1980 Mercedes 450SL Fuel Pump Relay Location & Essential Replacement Guide

For owners of the classic 1980 Mercedes-Benz 450SL (R107 chassis), the fuel pump relay is invariably located behind the main instrument cluster within the dashboard. This critical component, also known as the K-Jetronic Control Relay or often referred to by its common Mercedes part number 001 545 90 32, resides buried deep within the dashboard structure. Accessing it requires careful removal of the driver's side lower dash panels and potentially steering column shrouds. Its failure is a frequent cause of a non-starting condition, where the engine cranks but fails to fire due to the fuel pumps receiving no power.

Understanding This Vital Component

The fuel pump relay in your 1980 Mercedes 450SL serves a pivotal role within the Bosch K-Jetronic continuous fuel injection system. This electro-mechanical switch is not merely a passive component; it acts as the high-current conductor responsible for energizing both the primary fuel pump located near the fuel tank and the auxiliary pump often situated near the fuel accumulator in the engine bay. Without a functioning relay, the fuel pumps remain inactive, preventing fuel from reaching the injection system. Crucially, this relay also powers other essential elements of the fuel delivery circuit, including the thermostatic time switch and the cold start valve. Modern replacements might differ slightly in external design but maintain identical electrical functionality and pin configuration, ensuring compatibility with your classic SL.

Precise Location Details: Behind the Gauges

Pinpointing the relay requires focusing on the area directly behind the driver's instrument cluster. Specifically:

  1. General Area: It is mounted on the metal firewall/bulkhead, situated above and slightly to the left (towards the center of the car) of the driver's pedals. It sits deep within the dashboard structure.
  2. Mounting: The relay is secured to a metal bracket bolted to the bulkhead.
  3. Relationship to Cluster: It is located immediately behind and beneath the main instrument cluster. While the cluster itself occupies the visible space behind the steering wheel, the relay is mounted on the structural metal just below the cluster's mounting area.
  4. Physical Identification: Look for a roughly square or rectangular, mostly black plastic housing, approximately the size of two decks of cards stacked together (approx. 80mm x 80mm x 35mm). Classic original Bosch units often have a transparent cover revealing the internal circuitry and a distinct green or beige base, with part numbers like "0 190 545 " (the last two digits varying) or Mercedes number 001 545 90 32 printed on the case. Replacement relays from Bosch (modern number: 0 332 514 4**) or other brands might be slightly different in appearance but share the same footprint and pin layout.
  5. Wiring: It connects via a multi-pin plug socket directly onto its base.

Why This Location is Troublesome

The position chosen by Mercedes engineers presents significant challenges:

  1. Extreme Inaccessibility: Simply glancing under the dash won't reveal the relay. It is buried deep within the dashboard structure.
  2. Limited Physical Access: Human hands, even smaller ones, struggle to reach it comfortably. Tight working angles are unavoidable.
  3. Poor Visibility: Seeing the relay clearly without removing obstructive panels is nearly impossible. Diagnosis and manipulation are done largely by feel.
  4. Environmental Vulnerability: Its location exposes it to heat buildup behind the dash, vibrations from driving, and potential dust accumulation over decades. Proximity to the heater box can also contribute to heat cycling stress.
  5. Link to Other Issues: Physical shock or electrical surges caused by nearby components failing (like starter motor solenoids or ignition switches) can inadvertently damage the relay.

Symptoms of a Failing Fuel Pump Relay (1980 450SL)

Recognizing the signs is crucial for diagnosis:

  1. Sudden No-Start (Cranks, No Fire): The most definitive sign. Engine cranks perfectly healthy but shows absolutely no sign of firing. This occurs because the fuel pumps are not running.
  2. Silent Fuel Pumps: With the ignition switched to "Run" (before cranking), no audible humming or buzzing sound should be heard from the fuel tank area or engine bay near the fuel filter/accumulator. Important Note: After years, pump sounds might become faint. Use a mechanic's stethoscope to confirm silence.
  3. Intermittent Starting Failure: The relay works intermittently, leading to occasional "dead" mornings where the car refuses to start, only to work perfectly later, perhaps after the car has cooled down or been jostled.
  4. Car Stalling Unexpectedly: While driving, the relay can cut out, instantly killing the fuel pumps and causing the engine to die abruptly, as if the ignition was switched off. Often, the car will restart after a few minutes if the relay contact resets.
  5. Inoperative Cold Start System: If the cold start valve doesn't function during cranking in cold weather (a vital part of starting a K-Jet system cold), it can point to the relay not supplying power to the thermostatic time switch and cold start valve.

