How to Test a Fuel Pump Relay and Other Automotive Relays

You can use simple procedures to test a car relay.
You can use simple procedures to test a car relay. | Source

A fuel pump relay can fail in several ways. Inner terminals may break, creating an electrical open condition; corrosion may build up around electrical contacts; contacts may burn, creating unwanted resistance; or the coil wire may brake, rendering the relay useless.

Although a failed fuel pump relay — or any other automotive relay — will cause trouble occasionally, many car owners just go and replace a suspect relay and end up wasting money and time. So, before you decide to replace it, test the relay to make sure it has actually failed.

Let's go first over the basics of an electrical relay to help make sense of the following tests.

NOTE: Here, we'll deal with electro-mechanical relays, not those that incorporate solid state or integrated circuits.

If you've worked with automotive relays before, you may want to skip to the tests sections. Here's the table of contents.

Table of Contents

I. What is an Automotive Relay?
II. How It Works
III. Locating the Fuel Pump Relay
IV. The Fastest Way to Test a Fuel Pump Relay or Other Automotive Relays
V. Using a Special Relay Tester
VI. Using a Digital Multimeter to Test the Fuel Pump Relay

I. What Is an Automotive Relay?

Physically, a common 12-volt relay contains a series of electrical components set inside a small cube or cylinder box made up of hard plastic or aluminum.

A common fuel pump relay has four electrical terminals or pins sticking out of its base. The terminals in an automotive relay hook up to an internal coil, possibly one or more resistors or diode, and one or more switches or contacts, depending on the particular configuration.

You'll see relays with 3 or 4 pins often, but others with 5 and 6 terminals are not unusual. These automotive relays come in different sizes and ratings. And you'll find them under the hood inside the "block" or "power distribution center" box; inside the cabin, in fuse boxes under the dashboard; behind kick panels, and even mounted by themselves.

They switch power on and off circuits around the transmission, engine and accessories like the fuel pump, the headlights, the brake lights, the wipers and horn.

A common electromechanical relay.
A common electromechanical relay. | Source

II. How It Works

It helps to think of a relay as a regular switch, like the one you use at home to turn on and off the lights in a room.

A relay works in a similar way, but instead of you reaching into the relay to flip on the internal switch directly, you energize a control circuit that connects to a coil inside the relay — this happens to the fuel pump relay when you turn the ignition switch to fire up the engine, for instance.

The coil creates an electromagnetic filed that closes a pair of metal contacts inside the relay that connect to a controlled circuit that activates a load — the electric fuel pump, for example.

Why duplicate a simple switching function by having an extra switch inside a box?

Actually, this turned up to be a convenient set up in electrical circuits. You not only use a small, safer current circuit — the control circuit — to switch on and off a higher current circuit — the controlled circuit — but a relay can be placed in a strategic location to reduce the amount of — and expenses in — thicker gauge wire needed for high current circuits.

Testing Relays

III. Locating the Fuel Pump Relay

If you haven't located it yet, you'll need to find the fuel pump relay. Look inside the power center under the hood — a box with relays — or along the firewall or the fuse box(es) under the dashboard and side kick panels.

On the back of the lid of these boxes you may find markings showing the relays they house and a short description of each. On some vehicle models, you can find the fuel pump relay by itself somewhere underneath the dashboard or under the hood.

Consult the vehicle repair manual for your particular car, if necessary. Most auto parts stores carry aftermarket repair manuals for different vehicle makes and models, but you can buy it online too.

Once you locate the relay, you have three methods to test it. Choose the most convenient.

IV. The Fastest Way to Test a Fuel Pump Relay or Other Automotive Relays

By far, the easiest way to test a suspect fuel pump relay — or some other automotive relay — is to swap the suspect relay with a good one.

You may find another relay in your car with the same configuration — arrangement and number of terminals — as your fuel pump relay. Or you can borrow a relay from a relative's or friend's car. Just make sure the other relay has the same configuration.

Remove the suspect relay and install the good relay and check if your engine fires up. If it does, install a new relay. Otherwise, the problem lies somewhere else.

V. Using a Special Relay Tester

Another way to test your fuel pump relay is with a relay tester. You can find testers to help you troubleshoot the most common vehicle relays and more sophisticated testers to troubleshoot relays with different configurations.

The fact that you don't need schematics or wiring diagram speeds up the troubleshooting process. Just follow the instructions that come with the tool.

