Most electric fuel pumps emitted a reassuring hmm sound when in operation. If you’re missing your humm and the engine won’t start. Checking the fuel pump fuse is the best place to start. I’m a mechanic for twenty plus years, I fix a ton of fuel pump issues, you are in the right place and very shortly we have this figured out.
To check a fuel pump fuse, follow these three steps:
- Locate the fuse box and fuel pump fuse number by referencing the drivers manual
- Remove the fuse by using the fuse puller tool attached to fuse box cover
- Check if fuse conductive strip is broken by holding it to the light
By the end of this post you’ll know how to locate your fuel pump fuse, how to check it and how to replace it.
1 Locating The Fuse Box
Modern cars use a ton of electrics and computer modules to manage the various systems. As a result it’s not unusual for a car to have three, four or more fuse boxes. Most are well hidden from view and may require panels like glove boxes, trunk carpeting or kick panel removal to access.
It’s better to do a little research in the driver’s manual before removing covers in search of possibly the wrong fuse box. A good driver’s manual will show the location of the fuse box and the fuel pump fuse number, rating and color. It will likely show all this information in an easy to follow diagram form.
In most cases the fuse box under the hood will contain both the fuel pump fuse and relay.
Common locations for fuse boxes include:
- Under hood
- Side of dashboard
- Under kick panel drivers and passenger front
- Behind glove box
- Either side of trunk wall behind carpet
- Under seating
Locating the fuse:
Removing the fuse box cover reveals the fuses, which apart from color all look identical. Looking closely you may find the fuse location number printed on the fuse box floor at the foot of each fuse. This number represents the fuse location number you checked in the driver’s manual.
Finding the correct fuse is really only a matter of matching the representation in the drives manual or fuse box cover diagram to that of the fuse box.
2 Removing The Fuse
Most fuse boxes will supply a useful fuse pulling tool. It’s often fixed to the rear of the fuse box cover. Fuses can be difficult to grip without the puller tool or a small pointy nose plier works great too. Pull the fuse upwards to remove, simple!
It is possible to check the fuse without removing it, but you’ll need a test light or voltmeter (See below). In any event it is better to pull the fuse to check as older fuse terminals often corrode. If you find corrosion, cleaning
3 Checking The Fuse
With the fuse removed and in hand hold the fuse to the light. Looking through the opaque body you’ll notice a metallic strip. A fuse is said to be blown if the metallic strip is broken.
Replacing Fuel Pump Fuse
It’s important to replace the fuel pump fuse with the correct rating. Most fuel pumps will run a 15 (Blue) 20 (Yellow) or 25 (Clear) amp fuse. Using a fuse that’s too small will cause the fuse to blow again. This is a normal condition as the fuse isn’t rated for the higher amps of a fuel pump circuit.
Replacing the pump fuse with a fuse that’s too big can cause problems, if the circuit overloads. The higher rated fuse won’t blow and this may cause damage to other components and runs a real risk of starting a fire.
I don’t assume the fuse rating I’m removing is correct, best to check the rating in the driver’s manual or fuse cover diagram. Fuses are color coded and rated by amperage (amp).
Common Causes Of No Fuel Pump
So maybe you’ve checked your fuse and it’s okay, or you’ve replaced the fuse with the correct amp rating and it blows again, now what? You’ll need to do a little detective work, but don’t worry most of it is easy. Here’s a list of causes of a fuel pump that won’t work.
- Blown fuse
- Faulty relay
- Bad connector at pump
- Bad wiring between fuse box and pump
- Bad pump
- Bad crank sensor (CKP)
- Bad ECM/PCM
- Inertia switch activation or fault
Next we’ll diagnose some of the more common reasons, but first it’s helpful to have an idea of how your fuel pump operates.
How Fuel Pump Works
Fuel pump failure is very common, but they are expensive and obviously we want to get this right, replacing good parts isn’t fun.
Most fuel pumps are located in the gas tank, submerged in the gas and so access for testing directly is challenging.
How most fuel pumps work:
Fuel pumps generally turn on for two seconds to pressurize the fuel rail, known as priming. This creates enough pressure in the system to start the engine. Priming generally occurs when ignition is turned “On” (Position 2). More modern cars may prime the system when the drivers door is opened.
With the system primed, the engine will start but it won’t stay running unless the ECM/PCM (computer) receives a pulse signal from the crank sensor.
When the ECM receives the crank signal it activates the fuel pump relay by either controlling power to the control side of the relay but more commonly the ECM controls the relay by supplying a ground. (More on this later)
As a side note engine controllers go by a few different acronyms, ECU,ECM, PCM.
Your Crank position sensor (CKP) is a small sensor located at the harmonic balancer or at the flywheel. It’s job is to detect movement of the crankshaft (cranking of the engine) and report to the ECM.
Wiring the pump not to the ignition but instead to the crankshaft so that it only runs when the engine is actually turning is a safety feature. Most car engines will shut off after an accident but the ignition may remain on.
Your car may also have an inertia switch fitted, this is an additional safety feature, it activates on impact and kills power to the pump. Inertia switches do go bad and is another possible reason a fuel pump won’t run.
What Does A Relay Do?
Fuel pump circuits employ a relay to control a high amp circuit with a low amp circuit. They are either external and are fitted in the fuse box or are integrated in the ECM.
If your fuel pump has an external relay and most have, its location is generally detailed in the drives manual just like the fuses. (Engine compartment fuse box usually)
A relay contains two separate circuits, a control side which is controlled by the ECM and a load side which powers the pump.
