Electrical Diagnosis

Systematic electrical diagnosis procedure for the 2009 Aston Martin DB9, covering circuit testing, component verification, and diagnostic tool usage for troubleshooting electrical faults.

Warnings

Tools required

Parts

• Replacement fuses (assorted 5A-40A) × 1 — Use OEM specification
• Dielectric grease × 1 — Use OEM specification
• Electrical contact cleaner × 1 — Use OEM specification
• Heat shrink tubing assortment (if repair needed) × 1 — Use OEM specification

Preparation

1. Park vehicle on level surface and apply parking brake
2. Turn off all electrical accessories and remove key from ignition
3. Allow engine to cool completely if recently operated
4. Open hood and locate battery in engine bay
5. Check battery voltage with multimeter - should read 12.6V or higher for accurate diagnosis
6. If battery voltage is low, charge battery fully before proceeding
7. Gather all wiring diagrams and technical service bulletins relevant to the reported electrical concern
8. Document all customer complaints and symptoms in detail, including when fault occurs and environmental conditions

Procedure

1. 1

   Initial System Scan and Fault Code Retrieval
   Connect diagnostic scanner to the OBD-II diagnostic port (verify exact location per factory service manual; typically in the footwell/lower dash area). Turn ignition to position II (all systems powered, engine off). Perform full system scan covering all control modules including engine, transmission, ABS, airbag, body control, instrument cluster, and convenience modules. Record all diagnostic trouble codes (DTCs), freeze frame data, and pending codes. Note code frequency and environmental conditions. Check for communication faults between modules which may indicate CAN bus issues. Print or save complete scan results for reference throughout diagnosis.
2. 2

   Visual Inspection of Battery and Main Power Distribution
   Visually inspect battery terminals for corrosion, looseness, or damage. Clean terminals if corrosion present using battery terminal cleaner and wire brush. Check battery hold-down is secure. Inspect main battery cables for chafing, burns, or damaged insulation. Locate main fuse box in engine compartment and inspect for signs of water intrusion, corrosion, or burned fuses. Remove fuse box cover and visually inspect all fuses and relays. Check for signs of overheating (melted plastic, discolored components). Verify all relays are properly seated. Use fuse puller to extract and test any suspect fuses with multimeter continuity function.
3. 3

   Verify Ground Connections and Chassis Grounds
   The DB9 uses multiple ground points throughout chassis. Locate main engine ground connections on cylinder heads and engine block to chassis/body. Check primary ground strap from battery negative terminal to body. Inspect all visible ground connections for corrosion, looseness, or paint interference preventing proper contact. Use multimeter to measure resistance between battery negative terminal and various chassis ground points - should read less than 0.5 ohms. Pay special attention to grounds in footwell areas and trunk where moisture intrusion is common. Remove, clean, and reinstall any questionable ground connections, ensuring bare metal-to-metal contact.
4. 4

   Inspect Interior and Trunk Fuse Boxes
   Access interior fuse box located in driver-side footwell area. Remove fuse box cover (typically secured with plastic clips or Torx fasteners). Visually inspect for moisture, corrosion, or signs of component failure. Check fuses related to affected circuit using fuse puller and multimeter. Open trunk and locate trunk-mounted fuse and relay box. Inspect for water intrusion - this area is prone to moisture issues on DB9. Check condition of all fuses and relays in this location. Use wiring diagram to identify circuits related to customer complaint and verify fuse integrity and power distribution to affected systems.
5. 5

   Perform Circuit-Specific Voltage Drop Testing
   Based on diagnostic codes and symptoms, identify specific circuits requiring testing. With ignition on and circuit activated, use multimeter to perform voltage drop testing along suspected circuit paths. Back-probe connectors using terminal probe kit to avoid damaging connector seals. Measure voltage at component, then work backward toward power source checking voltage at each connector and splice point. Acceptable voltage drop across connections should be less than 0.2V; across switches less than 0.3V. Document all voltage readings and identify any excessive drops indicating high resistance or poor connections. For ground circuits, measure voltage drop from component ground to battery negative - should be less than 0.5V total.
6. 6

