1987 VW Cabriolet Fuel Pump Relay Location: Full Guide

For owners of the classic 1987 Volkswagen Cabriolet experiencing fuel delivery issues, the fuel pump relay is located within the relay plate panel mounted above the steering column on the driver's side, near the brake pedal. It is typically designated as position "12" or the relay number itself (often "173" or "175").

Finding a specific electrical component like the fuel pump relay in a classic car like the 1987 Volkswagen Cabriolet (also known as the Mk1 Cabriolet) can be surprisingly challenging, especially when performance problems arise. Understanding precisely where this critical relay resides, how to identify it, and how to access it is essential for efficient diagnosis and repair. This guide provides clear, step-by-step information to locate the fuel pump relay in your '87 Cabriolet.

Why the Fuel Pump Relay Matters

Before pinpointing its location, understanding the role of the fuel pump relay clarifies why its position is crucial information.

• The Middleman: Your Cabriolet's fuel pump requires significant electrical power to pump fuel from the tank to the engine. The relay acts as an essential intermediary. It allows a small, low-current signal from the ignition switch or engine control unit (ECU) to activate a switch inside the relay that safely carries the high current needed by the fuel pump. The relay uses an electromagnet to physically connect heavy-duty internal contacts. This method isolates the delicate ignition switch circuitry from the high-demand fuel pump circuit, offering significant protection against switch failure.
• Symptoms of Failure: When the fuel pump relay malfunctions, fails completely, or develops poor internal contacts, the fuel pump receives no power. This results in classic symptoms including the engine turning over normally when the starter motor activates but failing to start, a complete lack of fuel pump priming noise when you turn the ignition key to the "ON" position before cranking (listen near the rear of the car), and stalling immediately after starting. These symptoms are identical to a failed fuel pump, a bad ignition switch, or a blown fuel pump fuse, making the relay's condition a key diagnostic point. Identifying whether the relay is receiving the correct signals and passing power is a fundamental troubleshooting step.
• Critical Component: Because a healthy fuel pump relay is mandatory for engine operation, knowing how to access and inspect it is fundamental knowledge for any 1987 Cabriolet owner or mechanic. A properly functioning relay means the fuel pump gets power when commanded, primarily during the pre-cranking ignition stage and whenever the engine is rotating (cam or crank position sensors provide run signals). Failure means the engine simply cannot run, as vital fuel pressure never reaches the injectors.

Electrical System Layout in the 1987 VW Cabriolet

The Volkswagen Cabriolet of the late 1980s, sharing its core platform with the Volkswagen Golf Mk1 (Rabbit in North America), utilizes centralized relay and fuse panels within the driver's cabin for primary electrical control systems.

1. The Central Control Point: Unlike modern cars that distribute relays around the engine bay and cabin, the 1987 Cabriolet concentrates the majority of its critical relays and fuses in a primary control panel located conveniently for the driver.
2. Relay Plate Panel Position: The main relay panel assembly is mounted on the interior cabin side of the firewall (the steel partition separating the engine bay from the passenger compartment), positioned directly above the steering column and immediately behind and slightly above the brake pedal assembly. Some vehicle assemblies might also include related fuses nearby. This location consolidates power distribution and control circuits affecting engine operation and driver controls.
3. Access Requires Visibility: Seeing and accessing this relay panel necessitates looking upwards under the dash from the driver's side footwell area. The footwell offers primary physical access to the panel and its components. For most inspections and relay replacements, working from the driver's seat area with a suitable light source provides the needed access without requiring full disassembly, though some head positioning is necessary.
4. Driver-Centric Design: Placing the critical electrical controls inside the cabin offers protection from the harsh under-hood environment – extreme heat from exhaust components, vibration, moisture exposure, and road debris impacting the electrical connections. It prioritizes reliability through this physical protection strategy. While later models migrated more relays to the engine bay fuse box, the Mk1 design favours the cabin location for vital system relays like the fuel pump control.

Detailed Location: Finding the Fuel Pump Relay

Here's precisely where to find the vital component:

