Maintaining your 2005 Mercury Grand Marquis involves understanding its electrical system, and a crucial part of that is the fuse and relay system. While a detailed OBD2 wiring schematic can be complex, knowing your fuse box layout is the first step in diagnosing many electrical and diagnostic issues, including those related to your OBD2 system’s functionality. This guide provides a comprehensive look at the fuse box diagrams for your 2005 Mercury Grand Marquis, helping you pinpoint problems and ensure your vehicle runs smoothly.
Located for easy access, the passenger compartment fuse panel is your first point of inspection for interior electrical malfunctions.
Passenger Compartment Fuse Panel
You can find this fuse panel conveniently placed below and to the left of the steering wheel, near the brake pedal. Simply remove the panel cover to access the fuses and relays.
| Fuse/relay | Ampere rating [A] | Description |
|---|---|---|
| 1 | 15 | Cluster, Lighting Control Module (Interior Lighting) |
| 2 | 10 | Ignition (ON) – Electronic Automatic Temperature Control (EATC) module, A/C mode switch (manual A/C), A/C blower relay coil |
| 3 | 10 | EATC module |
| 4 | 10 | Ignition (ON) – Anti-lock Brake System (ABS) module, Rear Air Suspension Module (RASM), Variable Assist Power Steering (VAPS) |
| 5 | 10 | Speed control deactivation switch, Stop signal |
| 6 | 10 | Ignition (ON) – Cluster |
| 7 | 15 | LCM (Park lamps, Corner lamps) |
| 8 | 10 | LCM |
| 9 | 10 | LCM (Switch illumination) |
| 10 | 5 | Ignition (START) – Audio mute |
| 11 | 10 | Ignition (ON/ACC) – window relay coil |
| 12 | 10 | Ignition (START) – Starter relay coil, DTRS |
| 13 | 10 | Ignition (ON/ACC) – Wiper module |
| 14 | 10 | Ignition (ON) – BTSI (Floor-shift transmission) |
| 15 | 7,5 | Ignition (ON/ACC) – LCM, Door lock switch illumination, Heated seat switch illumination, Moonroof, Overhead console, Electrochromatic mirror |
| 16 | 15 | Ignition (ON) – Turn signals |
| 17 | 10 | Ignition (ON/ACC) – Audio |
| 18 | 10 | Ignition (ON) – A/C mode switch (manual A/C), Blend door, Heated seat modules |
| 19 | 10 | LCM (Left-hand low beam) |
| 20 | 10 | Ignition (ON/START) – Back-up lamps |
| 21 | 10 | LCM (Right-hand low beam) |
| 22 | 10 | Ignition (ON/START) – Restraint Control Module (RCM), Occupant Classification Sensor (OCS), Passenger Air bag Deactivation Indicator (PADI) |
| 23 | 15 | Multi-function switch (Flash-to-pass), LCM (High beams) |
| 24 | 10 | Ignition (ON/START) – Passive Anti-Theft System (PATS) module, Powertrain Control Module (PCM) relay coil, Fuel relay coil, Ignition coil relay coil |
| 25 | 10 | Autolamp/Sunload sensor, Power mirrors, Door lock switches, Mirror switch, Keypad switch, Decklid switch, Adjustable pedal switch, DDM |
| 26 | 10 | Ignition (ON/START) – Cluster, LCM, Overdrive cancel switch, Rear defroster relay coil |
| 27 | 20 | Cigar lighter, OBD II |
| 28 | 7,5 | Center High-Mounted Stop Lamp (CHMSL) |
| 29 | 15 | Audio |
| 30 | 15 | Stop lamps, MFS |
| 31 | 15 | Hazards |
| 32 | 10 | Mirror heaters, Rear defroster switch indicator |
| 33 | — | Not used |
| Relay 1 | Full ISO relay | Rear defroster |
Notice fuse #27, rated at 20A, is specifically designated for “Cigar lighter, OBD II”. This fuse is critical for providing power to your On-Board Diagnostics system. If you are experiencing issues with your OBD2 scanner not connecting or powering up, this fuse should be one of the first things you check. A blown fuse here can directly impact your ability to diagnose your 2005 Mercury Grand Marquis using an OBD2 scanner.
For components under the hood, the power distribution box is the main hub for higher amperage fuses and relays.
