If your fuel pressure gauge is reading 0 PSI, don't assume the fuel pump is toast just yet. Before replacing it, you need to make sure it's actually getting power.
The cool thing is that on the 1991-2010 4.0L V6 Ford Aerostar, Explorer, and Mercury Mountaineer, that's easier than you might think —because the fuel pump power wire runs through the inertia switch, which is easy to access.
In this tutorial, I'll show you how to check for power at the inertia switch using just a test light or multimeter.
This simple check will confirm whether the fuel pump relay, fuse, and wiring are doing their job. It could save you from replacing a good fuel pump!
Contents of this tutorial:
- Why You Should Check For Power Before Replacing The Fuel Pump.
- What Tools You'll Need.
- How To Check That Power Is Reaching The Fuel Pump (Via The Inertia Switch).
- Fuel Pump Inertia Switch Circuit Descriptions.
- More Fuel Pump Troubleshooting Help.
- More 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer Tutorials.
NO-START DIAGNOSTICS: If you're troubleshooting a no-start or fuel pump issue, be sure to check out the list of wiring diagrams, fuel pump tests, and related diagnostics at the bottom of this page:
It'll help you dig even deeper if the inertia switch isn't the problem.
Why You Should Check For Power Before Replacing The Fuel Pump
Whenever I run into an engine no-start with 0 PSI on the fuel pressure gauge, the next thing I do is check if the fuel pump is actually getting power —and most (if not all) techs do the same. In this section, I'll show you how to quickly rule out a bad relay, fuse, or wiring issue before pointing the finger at the pump itself.
On the Ford Aerostar, Explorer, and Mercury Mountaineer, this check is especially easy —because power to the pump runs through the fuel pump inertia switch, and that switch is right within reach.
If there's no power reaching the fuel pump inertia switch, then there's definitely no power reaching the fuel pump.
This fuel pump power check takes just a few minutes, but it can save you hours of frustration and keep you from replacing a perfectly good fuel pump and having the engine still not start.
Before we get into the step-by-step, let's go over the few tools you'll need to do the job right.
What Tools You'll Need
You don't need any expensive diagnostic tools for this test. Just a few basic items and you're ready to go:
- Test light or digital multimeter —Either one works fine. You'll be using it to check for 12 Volts DC at the inertia switch. Don't have one? Here are the ones I use and recommend:
- Tekpower TP8268 AC/DC Auto/Manual Range Digital Multimeter (Amazon affiliate link).
- Lisle 28400 Heavy Duty 12 Volt Test Light (Amazon affiliate link).
- Access to the fuel pump inertia switch —It's located behind the passenger side kick panel (under the glove box).
- Helper to crank the engine —You'll need someone to turn the key and crank the engine while you check for voltage.
Disclosure: As an Amazon Associate, I earn from qualifying purchases. Buying through these links helps support this site at no extra cost to you. Thanks for your support —it really means a lot!
How To Check That Power Is Reaching The Fuel Pump (Via The Inertia Switch)
Alright, this is what you signed up for:
- Locate the inertia switch: On the Aerostar, Explorer and Mountaineer, you'll find it behind the passenger side kick panel (below the glove box). Remove the trim panel to access the switch and its 2-wire electrical connector.
- Unplug the connector: You'll see two wires going to the switch. One wire is the power coming in from the fuel pump relay. The other wire carries power out to the pump. We're interested in the power-in wire.
- Turn the key to CRANK: Have your helper crank the engine while you check the power-in wire with your test light or multimeter. To identify the power-in wire, see:
- Check for 10–12 Volts DC: You should see battery voltage (or close to it) on one of the wires while the engine is cranking. That confirms the relay, fuse, and wiring up to this point are working.
- No voltage? Then your no-start isn't being caused by the pump —it's something upstream: a bad relay, blown fuse, burned fusible link, or maybe the PCM isn't grounding the relay.