Step-by-Step Guide to Accessing & Replacing the Relay (1980 450SL)

Disclaimer: Working behind the dash involves potential risks to vehicle electronics and personal comfort. Proceed with caution. Disconnect the negative battery terminal before beginning. This process requires patience.

  1. Prepare & Position:

    • Ensure the car is parked on level ground with the parking brake firmly engaged.
    • Disconnect the negative (-) battery cable and secure it away from the battery terminal. Wait 5-10 minutes for residual systems to discharge.
    • Adjust the steering wheel to its lowest position and tilt it fully upwards.
    • Move the driver's seat as far back as possible to create maximum legroom.
    • Gather tools: Medium Phillips screwdriver, 10mm socket/wrench, small flat-head screwdriver for trim clips, good flashlight or work light. A small mechanics mirror is very helpful.
    • Wear eye protection. Cover sharp edges under the dash with tape or towels if possible.

  2. Remove Lower Steering Column Shroud:

    • Locate the two or three Phillips screws securing the lower half of the plastic shroud surrounding the steering column. Typically, one is near the front center, and one is on each side towards the driver's knees. Remove these screws.
    • Carefully separate the lower shroud (sometimes in two pieces) and set it aside.

  3. Remove Upper Steering Column Shroud (If Necessary):

    • If visibility and access remain poor, remove the upper shroud piece. This usually involves finding hidden screws near the base where it meets the column and/or carefully prying apart clips holding it to the lower piece. Be gentle to avoid snapping clips.

  4. Remove Driver's Side Lower Dash Panel ("Kick Panel"):

    • Look upwards and inwards from the footwell. You will see a large plastic panel running vertically from the base of the dash down towards the carpet by the pedals.
    • Locate and remove all visible Phillips screws holding this panel – typically near the edges and towards the front/car front.
    • Carefully pull the panel away. It may have clips along its top edge or near the hood release. Wiggle gently until it releases. Set aside.

  5. Locate the Relay:

    • Shine your flashlight up and slightly left (towards the center console) from the now-exposed footwell area. Focus above the pedals, behind the space where the removed lower dash panel was.
    • Look upwards towards the back of the instrument cluster. Your target is the metal bulkhead/firewall. Scan for the rectangular relay (described earlier) mounted on a bracket bolted to this metal panel.
    • Use a small mechanics mirror held at an angle if direct line of sight is blocked. It is above the pedals, behind the cluster, and mounted on the bulkhead. Identify the wiring harness plugged into its base.

  6. Access & Removal:

    • This is the most challenging part due to space constraints.
    • Method 1 (Feasible for Many): Carefully reach your hand and arm up towards the relay, navigating around pedals and wiring. You may need to position your body lying on your back in the footwell. Feel for the relay body and the socket. The socket usually has a simple locking tab. Press this tab firmly with your thumb while gripping the relay body. Pull the socket straight off the relay. It might be tight. Once unplugged, unscrew or unclip the relay from its mounting bracket.
    • Method 2 (Improved Access - Steering Wheel Removal Recommended for best results): For significantly easier access, removing the steering wheel is highly recommended. This requires a special deep-well 24mm or 27mm socket (size varies slightly by year) and ideally a wheel puller. Disconnect the battery negative first. Remove the horn pad (usually clipped or screwed from behind) and disconnect horn wiring. Mark wheel and column shaft alignment. Remove the center nut/washer/wave washer. Use a puller to remove the wheel. THIS reveals a much clearer path to the relay location behind the cluster. Unplug the connector as above and remove the relay. Reinstallation requires correct torque on the steering wheel nut for safety.

