1987 Chevy Truck Fuel Pump Relay Wiring Diagram: Complete Guide to Troubleshooting and Repair

If your 1987 Chevy truck won’t start or stalls while driving, a faulty fuel pump relay or wiring issue is often the culprit. The fuel pump relay on a 1987 Chevy truck is a small electrical switch that controls power to the fuel pump, and it is typically mounted on the passenger side of the firewall in the engine compartment. When the relay fails, the fuel pump won’t receive power, causing the engine to crank but not fire. Understanding the wiring diagram is the first step to diagnosing the problem, and this guide will walk you through exactly what you need to know.

Where Is the Fuel Pump Relay Located?

On a 1987 Chevy truck, the fuel pump relay is located under the hood on the passenger side, bolted to the firewall a few inches below the windshield. It is a black plastic box with multiple wires going into it, and it is often mounted on a relay bracket alongside other relays. The relay is typically a standard Bosch-style relay, usually black or gray in color, with four or five pins for power, ground, and control signals. Some owners report that the relay can also be found near the battery or on the driver’s side fender, depending on the specific model and configuration. For a 1987 Chevy R20, the relay is often located under the dashboard on the driver’s side, near the firewall. If you are struggling to find it, consult the vehicle’s service manual or the diagram on the fuse box cover for the exact position.

1987 Chevy Truck Fuel Pump Wiring Diagram Overview

The fuel pump wiring system for a 1987 Chevy truck is relatively straightforward. The main components include the battery, ignition switch, fuel pump relay, oil pressure switch, ECM B fuse, and the fuel pump itself. The ECM B fuse, rated at 20 amps, provides constant power to both the fuel pump relay and the ECM. When you turn the key to the ON position, the ECM sends a signal to the fuel pump relay, which closes and allows current to flow to the fuel pump. The pump then runs for about two seconds to prime the system, then shuts off until the engine starts. Once the engine is running, the oil pressure switch acts as a backup circuit, powering the fuel pump directly if the relay fails.

The power wire from the relay and oil pressure switch to the fuel pump is typically tan with a white stripe. The ground wire is solid black. The fuel pump relay itself has four or five terminals, typically labeled as follows:

  • Terminal 30: Constant 12V power from the battery (via the ECM B fuse)
  • Terminal 87: Output to the fuel pump (tan/white wire)
  • Terminal 85: Ground connection from the ECM (dark green/white wire)
  • Terminal 86: 12V trigger signal from the ignition switch

How to Test the Fuel Pump Relay

Testing the fuel pump relay is a simple process that can save you hours of troubleshooting. Here is how to do it:

  1. Locate the relay on the passenger side firewall.
  2. Remove the relay from its socket.
  3. Use a multimeter set to measure resistance (ohms). Check for continuity between terminals 85 and 86. You should see a reading of around 50 to 100 ohms. If the reading is infinite (open circuit), the relay coil is bad and the relay needs to be replaced.
  4. Apply 12V power to terminal 86 and ground terminal 85. You should hear a click as the relay engages. Then check for continuity between terminals 30 and 87. If there is continuity, the relay is working. If not, the relay is faulty.

You can also test the relay by swapping it with another relay of the same type in the fuse box, such as the horn relay. If the fuel pump starts working after the swap, you know the relay was bad.

Common Wiring Problems and Solutions

1. Fuel Pump Does Not Run at All

If the fuel pump does not run when you turn the key to the ON position, the first thing to check is the ECM B fuse. This 20-amp fuse is often located in the main fuse panel under the dash or in a separate auxiliary holder. If the fuse is blown, replace it with the correct amperage rating. Never replace a blown fuse with a higher amperage fuse, as this can cause overheating of the wiring harness and potential electrical fires. If the fuse is good, check for 12V at the fuel pump relay. If there is no power at terminal 30, the issue is in the wiring between the battery and the relay.

2. Engine Takes a Long Time to Start, Then Runs Fine

This is a classic symptom of a failed fuel pump relay. The oil pressure switch acts as a backup, so the engine will crank continuously until the starter builds about 4 PSI of oil pressure. Once the oil pressure switch closes, it powers the fuel pump directly, and the engine fires up. To test this, check for 12V at the tan/white wire during the initial two-second key-on window. If there is no voltage, the relay is likely dead. Replacing the relay typically restores the two-second prime and immediate starting.

