TEST 5: Testing The Fuel Pump Voltage Output At The Relay
In this test section, you're gonna' check that the fuel pump relay is sending voltage to the fuel pump.
You'll check for this power (10 to 12 Volts DC) right at the fuel pump relay connector (and with the fuel pump relay connected to the connector).
There's one more very important suggestion that I'm gonna' make to you and this is to connect your fuel pressure gauge to the pressure test port on the fuel injector rail (this test port is known as the Schrader valve).
If you're in need to a fuel pressure gauge, I recommend this one:
Actron CP7838 Professional Fuel Pressure Tester.
OK, to get this test going, this is what you'll need to do:
- Connect your fuel pressure gauge to the Schrader valve on the fuel rail.
- Make sure that the fuel pump relay is connected to its connector.
- With your trusty multimeter in Volts DC mode:
- Probe the wire labeled with the number 3 with the red multimeter test lead and an appropriate tool like a wire piercing probe (to see what this tool looks like, go here: Wire Piercing Probe).
- Ground the black multimeter test lead to a good Ground point on the engine or directly on the battery negative (-) terminal.
- Have your helper crank the engine as you observe the multimeter.
- NOTE: When working around a cranking engine, take all necessary safety precautions!
- With the engine cranking, you should see the following:
- Your multimeter should register 10 to 12 Volts DC.
- Your fuel pressure gauge should register 35 to 45 PSI.
- You should hear the hum of the fuel pump (if you go around the back of the vehicle, close to the gas tank).
OK, let's interpret your test results:
CASE 1: The 12 V. test light came ON and the fuel pressure was at specification. This tells you that the fuel pump is OK.
This test result also indirectly tells you several very important things:
- That the fuel pump inertia switch is OK (not tripped).
- Otherwise, the fuel pressure would not have gone up to specification (35 to 45 PSI).
- That the wiring between the fuel pump and the fuel pump relay is OK.
- Otherwise, the fuel pressure would not have gone up to specification (35 to 45 PSI).
So, to make the long story short, the fuel pump, the fuel pump relay and the fuel pump inertia switch are NOT the cause of your Ford's no-start problem.
CASE 2: The 12 V. test light DID NOT come ON and the fuel pressure WAS NOT at specification. This tells you that the relay is bad and needs to be replaced.
This is only true, if you have done all of the 4 previous tests:
- The fuel pump relay's low current circuit (wire 2) is getting power (TEST 1).
- The fuel pump relay's high current circuit (wire 4) is getting power (TEST 2).
- That the fuel pump does come on and is producing the correct fuel pressure specification (35 to 45 PSI) (TEST 3).
- That the computer is Grounding the relay's low current circuit (TEST 4).
TEST 6: Fuel Pump Stays On All Of The Time
There are times that the fuel injection computer goes bad internally and it activates the fuel pump continuously as long as the key is in the RUN position (but with the engine OFF).
To understand this type of failure, you need to know that the fuel injection computer activates the fuel pump for a few seconds and then shuts it off if you don't crank the engine.
A sure sign that the fuel injection computer (EEC Electronic Control Module) is fried, is when it activates circuit 1, and then never shuts it off.
The key symptom, again, is that you turn the ignition key on (but you don't crank or start the engine) and the fuel pump comes on but never turns off. When everything is functioning normally, when you turn the key on (but don't crank or start the engine) the fuel pump should come on for a few seconds and then turn off.
This isn't something that happens everyday, but I have seen it enough these past couple of years that I thought it a good idea to make you aware of it.
NOTE: Before replacing the computer, you'll need to get a wiring diagram and make sure that circuit 1 is not shorted to Ground somewhere between the fuel pump relay connector and the fuel injection computer's connector.
Ford Fuel Pump Relay Test Summary
Well, you've come to the end of the article. It's my sincere hope the info helped!
I know this article is pretty narrow in its focus, so if there's any info you'd like to share with the rest of us (out here in internet land), you can use the contact form below.
Sharing you diagnostic and repair experience not only improves this article, but also helps anyone else with the same symptoms or problems.
Where To Buy A Fuel Pressure Gauge
You can walk into any Auto Parts Store in your neighborhood and buy a fuel pressure gauge, but if you want to save a few buck, here's where I recommend you buy it:
If this info saved the day, buy me a beer!