1. Driver's Seat: To access the critical relay panel assembly area, sit comfortably in the driver's seat. Position yourself such that your head is near the pedals for upward visibility under the dashboard structure. Some movement may be necessary to get a clear view.
2. Look Above Brake Pedal: Position your body for a clear view upwards into the space immediately above and behind the upper portion of the brake pedal assembly and the steering column base. You are looking towards the firewall section behind these control mechanisms. Proper illumination using a bright flashlight or work light is strongly recommended to see component details effectively and avoid mistakes.
3. Identify the Relay Panel: Focus your attention on the metal relay plate assembly attached directly to the cabin surface of the steel firewall. This panel holds multiple plug-in relays and potentially nearby fuse holders or mounting points for other controllers specific to the vehicle's options.
4. Relay Layout: This panel has several positions holding identical-looking relay modules. The specific positions correspond to different vehicle systems. Manufacturers label or number these positions for factory assembly accuracy. Clear identification relies on knowing the correct spot number or recognizing the relay designation markings directly on each unit.
5. Position 12: The standard location for the fuel pump relay in the 1987 Cabriolet is position "12" on this relay plate assembly. Some vehicle information diagrams may represent this as a numbered slot location.
6. Relay Numbers: The relay plugged into position 12 typically carries one of two identification markings directly molded onto its top plastic casing:
   • "173": Most commonly found.
   • "175": Also used, depending on specific build configuration factors like the engine type present in the car. Functionally, for the fuel pump activation task, these relays perform the same role. The number might indicate slight internal variations irrelevant to basic fuel pump control.
7. The Relay Unit: The fuel pump relay itself is a standard Bosch-style, four-terminal automotive relay. It has a rectangular plastic housing with an integral mounting bracket formed as part of the plastic case. Color can vary but is typically matte black or sometimes a translucent brown. The bottom has a multi-pin connector slot that plugs directly into a socket on the metal relay plate.
8. Visual Confirmation: Carefully locate the relay in position "12". Verify that the markings on its top surface correspond to either "173" or "175". Position numbers might be stamped or printed on the relay plate assembly near each socket.

Step-by-Step Access & Replacement

Safety and attention to detail are crucial when dealing with electrical components:

1. Safety First - Disconnect Battery: Crucially, always disconnect the negative (-) terminal of the vehicle's battery before handling relays or fuses. This action eliminates the risk of accidental short circuits and potential damage to the vehicle's sensitive electronics or personal injury from sparks. Secure the cable away from the battery post to prevent accidental contact during your work.
2. Gather Tools: Ensure you have a suitable light source (bright flashlight or work light), and possibly a small inspection mirror if visibility is tight. A pair of needlenose pliers might sometimes help extract a stubborn relay. Often, firm finger pressure pulling straight outward is sufficient for removal. Avoid damaging surrounding wires.
3. Position Yourself: Sit in the driver's seat. Slide the seat rearwards if possible to create more legroom for access. Recline the seat back slightly if needed to position your body lower for an upward sightline. Rest your shoulders on the seat cushion for stability while working in an awkward posture for an extended period.
4. Locate Relay: Using your light source, illuminate the relay panel above the steering column near the firewall surface. Identify the relay labeled "173" or "175" plugged into position "12" per the details outlined above. Mentally note its orientation and relationship to adjacent components.
5. Remove Old Relay: Firmly grasp the fuel pump relay unit by its plastic body. Avoid pulling wires! Apply steady pressure straight outward, away from the mounting panel. The relay is plugged into its socket base; a secure tug should release it. Wiggle slightly if necessary, but avoid excessive twisting force which could damage connector pins inside the relay base assembly.
6. Inspect Socket & Relay (Optional but Recommended): Visually inspect the plastic multi-pin socket on the vehicle side for any signs of burning, melting, corrosion, or bent/broken pins. This indicates excessive resistance or arcing, requiring further attention to avoid repeat failure. Examine the relay's terminals for significant corrosion or heat discoloration. Minor surface tarnish might clean off using electronic contact cleaner and gentle abrasion with fine sandpaper.
7. Install New Relay: Take your known-good replacement relay (new Bosch part recommended, specifying 173 or 175 as suitable for application). Orient it identically to how the old relay sat. Align the relay's plastic mounting bracket key with any slot on the metal panel. Push the relay directly onto its socket pins with firm, even pressure. Listen and feel for it seating completely. You should hear an audible click if retaining clips are present. Ensure no gaps remain between the relay body and the relay plate panel structure.
8. Reconnect Battery: Reconnect the negative (-) battery cable carefully to its original terminal position. Tighten the clamp bolt securely.
9. Test Operation: Turn the ignition key to the "ON" position (not Start). You should now distinctly hear the fuel pump operate with a humming/buzzing sound for approximately 1-2 seconds as the system primes. This noise usually originates near the fuel tank at the rear of the car. If you hear this priming sound, it confirms the relay is delivering power correctly. Attempt to start the engine.