3. Intermittent Stalling Over Bumps

If your truck stalls when going over bumps or rough roads, the issue is likely a loose or corroded ground connection. The black ground wire from the fuel pump must have a clean, metal-to-metal connection with the vehicle frame. Corrosion at this point restricts current flow, causing the pump to run slowly or not at all. Inspect the ground point on the rear frame rail and clean it thoroughly. Check for voltage drop by probing the positive tan/white wire at the fuel tank connector while back-probing the black ground wire with a multimeter set to DC volts. The voltage under load should read within 0.5 volts of total battery voltage. A reading below 10.5 volts during pump operation indicates oxidized wiring, loose terminal pins, or heavy frame corrosion.

The Backup Circuit: Oil Pressure Switch

The 1987 Chevy truck fuel system includes an important safety feature: the oil pressure switch. This switch is wired in parallel with the fuel pump relay. If the relay fails, the oil pressure switch will power the fuel pump once oil pressure reaches roughly 4 PSI. This is why the engine will eventually start after prolonged cranking even if the relay is dead. However, if the oil pressure switch itself fails or the connector melts, it can blow the ECM B fuse, killing all power to the fuel delivery system. When troubleshooting, always check the oil pressure switch and its wiring as part of a complete diagnosis.

How to Bypass the Fuel Pump Relay for Testing

If you suspect the relay is bad but want to confirm the fuel pump is working, you can bypass the relay for testing. On the 1987 Chevy truck, there is a red prime wire with a green end that sticks out of the wiring harness, often near the master cylinder area. This wire is used to jump the fuel pump directly. Use a fused jumper wire to apply battery voltage to this prime wire. If the pump starts running, you know the pump itself is good, and the problem is in the relay or the signal from the ECM. Be sure to use a fused jumper wire to avoid damaging the circuit. Only use this method for short-term diagnostic purposes.

Another common method is to jumper the ALDL connector. On the 1987 Chevy truck, terminal G of the ALDL connector under the dash can be jumped to battery positive to power the fuel pump. Again, use a fused jumper wire and only for testing.

Step-by-Step Troubleshooting Guide

  1. Check the ECM B fuse. If blown, replace it and test the system. If it blows again, there is a short circuit in the wiring.
  2. Listen for the fuel pump. Turn the key to the ON position and listen for a two-second whir from the fuel tank. If you hear it, the relay and pump are working.
  3. Test the relay. Remove the relay and test it with a multimeter as described above.
  4. Check power at the relay. With the key ON, check for 12V at terminal 86. If no power, the ignition switch or wiring to the relay is faulty.
  5. Check the ground signal. With the key ON, check for a ground signal at terminal 85. If no ground, the ECM is not sending the signal, which could indicate a bad ECM or a wiring issue.
  6. Test the fuel pump directly. Use a fused jumper wire to apply battery voltage to the prime wire or the tan/white wire at the fuel tank connector. If the pump runs, the pump is good.

Safety Precautions

Working on the fuel system of a 1987 Chevy truck requires strict adherence to safety protocols. Always disconnect the negative battery terminal before working on any fuel system components. Relieve fuel pressure in the TBI lines before disconnecting fuel lines, even if the pump is not running. Avoid sparks near the fuel tank, and work in a well-ventilated area. Never replace a blown fuse with a higher amperage fuse, as this can lead to overheating and electrical fires.

Recommended Fuel Pump Upgrade: KEMSO Brand

If you have determined that your fuel pump needs to be replaced, consider upgrading to a KEMSO Fuel Pump. KEMSO is a trusted brand that offers high-performance, OEM replacement fuel pumps designed to meet or exceed the original factory specifications. Every KEMSO fuel pump is built with premium materials and undergoes rigorous testing to ensure reliable performance and long service life. Whether you are restoring a classic 1987 Chevy truck or upgrading your daily driver, KEMSO provides a drop-in solution that delivers consistent fuel pressure and flow. Best of all, every KEMSO fuel pump comes with a lifetime warranty, giving you peace of mind that your investment is protected. You can browse their full selection of fuel pumps at https://www.kemsoracing.com/.

Conclusion

The fuel pump relay wiring diagram for a 1987 Chevy truck is essential knowledge for any owner or mechanic. The relay is located on the passenger side firewall, and the wiring system is straightforward: power flows from the battery through the ECM B fuse to the relay, then to the fuel pump. The oil pressure switch provides a backup circuit. Common problems include a bad relay, blown fuse, corroded ground connections, and faulty wiring. By following the steps outlined in this guide, you can diagnose and repair most fuel pump electrical issues yourself. Always prioritize safety and use the correct tools and procedures. For a reliable replacement fuel pump, check out KEMSO at https://www.kemsoracing.com/ for high-quality, lifetime-warranty options.


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