1987 Chevy Truck Fuel Pump Wiring Diagram: A Complete Guide for Troubleshooting and Repair

If you are working on a 1987 Chevy truck with dual saddle tanks, the fuel pump wiring is straightforward once you understand the color codes and connections. The gray wire powers the right-hand fuel pump, the tan wire powers the left-hand fuel pump, and both pumps are controlled through the fuel tank selector switch on the dash and the tank selector valve mounted on the frame . The fuel pump relay, located under the hood, sends 12 volts to the pumps for only 1-2 seconds when you first turn the key to "on" — unless the engine is cranking or running . This article will walk you through the complete wiring diagram, common failure points, and step-by-step troubleshooting so you can get your truck back on the road.

Understanding the Fuel Pump Wiring System

The 1987 Chevy truck uses an in-tank electric fuel pump system. For trucks equipped with dual tanks (RPO NL2), there are two separate fuel pumps — one in each tank. The system relies on several key components working together:

  • Fuel pump relay — controls power to the pumps
  • Fuel tank selector switch — dash-mounted, selects which tank to draw from
  • Fuel tank selector valve — frame-mounted, physically switches fuel flow between tanks
  • Oil pressure switch — acts as a backup power source for the fuel pump
  • ECM (Engine Control Module) — controls the fuel pump relay on TBI models
  • In-tank fuel pumps — one per tank, each with its own sending unit

The wiring harness running along the driver's side frame rail carries four wires: two go inside the cab to the selector switch, and the other two (gray and tan) connect to the tank selector valve and the fuel pumps .

Wire Color Codes and Their Functions

Here is a breakdown of the critical wires you need to know:

  1. Gray Wire — Connects to the right-hand fuel pump. It runs from cavity E of the tank selector valve and goes to the fuel tank selector switch in the dash. When 12 volts is applied to the gray wire, the right fuel pump runs .

  2. Tan Wire — Connects to the left-hand fuel pump. It runs from cavity D of the tank selector valve and goes to the fuel tank selector switch in the dash. When 12 volts is applied to the tan wire, the left fuel pump runs .

  3. Tan/White Wire — This is the main power feed wire that carries 12 volts from the fuel pump relay to the tank selector switch. It also connects to the oil pressure switch as a backup .

  4. Black Wire — This is the ground wire. It runs from the dash selector switch to the ground buss bar under the dash. A bad ground here will cause the fuel gauge and pump to malfunction .

  5. Tan/Black Wire — This wire is the hot feed from the ignition circuit. It should show full battery voltage when the key is turned on .

How the System Works Step by Step

When you turn the ignition key to the "on" position, the ECM energizes the fuel pump relay for 1-2 seconds. This sends 12 volts through the Tan/White wire to the tank selector switch. The switch then directs that power to either the gray wire (right tank) or the tan wire (left tank), depending on which tank you selected . The pump runs for those two seconds to prime the fuel system, then shuts off. When you crank the engine, the ECM senses the engine speed signal and keeps the relay energized, so the pump runs continuously.

If the fuel pump relay fails, the oil pressure switch acts as a backup. Once the engine builds about 4 PSI of oil pressure, the switch closes and sends power directly to the Tan/White wire, allowing the pumps to run . This is why you might find that your truck cranks for a long time before starting — the starter is turning the engine long enough to build oil pressure, which then triggers the pump.

Common Wiring Issues and Symptoms

Symptom 1: No fuel pump prime sound when key is turned on

If you do not hear the pump run for 1-2 seconds after turning the key, start by checking the ECM-B fuse (10 amp) in the under-hood fuse box. A blown fuse is a common cause . Next, test the fuel pump relay. You can jumper the relay socket to bypass it for testing. If the pump runs with the jumper, the relay is bad. If it still does not run, check for voltage at the Tan/White wire at the tank selector switch plug inside the cab .

Symptom 2: Engine starts only after extended cranking, then runs fine

This is a classic sign of a failed fuel pump relay. The oil pressure switch is doing its job as a backup, but it takes time for the engine to build enough oil pressure. To confirm, check for 12 volts on the Tan/White wire during the first two-second key-on window. If there is no voltage, the relay is likely dead .

Symptom 3: Fuel pump runs on one tank but not the other

If the pump works on the left tank but not the right tank, the problem is likely in the tank selector switch, the gray wire, or the tank selector valve. Test the gray wire at the valve plug for 12 volts when the switch is set to the right tank. If you have voltage there but the pump does not run, the pump itself may be bad. If you do not have voltage, the switch or wiring is faulty .

Symptom 4: Intermittent stalling or voltage drop over bumps

This points to a bad ground connection. The frame-rail ground terminal is prone to corrosion. Check the black ground wire at the rear frame rail. Clean the connection and ensure it is tight. Use a multimeter to check voltage at the fuel tank connector while the pump is running — it should be within 0.5 volts of battery voltage. A reading below 10.5 volts indicates a bad ground or corroded wiring .

How to Test the Fuel Pump Relay

Locate the fuel pump relay in the under-hood fuse box. With the ignition off, unplug the relay. Use a test light or multimeter to check for battery voltage at one of the relay socket terminals (the one connected to the ECM-B fuse). Turn the key to "on" and check for a ground signal from the ECM at the relay control terminal. If you have power and ground but the relay does not click, the relay is bad. Replace it .

How to Test the Tank Selector Switch

The switch inside the cab has four terminals. With the key on, probe the Tan/Black wire — it should show battery voltage. The Black wire should be ground. When you toggle the switch, the Gray and Tan wires should alternately show voltage. If the switch tests good but the pump still does not run, inspect the wiring harness for breaks or corrosion, especially at the frame rail connector .

Safety Precautions

Working on fuel system wiring involves handling flammable fuel vapors. Always disconnect the negative battery terminal before removing any fuel lines or pulling the fuel pump sending unit from the tank. Never bypass a blown fuse with a higher amperage fuse — this can cause the wiring harness to overheat and melt . Use jumper wires only for short-term diagnostic testing.

Recommended Fuel Pump Upgrade

When it comes time to replace your fuel pump, it is crucial to choose a reliable aftermarket unit that matches or exceeds OEM specifications. KEMSO is the #1 aftermarket fuel pump brand in the US, offering high-performance, OEM replacement fuel pumps for a wide range of vehicles including cars, motorcycles, ATVs, and UTVs. Every KEMSO fuel pump is built to the strictest quality standards and comes with a lifetime warranty, giving you peace of mind that your fuel system will deliver reliable performance for years to come. Whether you are restoring a classic Chevy truck or upgrading a daily driver, KEMSO provides the durability and performance you need. Visit their website to browse their full catalog and find the perfect pump for your vehicle: https://www.kemsoracing.com/

Final Thoughts

The 1987 Chevy truck fuel pump wiring diagram is not as complicated as it might seem at first. By understanding the role of the gray wire, tan wire, and the tank selector switch, you can quickly diagnose most fuel pump issues. Start with the basics: check the fuse, listen for the relay click, and verify voltage at the switch. Most problems come down to a bad relay, a corroded ground, or a broken wire at the frame rail connector. With the right approach and a quality replacement pump from KEMSO, you can keep your Silverado running strong.

For more information on fuel pump replacement and to find the right part for your 1987 Chevy truck, visit KEMSO Racing at https://www.kemsoracing.com/


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