Identifying the Correct Relay & Common Misconceptions

Given the presence of multiple identical-looking relays on the same panel, accurate identification is vital:

• Relay Numbers: As emphasized, look for the number molded into the relay case: "173" or "175". This is the primary identifier. Ignore adjacent relays unless specified otherwise. Consult a Cabriolet/Golf Mk1 wiring diagram for absolute confirmation if markings are ambiguous or missing due to age or previous repairs.
• Relay Appearance: The fuel pump relay is the standard Bosch type with a square-base, rectangular body. Earlier Cabriolet models might have all relays as this basic black box style. Later models or specific option packages might introduce different relay shapes for other functions, making the position number ("12") crucial for identification when visual labeling is difficult.
• Not the Same as...
  • Fuse: The relay is not the fuse. Fuses are designed to blow quickly if an electrical circuit experiences a severe short circuit or massive current overload, acting as a primary circuit protection mechanism. A fuse cannot switch power on and off repeatedly; its sole purpose is over-current protection. Fuses protect multiple circuits; locate those separately in the fuse box panel usually found near the driver's door hinge pillar or the relay panel assembly in the Cabriolet. The fuse for the fuel pump circuit (often fuse 28, rated at 15A or 20A depending on vehicle specification) must also be checked if the pump lacks power. Check both relay and fuse when diagnosing fuel pump failure conditions. The fuse protects the wiring downstream of the relay contact points.
  • Ignition Control Module (ICM): While also electrical, the ignition control unit manages spark generation specifically and is located elsewhere, typically near the distributor assembly or within the engine compartment. It handles triggering the ignition coil.
  • Coolant Thermo Switch Relay: This might be another relay near the fuel pump relay. The fuel pump relay solely controls fuel pump power, regardless of engine temperature conditions signaled elsewhere.

Diagnosing a Faulty Relay

Even after locating the relay, confirming its condition is essential. Symptoms might stem from other issues:

1. Listen for the Pump: Turn the ignition key to "ON". Do you hear a distinct "hum" or "buzz" from the rear of the car lasting 1-2 seconds? If Yes: The fuel pump relay and fuse are likely functioning correctly at that moment (power is getting to the pump), though intermittent relay failure can occur. Proceed to check fuel pressure at the engine fuel rail for an accurate pressure reading. If No: This strongly points to a problem in the circuit activating or powering the fuel pump (relay, fuse, wiring, pump, or switch inputs). Proceed to step 2.
2. The Swap Test (Simplest Check): If you have an identical spare relay (same Bosch "173" or "175"), swap the suspect fuel pump relay with a relay controlling a non-critical system that operates similarly (like the horn relay). Ensure the donor relay fits the same socket configuration (pin count and type). Turn the ignition "ON" again. If the pump NOW primes: The original fuel pump relay is faulty. If the pump still does NOT prime: The problem lies elsewhere (fuse, wiring, fuel pump itself, ignition switch, ECU, associated sensors). This swap test quickly isolates the relay from other potential causes of fuel pump inactivity.
3. Fuse Check: Locate the main fuse panel (often under dash near driver's door hinge pillar). Check the fuse designated for the fuel pump circuit (consult Cabriolet manual for precise location; often Fuse #28, rated 15A or 20A). Replace any blown fuse immediately. Determine why it blew (short circuit, pump failure); do not ignore the underlying cause. A blown fuse prevents power delivery even with a perfect relay component. Check fuse condition visually and with a multimeter set to continuity testing.
4. Testing with a Multimeter (Advanced): Requires electrical knowledge and a multimeter with continuity/resistance and DC voltage measurement capability.
   • Voltage Supply Test (Relay Removed): With ignition key in "ON" position (battery connected), probe the socket terminals corresponding to terminal 30 (constant battery power) and terminal 85 (relay coil ground, activated by the ECU/ignition switch). Terminal 30 should show battery voltage (approx. 12.6V with engine off). Terminal 85 should also show battery voltage briefly when ignition is first turned on (if the ECU provides the ground path signal). CAUTION: Risk of shorts. Be precise. Ideally, identify terminals using a wiring diagram.
   • Relay Output Test: If possible, connect the relay outside the socket using fused jumper wires. Apply battery voltage to terminals 85 and 86 to activate the coil. Test continuity between terminals 30 (power in) and 87 (power out). Continuity should exist when coil is powered. No continuity when unpowered. This tests the internal contact switching action.
   • Coil Resistance Test: Test resistance between terminals 85 and 86 (relay coil). A typical Bosch automotive relay coil reads 70-90 ohms. Infinite resistance indicates an open circuit (broken coil), 0 ohms indicates a shorted coil. Both mean the relay has failed internally.
5. Professional Help: If diagnosis becomes complex (no power on key inputs, complex wiring issues) or if you are uncomfortable testing electrical components, seeking professional automotive diagnostic services is prudent. They possess specialized tools like breakout boxes and in-depth wiring schematics that simplify fault tracing significantly in vintage vehicle electrical systems known for aging insulation and corrosion issues impacting signal integrity.