  7. Install New Relay:

    • Position the new relay onto its mounting bracket and secure it (clip or screw).
    • Firmly press the wiring connector straight onto the relay base until the locking tab clicks into place. Give it a gentle tug to confirm it's secure. The pins can only align one way.

  8. Reassembly:

    • Reverse the disassembly steps meticulously.
    • Reinstall the lower dash panel, aligning it carefully and tightening all screws firmly.
    • Reinstall the upper steering column shroud (if removed), ensuring clips engage.
    • Reinstall the lower steering column shroud, tightening screws.
    • If steering wheel was removed: Carefully place the wheel back on the splines using your alignment marks. Reinstall the wave washer, flat washer, and tighten the large center nut to the proper torque specification (consult R107 service manual - typically between 45-65 Nm / 33-48 ft-lbs, but varies - CRITICAL SAFETY STEP). Reconnect horn wiring and secure horn pad.
    • Reconnect the negative battery cable.

  9. Testing:

    • Before cranking, turn the ignition key to the "Run" (ON) position. You should immediately hear a distinct humming or buzzing sound emanating from the rear of the car (main fuel pump in/near the tank) and/or near the engine bay fuel filter/accumulator (auxiliary pump).
    • If the pumps prime audibly, crank the engine. It should start normally.
    • If no pump sound occurs: Double-check the battery connection. Re-verify the relay plug is fully seated. Consider re-checking fuse 1 (Fuel Pump Fuse - 15A) located in the main fuse box under the hood near the battery. If still silent, the new relay itself could be faulty (rare but possible), wiring damage exists, or the fuel pumps have coincidentally failed (less likely but testable by applying direct 12V power to them).

Testing the Relay (Bench Test) - Recommended Step

Before installing a replacement, or to confirm the old one is truly faulty, perform a bench test:

  1. Identify Terminals: Locate the pin numbering (usually tiny numbers molded into the plastic base near the pins - or see diagram below).
  2. Basic Function Test (Activation):
    • Apply GROUND to terminal 85 (typically).
    • Apply +12 Volts to terminal 86 (typically). You should hear/feel a distinct CLICK from inside the relay as the internal electromagnet activates.
  3. Contact Test (Load Carrying):
    • Use a multimeter in Ohms/Continuity mode.
    • Identify the high-current paths:
      • Terminal 30: Common Input (Usually fused constant Battery +12V)
      • Terminal 87: Normally Open Output (Sends power to Fuel Pumps when activated)
      • Terminal 87a: Normally Closed Output (Not usually used in this relay on a 450SL).
      • Terminal 88: Often Auxiliary Output (sometimes powers Cold Start Valve circuit). Check continuity with terminal 30.
    • Without Power Applied (Relay Off): Continuity should exist between terminal 30 and terminal 88. There should typically be NO CONTINUITY between terminals 30 and 87. (Confirm behavior for your specific pinout).
    • With Power Applied to 85 & 86 (Relay On - Clicked): Continuity should exist between 30 and 87. Continuity between 30 and 88 may now be open (depends on design - some retain 30-88 even when 30-87 is active). The critical requirement is continuity between 30 and 87 when activated. Lack of continuity here confirms a faulty relay. Failure to click also indicates a fault.

Finding a Quality Replacement Relay

Avoid generic auto parts store relays. Seek these options:

  1. Genuine Mercedes-Benz: Parts counters still often stock these vital relays (e.g., A0015459032). Most expensive, highest reliability assurance.
  2. Bosch Original Spec: Bosch produces replacement relays (e.g., Bosch 0 332 514 4). Excellent quality, OEM manufacturer. Widely available online retailers and European auto parts specialists.
  3. Reputable Aftermarket Brands: Hella, Beru, etc., offer specific replacements. Ensure it lists compatibility with the 1980 450SL (R107) and Bosch K-Jetronic fuel injection.
  4. Salvage Yard/Used: Exercise extreme caution. Old relays are prone to failure. Test rigorously before purchase/installation.

