Introduction

The Porsche 911 Capacitor Discharge Ignition (CDI) system was introduced in 1969 with a 3-pin connector and points-type distributor.
In 1978, the 911 SC received a 6-pin CDI for a reluctor pickup distributor.
This manual provides step-by-step instructions for diagnosing and troubleshooting both CDI variants.

CDI System Overview

• 3-Pin CDI (1969–1977): Points-type distributor
• 6-Pin CDI (1978+ 911 SC): Reluctor pickup distributor
• Main Components:
  • CDI Box (red arrow)
  • Coil (blue arrow)
  • Distributor (green arrow)

Safety Precautions

• Never leave the high-voltage coil lead unterminated—risk of permanent coil damage
• Ensure spark plug is well grounded
• Do NOT hold the wire or spark plug during tests—risk of electric shock
• Disconnect power before removing CDI box

Troubleshooting the 3-Pin CDI System

A. Visual Inspection

Inspect the underside of the distributor cap for loose pots or debris. Damage between rotor and cap can destroy both—check if experiencing ignition issues.

B. Coil Spark Test

1. Remove the distributor end of the high-voltage cable
2. Plug into a grounded spark plug (red arrow)
3. Ensure spark plug body is well grounded (blue arrow)
4. Attach the other cable end to engine ground (green arrow)
5. Crank engine and observe for spark
6. If no spark: Check cable continuity

C. Distributor Rotor Check

1. Verify rotor turns when engine is cranked
2. Check rotor continuity: Resistance should be <6,000 ohms for noise suppression
3. >6,000 ohms or open circuit: Rotor resistor likely burnt open (no spark at plug wires)

D. Coil Continuity Test

Readings do not guarantee coil is good; internal arcing may still occur.

E. Substitution Test

• Substitute with a known good coil (an inductive coil is sufficient for testing)
• Bosch CDI coils (silver, Brazil) are prone to failure
• If still no spark after substitution, proceed to wiring tests

F. CDI Wiring Harness Test

1. Identify harness pin locations for the 3-pin CDI
2. Ensure CDI is grounded via tabs (one to chassis, one through harness for coil ground)
3. Remove CDI cable plug
4. Check for 12V at terminal B (center pin) with ignition ON and START positions
5. Verify point signal at pin C:
   • Points open: Open circuit
   • Points closed: Continuity to ground
6. Check continuity:
   • Plug A to coil positive terminal
   • Coil negative terminal to engine chassis ground
7. Reconnect harness to CDI box

G. CDI Box Basic Function Test

1. Ensure earth ground
2. Apply 12V power
3. Listen for CDI ‘singing’ sound (easier with rear cover removed)
4. Touch orange trigger wire (pin C) to ground, then release
5. Observe for spark at spark plug

H. CDI Box Removal

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

Troubleshooting the 6-Pin CDI System (911 SC)

A. Coil Tests

• Original coil (black, part #0 2 2 1 1 2 1 0 0 1): Rarely fails
• Silver replacements (Brazil): Prone to failure
• Measure primary resistance: ~0.5 ohms (positive to negative)
• Measure coil negative to center terminal: ~650 ohms
• If no spark: Substitute with a Bosch VW Blue 3-ohm coil for testing

B. Distributor Rotor Test

• Resistance for RFI suppression: Up to 5,000 ohms
• Open circuit: No spark at plug wires
• Measure from rotor center to outer edge for continuity

C. Distributor Signal Wire Test

• Green coax cable from distributor: Prone to internal shorts or opens due to heat, vibration, or flexing
• Check continuity of inner conductor and shield; no shorting between
• Manipulate cable near distributor at idle: If engine stumbles/dies, cable is faulty
• Should measure ~600 ohms between two pins on distributor connector

D. CDI Box Power and Signal Test

• Remove electrical connection from CDI box
• Verify power at tab 15 (12V) during cranking and run positions

E. Coil Spark Test

• Same procedure as for 3-pin system

F. CDI Box Basic Function Test

1. Ensure earth ground
2. Apply 12V power
3. Listen for CDI ‘singing’
4. Briefly touch and release green trigger wire (pin 7) to ground
5. Observe for spark at spark plug

G. CDI Box Removal

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

Common Failures & Recommendations

• Distributor rotor resistor open: No spark to plug wires (especially in early models)
• Bosch silver coils (Brazil): Commonly fail; use substitution to confirm
• Distributor green coax cable: Internal breakage/shorting causes misfires or no spark
• Never leave high-voltage coil lead unterminated
• Always check for proper grounding