1999 GMC Sierra Fuel Pump Wiring Diagram: A Complete Troubleshooting Guide
Understanding the Fuel Pump Electrical System
The 1999 GMC Sierra fuel pump electrical system is designed with two separate power sources. The primary power source is the fuel pump relay, which is controlled by the PCM. The secondary power source is the oil pressure sending unit, which can keep the fuel pump running if the relay fails. This dual-path system is a safety feature that ensures the fuel pump only runs when the engine is running or cranking.
The fuel pump itself is located inside the fuel tank, mounted on the fuel pump module assembly. The module includes the fuel pump, sending unit, and fuel pressure sensor . The wiring harness runs from the fuel pump module up along the frame rail to the engine compartment. The main components you need to know about are:
- Fuel pump fuse – Located in the underhood fuse box
- Fuel pump relay – Also in the underhood fuse box
- Oil pressure sending unit – Mounted on the engine block
- PCM – Controls the fuel pump relay
- Fuel pump module – Contains the pump and sending unit
Locating the Fuel Pump Fuse and Relay
The fuel pump fuse and relay are both located in the underhood fuse box, also called the Power Distribution Center. On a 1999 GMC Sierra, this fuse box is located on the driver's side of the engine compartment, near the battery.
Fuel pump fuse location: The fuel pump fuse is a 20-amp mini fuse. On the 1999 GMC Sierra 1500, 2500, and 3500 models, the fuel pump fuse is typically labeled "FUEL PUMP" or "FP" in the fuse box diagram. According to the fuse box diagram for the 1999 GMC Sierra 2500, the fuel pump fuse is located in position F7 and is a 20-amp fuse . For the 1999 GMC Sierra 1500, the fuse box diagram shows the fuel pump fuse in a similar position, often labeled as "FUEL PUMP" or "F/PUMP" .
Fuel pump relay location: The fuel pump relay is also inside the underhood fuse box. On the 1999 Sierra, it is usually a standard 4-pin or 5-pin relay. The relay is labeled "FUEL PUMP" or "FP RELAY" on the fuse box cover. If you don't have a cover diagram, you can find the relay by looking for a relay that is larger than the standard micro relays, typically a standard ISO relay.
Fuel Pump Wiring Diagram for 1999 GMC Sierra
The wiring diagram for the 1999 GMC Sierra fuel pump system is similar across all models (1500, 2500, 3500) with either 2WD or 4WD. The complete electrical schematic for the 1999 GMC Sierra 2500 shows the power distribution system in detail . Here is the basic wiring path:
Power flow from battery to fuel pump:
- Battery positive terminal connects to the underhood fuse box
- From the fuse box, power goes to the fuel pump relay (pin 30 on the relay)
- When the relay is activated, power flows from pin 87 to the fuel pump fuse
- From the fuse, power goes to the fuel pump via the wiring harness
- The fuel pump ground wire connects to the chassis ground near the fuel tank
Control circuit wiring:
The PCM controls the fuel pump relay by grounding the relay coil (pin 85). When the ignition key is turned to the ON position, the PCM sends a 2-second ground signal to the relay to prime the fuel pump. If the engine does not start, the PCM stops grounding the relay after 2 seconds. Once the engine is running, the PCM keeps the relay grounded continuously.
The oil pressure sending unit provides an alternative power path. When oil pressure reaches about 4 psi, the sending unit closes a switch inside the sensor, which provides 12V power directly to the fuel pump. This is why the fuel pump can still run if the relay fails, as long as the engine is running and has oil pressure .
How to Test the Fuel Pump Fuse
Testing the fuel pump fuse is the first step in diagnosing a fuel pump problem. Here is a simple procedure:
Step 1: Locate the underhood fuse box and remove the cover. Find the fuel pump fuse. It should be a 20-amp fuse, usually colored yellow or clear.
Step 2: Remove the fuse using a fuse puller. Look at the metal strip inside the fuse. If the strip is broken or melted, the fuse is blown.
Step 3: Use a multimeter to test the fuse for continuity. Set the multimeter to the ohms setting or continuity setting. Touch one probe to each terminal of the fuse. If the multimeter shows a reading near 0 ohms (or beeps on continuity mode), the fuse is good. If it shows infinite resistance or no beep, the fuse is blown.
Step 4: If the fuse is blown, replace it with a new 20-amp fuse. Never use a higher amperage fuse, as this can cause wiring damage or fire. If the new fuse blows immediately, there is a short circuit in the wiring that needs to be found.
Testing the Fuel Pump Relay
The fuel pump relay is a common failure point. You can test it with a multimeter or by swapping it with a known good relay of the same type.
Method 1: Swap test – Find another relay in the underhood fuse box that is the same type and part number. Common relays that are the same type include the horn relay, A/C compressor relay, or starter relay. Swap the fuel pump relay with one of these. If the fuel pump now works, the original relay was bad.
Method 2: Multimeter test – Remove the fuel pump relay. Set your multimeter to measure resistance (ohms). Check the coil terminals (typically pins 85 and 86) – you should see a resistance reading between 50 and 120 ohms. If the reading is infinite (open circuit) or very low (shorted), the relay coil is bad. Next, check the switch terminals (pins 30 and 87). With the relay not energized, there should be no continuity between pins 30 and 87. If there is continuity, the relay is stuck closed.
