubleshooting (1969–1977)

Step 1: Visual inspection

Inspect the underside of the distributor cap for carbon tracking, debris, and loose/broken terminals. Damage between rotor and cap can quickly destroy both.

Step 2: Coil spark test (at the coil lead)

1. Remove the distributor end of the high-voltage cable.
2. Insert a known good spark plug into the lead.
3. Ground the plug body securely to engine ground.
4. Ensure the other end of the cable is grounded properly.
5. Crank the engine and check for spark.
6. If no spark: verify the cable’s continuity.

Step 3: Distributor rotor check

1. Confirm the rotor turns while cranking.
2. Measure rotor resistance (noise suppression rotor):
   • Expected: < 6,000 Ω
   • > 6,000 Ω or open circuit: rotor resistor likely burned open → no spark at plug wires.

Step 4: Coil resistance check

Resistance checks do not guarantee the coil is perfect (internal arcing can still occur), but they are a useful screen.

Step 5: Substitution test (recommended)

• Substitute a known-good coil (even an inductive coil is fine for testing).
• Common issue: Bosch silver “Brazil” CDI coils often fail.

If still no spark, continue to wiring checks.

Step 6: CDI wiring harness checks

1. Confirm CDI box grounding:
   • One tab to chassis
   • One through harness for coil ground
2. Unplug CDI harness.
3. Check for 12 V at terminal B (center pin) with ignition in ON and START.
4. Verify point signal at pin C:
   • Points open: open circuit
   • Points closed: continuity to ground
5. Check continuity:
   • Plug A → coil positive
   • Coil negative → engine/chassis ground
6. Reconnect the harness.

Step 7: Basic CDI box function test

1. Ensure solid chassis ground.
2. Apply 12 V power.
3. Listen for CDI “singing” (more obvious with rear cover removed).
4. Momentarily touch the orange trigger wire (pin C) to ground, then release.
5. Check for spark at the plug.

Step 8: CDI box removal

1. Switch power off.
2. Disconnect the wiring harness.
3. Remove the three 5 mm Allen screws.

Part 2 — 6‑Pin CDI Troubleshooting (1978+ 911 SC)

Step 1: Coil checks

• The original black coil (part #0 221 121 001) rarely fails.
• Silver “Brazil” replacements fail more often.

• Primary (+ to –): ~0.5 Ω
• – to center terminal: ~650 Ω

If no spark: substitute with a Bosch VW Blue 3‑Ω coil for testing.

Step 2: Distributor rotor test

• Typical rotor suppression resistance: up to 5,000 Ω
• Open circuit: no spark at plug wires
• Test from rotor center to outer edge

Step 3: Distributor signal wire (green coax) test

The green coax cable is a known weak point due to heat and vibration.

• Confirm continuity of inner conductor and shield.
• Confirm no short between conductor and shield.
• At idle, gently move the cable near the distributor:
  • If engine stumbles/dies → cable likely broken internally.
• Expected: ~600 Ω between the two pins on the distributor connector.

Step 4: CDI box power verification

• Unplug the CDI box connector.
• Check for power at tab 15 (12 V) during crank and run.

Step 5: Coil spark test

Use the same coil-lead spark test procedure described under the 3‑pin system.

Step 6: Basic CDI box function test

1. Ensure chassis ground.
2. Apply 12 V power.
3. Listen for CDI “singing”.
4. Briefly touch and release the green trigger wire (pin 7) to ground.
5. Check for spark.

Step 7: CDI box removal

1. Power off.
2. Disconnect wiring harness.
3. Remove three 5 mm Allen screws.

Common failures & workshop notes

• Rotor resistor open → spark at coil but none at plug wires
• Bosch silver CDI coils (Brazil) → common no-spark culprit
• Green coax distributor trigger wire (6‑pin) → intermittent misfire/no-start
• Always confirm clean grounds and never leave coil HV lead unterminated