1999 Chevy S10 Fuel Pump Relay Location: A Complete Guide for DIY Owners
If you own a 1999 Chevrolet S10 and your truck won't start, the fuel pump relay is one of the first things you should check. The fuel pump relay on a 1999 Chevy S10 is located inside the glove box on the passenger side, behind the glove box compartment. It is the last relay on the left in the relay center mounted to the back of the glove box. Once you know exactly where to find it and how to test it, you can save yourself a tow truck bill and get back on the road quickly.
This guide covers everything you need to know about the 1999 S10 fuel pump relay location, how to identify a bad relay, step-by-step testing procedures, and what to do if the relay is not the problem. We'll also explain the unique dual-path fuel pump circuit that GM designed for these trucks, so you understand how the system actually works.
The Bottom Line: Where to Find the Fuel Pump Relay
For the 1999 Chevrolet S10, the fuel pump relay is not under the hood. It is located inside the passenger compartment. Here is the exact location:
Open the glove box. Remove the glove box liner by squeezing the sides and pulling it downward. Behind the glove box, you will see a relay center mounted to the firewall side. The fuel pump relay is the last relay on the left side of this relay center.
Some older guides or general automotive articles suggest the fuel pump relay is under the dashboard near the steering column or in the under-hood fuse box. For the 1999 S10 specifically, those locations are incorrect. The 1999 model year places the relay behind the glove box. If you are working on a different year S10 (1994-2004), the location may vary slightly, but the 1999 model consistently uses the glove box location.
Understanding the S10 Fuel Pump Circuit
Before you start testing, it helps to understand how the fuel pump circuit works on your 1999 S10. GM designed a clever system with two separate paths to keep your truck running.
Primary Path: When you turn the key to the "ON" position, the Powertrain Control Module (PCM) grounds the fuel pump relay coil for exactly 2 seconds. This primes the fuel system. Once the engine starts turning over, the PCM senses the RPM signal and keeps the relay grounded to maintain fuel pump power.
Backup Path (Oil Pressure Switch): If the fuel pump relay fails, the system has a backup. The oil pressure sender switch provides an alternate power path. Once oil pressure reaches roughly 4 PSI, the mechanical switch closes, sending power directly to the fuel pump. This keeps the engine running even with a dead relay.
This backup path explains a common symptom: if your S10 cranks for a long time (10+ seconds) before starting, and you hear the fuel pump kick on after the engine begins firing, the primary fuel pump relay is dead. The truck is running solely on the oil pressure backup circuit.
Symptoms of a Bad Fuel Pump Relay
Knowing the symptoms helps you confirm that the relay is the likely culprit before you start digging behind the glove box.
1. Engine cranks but does not start. This is the most common symptom. You turn the key, the starter engages, the engine spins, but it never fires up. You may not hear the fuel pump prime when you turn the key to "ON".
2. No fuel pump prime sound. When you turn the key to the "ON" position (without cranking), you should hear a 2-3 second humming sound from the rear of the truck near the fuel tank. If you hear nothing, the relay, fuse, or pump may be at fault.
3. Long crank time before starting. As mentioned above, if the relay is dead but the oil pressure backup works, the truck will crank for a long time before finally starting. This is a telltale sign of a failed relay.
4. Intermittent stalling. A failing relay may work sometimes and fail other times. Your truck might run fine for a while, then suddenly stall without warning. After a few minutes, it may restart.
5. Engine dies and then starts a few minutes later. This pattern is often associated with a relay that overheats and fails, then cools down and works again.
How to Test the Fuel Pump Relay
Once you have located the relay behind the glove box, you can test it using several methods. Always disconnect the negative battery terminal before working with electrical components to avoid shorts or shocks.
Method 1: The Swap Test (Easiest)
This is the quickest way to check if the relay is bad. The relay center behind the glove box contains multiple relays of the same type. Look for a relay that is identical in shape and size, such as the horn relay or another non-critical relay. Swap the fuel pump relay with that relay. If the fuel pump now works (you hear the prime sound), the original relay was bad. If the swapped relay also fails to make the pump work, the problem is elsewhere.
Method 2: Listen for the Prime Sound
Turn the key to the "ON" position. Do not crank the engine. Listen carefully. If you hear a 2-3 second hum from the rear of the truck, the relay is working, and the pump is receiving power. If you hear nothing, the relay or another component may be failing.
Method 3: Use a Multimeter
If you have a digital multimeter, you can test the relay circuit directly.
Locate the relay and identify the terminals. Typical relay terminals are numbered:
- Terminal 30: Constant 12V power from the battery (via the fuel pump fuse)
- Terminal 87: Output to the fuel pump
- Terminal 85: Ground control from the PCM (black/white wire)
- Terminal 86: 12V ignition feed (small orange wire)
First, test for power at terminal 30. Set your multimeter to DC volts. With the key off, probe terminal 30 and ground the black lead. You should see about 12V. If not, check the fuel pump fuse.
Second, test for ignition power at terminal 86. Turn the key to "ON." Probe terminal 86 and ground. You should see 12V. If not, the ignition-switched circuit has a problem.