Power Distribution Box
The power distribution box in your 2005 Mercury Grand Marquis is located in the engine compartment. This box contains fuses and relays for many of the vehicle’s major systems.
| Fuse/relay | Ampere rating [A] | Description |
|---|---|---|
| 1 | 25* | Ignition switch (Key in, RUN 1, RUN 2), Hazards |
| 2 | 25* | Ignition switch (RUN/START, RUN/ACC, START) |
| 3 | 10* | Powertrain Control Module (PCM) keep alive power |
| 4 | 20* | Fuel relay feed |
| 5 | 10* | Rear Air Suspension Module (RASM), VAPS |
| 6 | 15* | Alternator regulator |
| 7 | 30* | PCM relay feed |
| 8 | 20* | Driver’s Door Module (DDM) |
| 9 | 15* | Ignition coil relay feed |
| 10 | 20* | Horn relay feed |
| 11 | 15* | A/C clutch relay feed |
| 12 | 25* | Audio |
| 13 | 20* | Instrument panel power point |
| 14 | 20* | Stop lamp switch |
| 15 | 20* | Heated seats |
| 16 | 15* | Foglamps |
| 17 | — | Not used |
| 18 | — | Not used |
| 19 | 15* | Injectors |
| 20 | 15* | PCM, Mass Air Flow (MAF) sensor, IAT |
| 21 | 15* | Powertrain loads and sensors, A/C clutch relay coil |
| 22 | — | Not used |
| 23 | — | Not used |
| 24 | — | Not used |
| 101 | 40** | Blower relay feed |
| 102 | 50** | Cooling fan |
| 103 | 50** | Instrument panel (I/P) fuse box feed #1 (I/P fuses 19, 21, 23, 25 and 27) |
| 104 | 40** | Instrument panel (I/P) fuse box feed #2 (I/P fuses 1, 3, 5, 7, 8 and 9) |
| 105 | 30** | Starter relay feed |
| 106 | 40** | Anti-lock Brake System (ABS) module (Pump) |
| 107 | 40** | Rear defroster relay feed |
| 108 | 20** | Moonroof |
| 109 | 20** | ABS module (Valves) |
| 110 | 30** | Wiper module |
| 111 | — | Not used |
| 112 | 30** | Air suspension compressor |
| 113 | — | Not used |
| 114 | — | Not used |
| 115 | — | Not used |
| 116 | — | Not used |
| 117 | — | Not used |
| 118 | — | Not used |
| 201 | 1⁄2 ISO relay | A/C clutch |
| 202 | — | Not used |
| 203 | 1⁄2 ISO relay | Ignition coil |
| 204 | 1⁄2 ISO relay | PCM |
| 205 | 1⁄2 ISO relay | Fog lamps |
| 206 | 1⁄2 ISO relay | Fuel |
| 207 | — | Not used |
| 208 | — | Not used |
| 209 | 1⁄2 ISO relay | Horn |
| 301 | Full ISO relay | Starter |
| 302 | Full ISO relay | Air compressor |
| 303 | Full ISO relay | Blower |
| 304 | Full ISO relay | Power windows relay (RUN/ACC) |
| 501 | Diode | A/C clutch |
| 502 | Diode | PCM |
| 503 | Diode | Horn, Door latch |
| 601 | 20A Circuit breaker | Power seats, Lumbar |
| 602 | 20A Circuit breaker | Power windows relay feed (RUN/ACC) |
| * Mini fuse ** Cartridge fuse |
Several fuses and relays in the power distribution box are directly related to the engine management system and overall vehicle diagnostics. For example, fuses linked to the Powertrain Control Module (PCM) such as fuses #3, #7, and #20, and relays like #204 (PCM relay) and #206 (Fuel relay) are essential for the engine’s operation and the diagnostic system’s ability to report issues. Problems within the OBD2 system can sometimes be traced back to these power feeds.
Utilizing Fuse Diagrams for OBD2 Diagnostics
While this article provides fuse box diagrams rather than a complete 2005 Mercury Grand Marquis Obd2 Wiring Schematic, understanding the fuse and relay layout is a fundamental step in diagnosing electrical problems that can affect your OBD2 system. Before seeking out complex wiring schematics, always start with the basics:
- Locate the OBD2 Port: Typically found under the dashboard on the driver’s side.
- Check Fuse #27 (Passenger Compartment): As highlighted, this fuse directly powers the OBD2 port.
- Inspect PCM Related Fuses and Relays (Power Distribution Box): Issues with PCM power can prevent proper OBD2 communication.
- Consult a Full Wiring Schematic (If Necessary): If fuse checks don’t resolve the issue, a detailed wiring schematic becomes invaluable for tracing circuit faults.
By using these fuse box diagrams in conjunction with your understanding of basic automotive electrical troubleshooting, you can effectively diagnose and address many electrical and OBD2 related problems in your 2005 Mercury Grand Marquis. For more complex diagnostic procedures, especially those requiring detailed wiring schematics, always consult a qualified automotive technician or repair manual.