Checking the Wiring Harness
The wiring harness that runs from the underhood fuse box to the fuel pump is a common source of problems. Over time, the wires can become chafed, corroded, or broken. The most vulnerable areas are where the harness runs along the frame rail and near the fuel tank.
Check for voltage at the fuel pump connector: The fuel pump connector is located on top of the fuel tank, accessible by removing the bed or lowering the fuel tank. With the ignition key turned to the ON position (engine not running), use a multimeter to check for 12V at the fuel pump connector. The power wire is typically the dark green wire with a white stripe, and the ground wire is usually black. If you have 12V at the connector but the pump does not run, the pump itself is bad.
Check for ground continuity: With the multimeter set to ohms, check for continuity between the black wire at the fuel pump connector and a clean chassis ground. You should have near 0 ohms. If the reading is high, there is a poor ground connection.
Visual inspection: Carefully inspect the wiring harness along the frame rail. Look for any signs of rubbing, chafing, or corrosion. Pay special attention to areas where the harness passes through holes in the frame or near sharp edges. Also check the connector at the fuel tank for corrosion or damaged pins.
Common Problems and Solutions
Based on the electrical system design, certain problems are more common than others on the 1999 GMC Sierra.
Problem 1: Fuel pump runs for 2 seconds then stops, engine will not start. This often indicates a problem with the ignition signal or the PCM not receiving a crank signal. The fuel pump relay is being powered for the initial prime, but the PCM does not sense the engine cranking, so it stops the relay. Check the crankshaft position sensor and the ignition switch.
Problem 2: Fuel pump does not run at all. This can be caused by a blown fuse, a bad relay, a failed fuel pump, or a broken wire. Start by checking the fuse and relay. If they are good, check for voltage at the fuel pump connector. If there is no voltage, the problem is in the wiring or the PCM. If there is voltage, the fuel pump itself is likely bad.
Problem 3: Fuel pump runs continuously even with the key off. This is usually caused by a stuck relay or a shorted wire in the control circuit. Remove the fuel pump relay to see if the pump stops. If it does, the relay is stuck closed. If it does not stop, there is a short to power in the wiring between the relay and the fuel pump.
Problem 4: Intermittent fuel pump operation. This is often caused by a poor connection or a corroded wire. Check the connector at the fuel tank, the ground connections, and the relay socket for corrosion. Also check the wiring harness for chafing that may only make contact when the vehicle hits a bump.
Using the Oil Pressure Sensor as a Backup
The oil pressure sending unit on the 1999 GMC Sierra serves a dual purpose. It sends the oil pressure reading to the instrument cluster, and it also provides a backup power source for the fuel pump. Inside the sending unit is a switch that closes when oil pressure reaches about 4 psi. When this switch closes, it connects battery power directly to the fuel pump, bypassing the relay.
This is a safety feature that allows the fuel pump to continue running even if the relay fails. However, it also means that if the oil pressure sensor is faulty, the fuel pump may not run when the engine is running. If your fuel pump works during the initial prime (2 seconds) but stops when the engine starts, the oil pressure sensor may be the problem.
To test the oil pressure sensor, you can check for voltage at the fuel pump while the engine is running. If the relay is working but the pump stops, the sensor may not be closing the switch. Replacing the oil pressure sensor is a relatively inexpensive repair that can solve this issue.
Replacing the Fuel Pump
If you have determined that the fuel pump itself is bad, replacing it is the next step. The fuel pump is located inside the fuel tank, which requires either lowering the fuel tank or removing the pickup bed. On a 1999 GMC Sierra, removing the fuel tank is the most common approach.
Safety precautions: Before working on the fuel system, always relieve the fuel pressure. The fuel system on the 1999 Sierra is pressurized to about 30-40 psi. To relieve pressure, remove the fuel pump fuse and crank the engine for a few seconds. The engine will start and then stall, indicating the pressure is relieved. Disconnect the negative battery cable before starting any work.
Removal process: Drain the fuel tank into an approved container. Disconnect the fuel lines, the electrical connector, and the filler neck. Support the tank with a jack and remove the straps. Lower the tank carefully. The fuel pump module is accessible from the top of the tank. Remove the locking ring (use a special tool or a hammer and punch) and lift the module out. Replace the entire module assembly if possible, as it includes the sending unit and pressure sensor .
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Final Thoughts on the 1999 GMC Sierra Fuel Pump Wiring
Understanding the fuel pump wiring diagram for your 1999 GMC Sierra can save you time and money when diagnosing fuel system problems. The system is relatively simple, with a fuse, relay, PCM control, and an oil pressure sensor backup. By following the troubleshooting steps outlined in this guide, you can quickly identify the root cause of a fuel pump failure.
Remember to always start with the simplest checks: the fuse and relay. These are the most common failure points and the easiest to test. If those are good, move on to checking for voltage at the fuel pump connector. If you have power but the pump does not run, the pump itself is bad. If you do not have power, the problem is in the wiring or the control circuit.
When replacing the fuel pump, invest in a quality product like KEMSO. The lifetime warranty and OEM-quality construction make it a smart choice for any 1999 GMC Sierra owner. For more information and to purchase, visit https://www.kemsoracing.com/ today.