Preventative Maintenance & Tips

• Quality Replacement: If replacing the fuel pump relay, invest in a high-quality Bosch replacement part or an Original Equipment equivalent specified for the Cabriolet. Avoid cheap, generic automotive relays; they often have inferior internal components like contacts that weld shut or fail early under load. Stick with "173" or "175".
• Spare Relay: Considering the relay's critical role and relatively low cost, carrying a spare Bosch fuel pump relay in the glove compartment is a sensible precaution, especially for older vehicles operating beyond typical component life expectancy. This simple spare part can save significant roadside hassle should the primary relay fail unexpectedly during travel.
• Clean Connections: While the relay is accessible, periodically inspecting and cleaning the relay socket terminals and the relay's metal pins ensures optimal electrical conduction. Use dedicated electronic contact cleaner spray and a small brush. Light corrosion can cause high resistance leading to voltage drop and eventual failure, often intermittent in nature. Ensure the relay connector pins fit tightly into the socket receptacles; any looseness indicates worn internal connector contacts.
• Address Underlying Electrical Issues: Repeated fuel pump relay failures could indicate a deeper problem within the electrical system. Examine related wiring harness sections for chafing (worn insulation allowing shorts), exposed copper strands, previous non-standard modifications, or severe corrosion buildup compromising connections. Check the fuel pump itself for excessive current draw indicating mechanical resistance (impeller binding, worn motor brushes) placing high stress on the relay contacts. An aging pump with rising amperage demand can accelerate the degradation of the relay contact points.

1987 VW Cabriolet Fuel Pump Relay Frequently Asked Questions (FAQs)

• Q1: Is the fuel pump relay located inside the car or under the hood?
  • A: Inside the car. It’s mounted to the firewall above the brake pedal / steering column within the relay plate assembly. Unlike many modern vehicles where engine bay fuse boxes dominate, the 1987 Cabriolet centralizes control relays in the cabin driver zone specifically for protection from engine heat and moisture ingress issues.
• Q2: How do I tell the difference between relay 173 and 175?
  • A: The number "173" or "175" is molded directly into the plastic case on the top surface. Look closely at the relay in position "12". Both numbers are used, and both function as the fuel pump relay in different Cabriolet configurations. Functionally, for fuel pump control purposes, they are interchangeable components fulfilling the same basic electrical switching task.
• Q3: What happens if I install a relay in the wrong socket?
  • A: Installing the wrong relay type, or installing the correct relay type into the wrong position, can cause unpredictable results. At best, a system won't work (e.g., horns don't blow). At worst, it could potentially damage other components or cause a short circuit if incompatible pinouts energize circuits incorrectly. Always replace relays exactly where they were removed from. Label or take photos before removal if unsure. Position number "12" is designated solely for fuel pump control functions.
• Q4: Why does my car start sometimes but not others? Could it be the relay?
  • A: Absolutely. An intermittent fuel pump relay is a very common cause of random no-start conditions, especially in older VWs. Internal relay contacts become worn, carbonized, or sensitive to temperature or vibration. When they fail to make a clean connection, the fuel pump gets no power. Heat soak after driving can worsen intermittent relay failures. Testing or swapping the relay is the primary diagnostic step for such sporadic symptoms.
• Q5: After replacing the relay, I still don’t hear the fuel pump prime. What next?
  • A: Confirm the new relay is fully seated and in the correct position ("12"). Double-check the fuel pump fuse condition (visual or multimeter continuity test). Use a multimeter to verify power arriving at the fuse and then at the fuel pump wiring connector near the fuel tank. Check for a grounded activation signal at relay socket terminal 85 briefly when ignition turns on. Inspect wiring visually for obvious damage or disconnections. Check the integrity of the fuel pump ground connection (usually near the fuel tank or pump mounting bracket, prone to corrosion). The fuel pump motor itself could also be the point of failure requiring replacement. Systematically check power flow from the battery through to the pump motor windings.
• Q6: Can I bypass the fuel pump relay to test?
  • A: Temporary bypass is possible but requires understanding the specific relay terminal purposes and using fused jumper wires to avoid creating a fire hazard from sustained high current flow through undersized test wiring. Improper bypass can damage wiring, the pump, or cause fire. Mechanically shorting pins inside the relay socket creates sparks and risks severe damage. The preferred diagnostic path involves multimeter voltage tests at the relay socket or fuel pump connector rather than potentially hazardous relay bypass attempts without proper fused safety mechanisms in place.

Conclusion: Key to Restoration Success

Mastering the location and function of the fuel pump relay – firmly positioned in slot "12" on the driver-side firewall relay plate – transforms troubleshooting for your cherished 1987 VW Cabriolet. When the engine refuses to start or suddenly stalls without warning, this compact component is a top suspect demanding immediate inspection. While its position requires maneuvering within the footwell cavity, the methodical steps for accessing and replacing it are achievable with careful preparation. Always prioritize safety by disconnecting the battery before starting any electrical work. Utilize simple diagnostics like listening for the fuel pump's priming sound or swapping relays before undertaking more complex testing procedures. By incorporating periodic relay checks and maintaining clean electrical connections in this crucial engine control system, you ensure that your classic Cabriolet continues its journey reliably for many years to come.