When the ECM powers the control side of the relay it sends voltage through a small coil which causes a magnetic field. The load side contains a small spring loaded armature that’s attracted to the magnet. The attraction causes the armature to close which completes the load side circuit and powers the pump.
When the ECM kills the power to the coil, it’s no longer magnetized and the small spring on the load side armature pulls it to the open position killing the power supply to the pump as it does so.
How to check a relay:
When relays fail they may fail intermittently or fail completely. Removing and shaking the relay often emits a loose sounding rattle. This is a sign of a bad relay.
Although relays differ, many relays in the same fuse box are identical. Swapping identical relays is a fast hack for troubleshooting a faulty relay.
Two other methods include using a voltmeter and wiring the relay up as per the diagram below or alternatively use a relay tester or a power probe, both are listed on the Auto electrical repair tools page.
Checking The Fuel Pump
If your car engine is running the pump should be too, that means your pump is working really hard and it’s no surprise they fail a lot. A faulty pump is very likely your problem, I replaced a ton.
That said we can’t know for sure until we test it.
Before embarking on a journey of discovery with your vehicle, I strongly advise the use of proper electrical repair tools and a wiring diagram, both will save you a ton of time and heart ache. You can check out all the tools I use here on the Electrical repair tools page.
The pump lives in the gas tank and a quick test is can you hear it when you turn the ignition on, but remember you’ll only hear it for about two seconds. That’s normal, it’s priming the system.
Bang on the bottom of the gas tank hard with just the heel of your hand while a helper turns the key on. This often encourages the pump to fire and is a sign all is well with the wiring but your fuel pump is sadly failing.
Because your fuel pump is likely submerged inside the gas tank, access to the wiring is challenging. I like to fast track the process to see which end of the system the fault lies.
For this I use a power probe to power the pump from the relay socket. You’ll need a power probe or use a fused jumper wire. You can check out all the tools I use here on the Auto electrical repair tools page.
A power probe works best and is the safest tool to use when working on circuits. Blasting the PCM with a jumper wire will cause damage.
Remove the relay which as you know can be found in the fuse box. Locate pin socket 87 and add power. Be sure you are on pin socket 87, powering the wrong circuit can cause module damage.
I use a fine test probe to supply power, jamming large objects into the delicate sockets can cause them to spread, which creates high resistance.
Listen for the pump, if the pump runs, all is good on the load side of the circuit and I know I can direct my attention to the control side. See checking the fuel pump circuit below.
If on the other hand the pump doesn’t emit a humm noise when powered I know the pump is most likely at fault. I pull the cover off to access the wiring harness (under the rear seat or trunk floor).
I’ll repeat the test and check for power and ground at the pump connector to confirm failure. See replacing the pump below.
Checking The Fuel Pump Circuit
The fuel pump circuit has a control side and a load side. They are two separate circuits. We’ll run a few tests directly from the relay socket as it’s easy to access. These tests will identify the circuit at fault but will then require further investigation in order to pinpoint the fault.
We’ll look at the control side first.
The control side is controlled by the ECM and may do so by controlling the power side supply to the relay or the ground side. Manufacturers differ about which circuit does the switching on and off. The control side of a relay circuit is pin sockets 85 (ground) and 86 (power).
To test the control side:
Remove the relay and check for a ground at pin 85 socket. If no ground, test with ignition “On”. If ground is present for two seconds – your relay is controlled/switched by the PCM on the ground side.
Test for power at pin socket 86 – If no power, turn ignition “On”. If power is present for two seconds – your relay is controlled/switched by the PCM on the power side.
After identifying if your PCM is controlling the ground side or power side it’s a ton easier to narrow down the missing circuit. Verify you have both ground and power circuits. If not chase whichever circuit you’re missing.
If all checks out move on and check the load side of the relay.
To test the load side:
The load side is the second circuit and it’s tasked with supplying power to the pump. The load side of the circuit is open until closed by the control side. The open pin sockets of the load side are pins 30 (Power) and 87 (Pump).
Remove the relay and check for power at pin 30 socket. If there’s no power, check the circuit back to the power supply for an open.
Pin 87 socket carries power from the front of the vehicle to the rear where the pump is located, but doesn’t carry power until powered by pin 30.
Use a power probe or fused jumper wire to power this circuit, if the pump and circuit are good the pump will run.
If not check for power at the pump using a test light or voltmeter. If there is no power at the pump check for a broken wire along the way, (open circuit).
If all tests okay, then go ahead and replace the fuel pump.
You can check out the power probe I use on the Auto electrical repair tools page.
Replacing The Fuel Pump
For most modern cars replacing the fuel pump means opening the gas tank and removing the fuel pump. Many cars will leave an access port on the trunk floor or under the rear seat of the vehicle, this saves dropping the gas tank.
Other vehicles like trucks may however require the gas tank dropped. Which is a pain in the ass when the tank is full.
Opening the tank exposes raw gas. Hot working lamps are a no no, as are striking tools together and wearing synthetic fibers. All can create a spark strong enough to ignite fuel.
This is a job best tackled in a well ventilated area, wearing cotton and using an LED light.
See the tools I use here on the Mechanics tools page.
The fitting process looks something like this:
- Locate access port, a few screws removes the cover
- Remove the wiring connector
- Remove the fuel lines
- Clean the cap area
- Release the ring fastener
- Lift the tank unit from the tank (careful not to bend the fuel sender arm)
- Remove the tank O-ring seal
- Dismantle the fuel pump assembly to extract the pump
- Fit new pump to assembly
- Lube and fit new O-ring seal to tank
- Fit fuel pump assembly
- Tighten the keeper fastener
- Fit fuel lines
- Fit wiring
- Fit cover
That wasn’t so bad, Nice work!
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