Third, test the PCM ground signal at terminal 85. Set your multimeter to DC volts. Backprobe terminal 85 (black/white wire) and ground the black lead. Turn the ignition to "ON." You should see about 12V for 2-3 seconds, then it drops to 0V. This indicates the PCM is providing a ground to activate the relay. If you see no voltage, the PCM or wiring to it may be faulty.
Method 4: Check the Relay Coil Resistance
With the relay removed from the socket, set your multimeter to ohms. Measure the resistance between terminals 85 and 86 (the coil terminals). The resistance should typically be between 40 and 90 ohms. If the reading is infinite (open circuit) or significantly outside this range, the relay coil is damaged.
What to Do If the Relay Is Good But the Pump Still Doesn't Work
If the relay tests good or swapping it makes no difference, the problem is likely elsewhere in the fuel system. Here are the next steps to check.
Check the Fuel Pump Fuse
The fuel pump fuse is located in the under-hood fuse box. Look for a fuse labeled "FUEL PUMP," "F/P," or "FUEL." It is typically a 15A or 20A fuse. Remove the fuse and inspect the element visually. If it is blown, replace it with a fuse of the same amperage. If the new fuse blows immediately, there is a short circuit in the wiring or the pump itself.
Check the Oil Pressure Sender Switch
If the relay is good but the pump still does not work, the oil pressure switch may be faulty. This switch is located near the oil filter. On some S10 models, a common problem is visible oil leaking around the switch, which can cause it to fail and cut fuel delivery. Inspect the switch for oil leaks or damage.
Check the Fuel Pump Ground
A poor ground is a common cause of fuel pump failure. The fuel pump ground wire (usually black) connects to the chassis frame near the rear of the truck. Locate the ground connection, remove it, clean the contact surfaces with a wire brush, and reattach it securely.
Check for Power at the Fuel Pump Connector
If all fuses and relays check out, you need to verify power reaches the pump itself. The fuel pump connector is located on top of the fuel tank. You may need to drop the tank or remove the truck bed to access it. Use a multimeter to check for 12V at the connector when the key is turned to "ON." If you have power and ground at the pump but it does not run, the pump is faulty and needs replacement.
Common Wiring Colors for the 1999 S10 Fuel Pump
Understanding the wire colors helps when testing the circuit. For the 1999 Chevy S10, the typical fuel pump wire colors are:
- Orange (ORN): Constant 12V power to the fuel pump relay (terminal 30). This wire comes from the fuel pump fuse.
- Gray (GRY): Output from the relay to the fuel pump (terminal 87). This wire carries power to the pump.
- Black (BLK): Fuel pump ground. This connects to a frame ground stud near the tank.
- Black/White (BLK/WHT): PCM ground control to the relay coil (terminal 85). The PCM provides ground here to activate the relay.
- Small Orange (ORN): 12V ignition feed to the relay coil (terminal 86). This comes from an ignition-switched fuse.
Does the 1999 S10 Have an Inertia Switch?
A common question is whether the Chevy S10 has an inertia switch (a safety switch that cuts power to the fuel pump in a collision). The answer is no. The Chevy S10 does not have an inertia switch. This is a feature found on some Ford Rangers and other vehicles, but not on the S10. Instead, the S10 relies on the oil pressure backup circuit for safety redundancy. Do not waste time looking for an inertia switch under the carpet or behind the kick panels.
A Note on the Prime Connector
Your 1999 S10 has a fuel pump prime connector. This is a single white female connector with a red wire, typically located near the relay center. This connector allows you to apply direct battery power to the fuel pump for testing or priming the system without using the key. It is very useful for running fuel pressure tests or draining the fuel tank for removal. If you need to verify the pump circuit from battery to pump, you can apply 12V to this connector and listen for the pump to run.
Why You Should Consider a KEMSO Fuel Pump for Your S10
If you have diagnosed the problem and determined that the fuel pump itself needs replacement, you want a reliable, high-performance option. The fuel pump is a critical component, and choosing a low-quality replacement can leave you stranded again in a few months.
KEMSO offers high-performance, OEM replacement fuel pumps designed to meet or exceed factory specifications. Whether you are replacing the pump in your 1999 S10 or any other vehicle, KEMSO fuel pumps are engineered for durability, consistent fuel pressure, and long service life. Every KEMSO fuel pump comes with a lifetime warranty, giving you peace of mind that your investment is protected. You can browse the full line of KEMSO fuel pumps, including options for your Chevy S10, at their official website: https://www.kemsoracing.com/. KEMSO is dedicated to fueling your adventure with confidence.
Final Thoughts
Finding the fuel pump relay on your 1999 Chevy S10 is straightforward once you know where to look: behind the glove box, last relay on the left. Start with the simple swap test to rule out a bad relay, then move through the rest of the troubleshooting steps if needed. Remember that your S10 has a backup oil pressure circuit that can keep the truck running even with a dead relay, but it will cause long crank times. Do not forget to check the fuel pump fuse and ground before condemning the pump itself. And if you do need a new fuel pump, consider a KEMSO replacement for reliable performance backed by a lifetime warranty.
For more information on high-quality fuel pumps and other automotive components, visit https://www.kemsoracing.com/.