   Test CAN Bus Communication
   If communication faults were detected in initial scan, locate CAN bus data lines. The DB9 uses high-speed and low-speed CAN networks. With ignition off and battery disconnected, measure resistance across the high-speed CAN High and CAN Low lines at the appropriate pins - a properly terminated high-speed CAN bus reads approximately 60 ohms (two 120-ohm terminators in parallel). Note that low-speed/body CAN networks use different termination and will not read 60 ohms; consult the wiring diagram for the specific network being tested. Turn ignition on and measure voltage on CAN High (should be approximately 2.5-3.5V) and CAN Low (approximately 1.5-2.5V) relative to ground. Both lines should show activity (voltage fluctuations) during communication. If no activity present, disconnect modules one at a time to isolate short circuit or module pulling bus down. Check wiring diagrams for CAN bus routing and inspect for damaged wiring, especially in areas where harnesses flex or pass through bulkheads.
7. 7

   Component-Level Testing
   Once circuit fault is isolated to general area, test specific components. For sensors: disconnect sensor connector and measure resistance or voltage output according to specifications in service manual. Compare readings to known good values for temperature, position, or pressure sensors. For actuators (motors, solenoids): remove component connector and apply direct battery voltage (with inline fuse protection) to verify mechanical operation. Measure actuator resistance and compare to specifications. For switches: test continuity through switch in both positions. Use oscilloscope for complex sensors like crankshaft position or wheel speed sensors to verify signal pattern quality. Document all component test results.
8. 8

   Connector and Wiring Harness Inspection
   Physically inspect all connectors in suspect circuit path. Disconnect each connector and examine terminals for corrosion, bent pins, pushed-back terminals, or moisture intrusion. Check connector locking mechanisms are functional. Inspect wiring harness for chafing against sharp edges, heat damage near exhaust components, or damage from previous repairs. Pay special attention to harness routing through doors, under carpet, and in engine bay where movement or heat exposure is greatest. For DB9, check harness routing near footwell areas where water leaks are common. Use continuity testing to verify wire integrity from connector to connector if visual inspection reveals no obvious damage.
9. 9

   Module Power and Ground Verification
   If fault is isolated to specific control module, verify module has proper power and ground before condemning module. Disconnect module connector and check for battery voltage at power supply pins with ignition on (use wiring diagram to identify correct pins). Verify ground circuit continuity from module ground pins to battery negative - should read less than 1 ohm. Check for voltage on signal wires that should be at 0V - this indicates short to power. Verify 5V reference voltage output from module if applicable. If all power, ground, and communication circuits test good but module does not function, module replacement may be necessary. Before replacement, check for technical service bulletins addressing module software updates or known issues.
10. 10

    Perform Functional Testing and Monitoring
    After identifying and addressing fault, reconnect all components and restore power. Clear all diagnostic trouble codes using scan tool. Start engine if applicable and allow all systems to initialize. Operate affected system while monitoring live data on scan tool. Verify proper voltage, signal patterns, and communication between modules. Perform operational tests specific to complaint - cycle windows, test lighting circuits, operate climate control, etc. Monitor for code reoccurrence. For intermittent faults, use scanner data logging or oscilloscope recording to capture fault as it occurs. Perform road test if necessary to recreate conditions under which fault appears.
11. 11

    Final System Verification and Documentation
    Perform complete system scan to verify no new faults have been introduced and original faults do not return. Check all related systems for proper operation. Verify all connectors are properly secured and locked. Ensure no tools or test equipment remain in vehicle. Apply dielectric grease to any connectors that were opened during diagnosis to prevent future corrosion. Document all findings, tests performed, measurements taken, and repairs completed. If fault was repaired, note root cause. If fault was intermittent and not reproduced, document conditions attempted and recommend customer return if fault reoccurs. Update scan tool with any necessary module configurations or adaptations if components were replaced.

Reassembly

1. Reinstall all fuse box covers ensuring proper seating of retaining clips or fasteners
2. Verify all electrical connectors are fully seated and locking tabs engaged
3. Reconnect battery negative terminal and torque to proper specification
4. If battery was disconnected, allow vehicle to sit with ignition on for 2 minutes to allow modules to initialize
5. Reset windows, sunroof, and any other systems requiring reinitialization after battery disconnect
6. Verify clock, radio presets, and other volatile memory settings are restored

Verification

• Perform final diagnostic scan showing no active or pending fault codes related to diagnosed issue
• Verify affected system operates correctly through multiple cycles
• Confirm all electrical systems function normally including lights, windows, HVAC, instrument cluster, and infotainment
• Check battery voltage with all accessories on - should maintain above 12.0V with engine off
• Verify no abnormal voltage drops or parasitic draws exist if battery drain was concern
• Road test vehicle if applicable to confirm fault does not reoccur under driving conditions
• Document baseline readings and component values for future reference

Related trouble codes on this vehicle

Codes that commonly send this job to the bay — tap one for symptoms, causes, and diagnostic steps.

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More procedures for this vehicle