- Voltage is present? That means the pump is getting power. If your gauge still reads 0 PSI, the pump is either dead or the fuel line is completely blocked (which is rare).
Voltage is present: Just because power is coming in doesn't mean it's making it out. To confirm that the inertia switch is actually doing its job, you'll need to test the power-out wire too.
- Reconnect the connector: Plug the 2-wire connector back into the fuel pump inertia switch.
- Backprobe the power-out wire: Using your red multimeter test lead, backprobe the other wire —the one that sends power to the pump. To ID the power-out wire, see:
- Ground your black lead: Touch the black multimeter lead to a clean, rust-free and paint-free metal spot near the inertia switch —you'll need a good Ground for an accurate reading.
- Crank the engine: Have your helper crank the engine while you monitor the multimeter display.
- Read the voltage: If the inertia switch is working properly, you should see 10–12 Volts on that wire —that confirms it's passing power through to the pump.
IMPORTANT: Keep in mind that we're not testing the fuel pump's pressure output —this is just checking that the pump is being powered. It's a crucial step before condemning the pump itself (after checking its pressure output with a fuel pressure test gauge).
Fuel Pump Inertia Switch Circuit Descriptions
Below you'll find the wire color info for both terminals of the fuel pump inertia switch connector —organized by model year so you know which wire to test for power in and power out.
| 1991-2003 Aerostar, Explorer, And Mountaineer | |
|---|---|
| Wire | Description |
| Dark green with yellow stripe (DK GRN/YEL) | 12 Volts IN (from Fuel Pump Relay) |
| Pink with black stripe (PNK/BLK) | 12 Volts OUT (to fuel pump) |
| 2004-2005 Explorer And Mountaineer | |
|---|---|
| Wire | Description |
| Red (RED) | 12 Volts IN (from Fuel Pump Relay) |
| White (WHT) | 12 Volts OUT (to fuel pump driver module) |
| 2006-2010 Explorer And Mountaineer | |
|---|---|
| Wire | Description |
| Violet with green stripe (VIO/GRN) | 12 Volts IN (from Fuel Pump Relay) |
| Violet with white stripe (VIO/WHT) | 12 Volts OUT (to fuel pump driver module) |
More Fuel Pump Troubleshooting Help
FUEL PUMP PRESSURE TESTS:
- How To Test The Fuel Pump (1993-2000 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
- How To Test The Fuel Pump (2001-2003 4.0L V6 Ford Explorer And Mercury Mountaineer).
ENGINE NO-START DIAGNOSTICS:
INERTIA SWITCH (FUEL PUMP) CIRCUIT DIAGRAMS:
- Fuel Pump Circuit Wiring Diagram (1993-1994 4.0L V6 Ford Explorer).
- Fuel Pump Circuit Wiring Diagram (1995 4.0L V6 Ford Explorer).
- Fuel Pump Circuit Wiring Diagram (1996 4.0L V6 Ford Explorer).
- Fuel Pump Circuit Wiring Diagram (1997 4.0L V6 Ford Explorer).
- Fuel Pump Circuit Wiring Diagram (1998-2001 4.0L V6 Ford Explorer).
- Fuel Pump Circuit Wiring Diagram (2002 4.0L V6 Ford Explorer And Mercury Mountaineer).
- Fuel Pump Circuit Wiring Diagram (2003 4.0L V6 Ford Explorer And Mercury Mountaineer).
More 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer Tutorials
If this guide helped, I've got a lot more of 4.0L V6 Explorer, Aerostar, and Mountaineer write-ups. You can find them in this index:
Here's just a taste of what's inside that index:
- How To Test Engine Compression (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
- How To Test The Fuel Injectors (1991-1996 4.0L Ford Explorer And Aerostar).
- How To Troubleshoot A No-Start Problem (1991-2010 4.0L V6 Ford Explorer, Aerostar, And Mercury Mountaineer).
If this info saved the day, buy me a beer!