Preventative Maintenance & Reliability Tips

  1. Avoid Water: Keep the driver's side floor dry. Wet carpets contribute to humidity buildup behind the dash, accelerating relay corrosion.
  2. Dash Heat: Use sunshades during hot weather to reduce cockpit temperature stress.
  3. Spare Relay: Purchase a known good replacement and keep it securely in the glove compartment or tool kit. Being stranded is frustrating; a 5-minute relay swap gets you moving again.
  4. Cleanliness: Periodically vacuuming under the driver's dash removes dust/dirt accumulation around components.
  5. Electrical System Health: Ensure your battery terminals are clean and tight, and the alternator is charging correctly (around 13.8-14.4V running). Voltage spikes stress electronics.
  6. Avoid Aftermarket Start/Immobilizers: Poorly installed security systems are notorious for damaging original electrical components like this relay. Keep wiring modifications professional.
  7. Diagnosis First: While the relay is a common suspect, verify fuel pressure and pump operation conclusively before condemning it. Check fuse 1 under the hood!

Understanding Related Components

  • Main Fuel Pump: Located inside or adjacent to the fuel tank. Primarily responsible for supply.
  • Auxiliary Fuel Pump: Often found near the fuel filter/accumulator in the engine bay, it primarily aids priming and high-pressure delivery.
  • Fuel Accumulator: Maintains residual pressure in the injection lines after shutoff to prevent vapor lock and assist hot restarts.
  • Fuel Filter: Critical for protecting the expensive injection components from debris. Replace regularly.
  • Control Pressure Regulator: A key component atop the fuel distributor regulating pressure based on engine temperature and load.
  • Thermostatic Time Switch: Controls cold start enrichment duration based on coolant temperature.
  • Cold Start Valve: Injects extra fuel during cranking when cold.
  • Fuse 1 (15A "Fuel Pumps"): Located in the under-hood fuse box near the battery. Protects the fuel pump circuit. Always check this first if pumps are silent.

Troubleshooting Beyond the Relay

If replacing the relay doesn't fix the issue, proceed systematically:

  1. Check Fuses Again: Reconfirm fuse 1 in the under-hood box. Also, check other critical fuses like ignition (fuse 2, 12A).
  2. Verify Pump Operation (Direct Power Test): Disconnect the wiring plug at the main fuel pump. Connect fused jumper wires directly from the battery (Positive to pump terminal, Negative to pump body/ground). If the pump doesn't run, it has failed. Use caution - fuel vapors can be present.
  3. Inspect Wiring: Visually trace the wiring harness running from the relay plug towards the pumps, especially in the engine bay near the fuel accumulator/filter area and under the car near the tank. Look for chafing, corrosion, pinched wires, or rodent damage. Pay attention to connectors - clean terminals with contact cleaner and ensure tight fits. Check the grounds for the pump motors and the relay itself.
  4. Check Ignition Switch: Power to terminal 86 of the relay is usually switched via the ignition key. A failing ignition switch might not be sending the "Run" signal to the relay coil. Test for power at terminal 86 (with ignition ON, key in Run position, before cranking) when attempting to start. If missing, the issue might be upstream (ignition switch, wiring).
  5. Measure Fuel Pressure: Use a dedicated K-Jet pressure gauge set to measure residual pressure (should hold for several minutes after shutdown) and running pressure. Lack of pressure confirms a fuel delivery failure even if pumps run (clogged filter, bad accumulator, leaks, failing fuel distributor).
  6. Consult Wiring Diagrams: A high-quality R107 wiring diagram is invaluable for tracing circuits. Identify the specific wire colors for power feeds and grounds.

Knowing precisely where the 1980 Mercedes 450SL fuel pump relay hides – behind the instrument cluster, mounted on the firewall above the pedals – and possessing the methods to access, test, replace, and troubleshoot it, empowers every R107 owner. While the location is famously awkward, mastering its service is essential for reliable operation of these iconic German roadsters. Keep a tested spare relay readily available as your first line of defense against a frustrating crank-no-start scenario. Patience, the right tools, and this detailed guide will see you through.

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