You can use a digital multimeter to test relays.
You can use a digital multimeter to test relays. | Source

VI. Using a Digital Multimeter to Test the Fuel Pump Relay

But, more likely you won't be testing relays that often, so probably you don't want to buy a relay tester. You still can use a digital multimeter for the same purpose. If you don't have a digital multimeter, though, buy one.

They come in handy for many car and home troubleshooting tasks. You may find a useful, quality and inexpensive digital multimeter in your local electronic parts store, hardware store, department store or online.

1. Visual Inspection

  • Before you start making any tests, pull out the relay and visually inspect the terminals and socket for signs of corrosion and overheating. Corrosion prevents proper current flow, and overheating indicates problems with the relay or circuits it connects to. Check the circuit, if necessary.
  • Clean corroded terminals and socket with electrical contact cleaner.

2. Identifying the Fuel Pump Relay Terminals

Now, you need to identify which pins or terminals belong to the control circuit and which to the load or controlled — aka power — circuit.

On an automotive relay, the control circuit is the one you or the computer activates. The load or power circuit is the one with the load or accessory that becomes activated (fuel pump, radiator fan, headlights, horn).

You'll find that many car relays have a diagram stamped on the case itself to identify each pin or terminal with a number. Now, if you look at the actual pins or terminals, chances are that you'll see these same numbers printed next to them as well.

A manufacturer standard is to use the numbers 85 and 86 for the terminals connected to the control circuit, and numbers 87 and 30 for the load or power circuit terminals. On some smaller relays (micro relays) a common number system for these terminals would use the numbers 1, 3, 2 and 4 respectively. However, they may vary with each configuration. Double check the markings on the relay itself with the wiring diagram in your car repair manual for the correct terminals.

Still, you'll find relays with no numbers or diagrams to identify the terminals. To identify the terminals, consult the wiring diagram in your repair manual; or, if you have access to the underside of the relay's socket, locate the thick and thin wires that connect to the socket. The thick wires connect to the power circuit terminals. The thin wires connect to the control circuit terminals.

Common Relay Terminal Designations

Common Relay
Control circuit
Power circuit
(normally closed)
Micro Relay
Control circuit
Power circuit
Markings may change with each configuration.

3. Checking for Continuity

Once you've identified the terminals, you need to check for continuity between the power circuit terminals.

  • Set your digital multimeter to the lowest range on the Ohms scale or set it to 'continuity.'
  • Then, connect one lead to one of the power circuit pins, and connect the other lead to the other power terminal. Your meter should read infinite (OL, open load) resistance. Or, if you are using the continuity setting, you shouldn't hear a beeping sound.
  • If you detect zero ohms or any resistance value in the hundredths or thousandths — instead of infinity — or hear a beeping sound, your relay's power pins have shorted. Replace the relay.


Often — as with a fuel pump relay — the relay switch for the power circuit has a normally open position when not energized.

When testing another automotive relay, check the diagram on the relay, or the wiring diagram in your car repair manual, because some relay power switches come normally closed and should open when energized — usually, these terminals are marked 30 and 87a.

On these type of relays, your meter should read zero Ohms or produce a beeping sound — if it's in good condition — when testing for continuity as in the previous step.

. . . some relay power switches come normally closed and should open when energized . . .

4. Checking that Your Relay Works Properly

  • Now, connect a jumper wire (a fused wired, if possible, which you can find in most auto parts stores) from the battery positive (+) terminal to one of the control-circuit terminals of the relay.
  • Connect another jumper wire from the negative (-) battery terminal to the other control-circuit terminal of the relay.
  • As you connect the second jumper wire on the relay, you should hear the contacts (switch) in the power circuit of the relay make a clicking sound as they close. If you don't, swap the jumper wire connections on the relay. That is, connect the jumper wire from the positive terminal of the battery to the other terminal on the control circuit and do the same with the negative terminal, just in case the control circuit has a critical polarity. If you still don't hear a click, the control circuit connection in the relay has failed. But continue ahead with the rest of the test to make sure there's a problem with the relay.
  • Leave the jumper wires connected and check for continuity between the power terminals as you did previously. This time, your meter should read 0 ohms or close to zero — in the hundredths or thousandths — or make a beeping sound, indicating there's continuity between both terminals. If so, your relay works fine (just remember that if your relay switch has a normally closed position, this time your meter should read infinite resistance or produce no beeping sound).
  • Instead, if your meter reads infinite resistance or you hear no beeping sound, replace the relay.
  • If your tests prove your relay is in good shape, though, check for bad wires, corroded terminals, corroded connections, disconnected wires (opens), or short circuits around the control and power circuits. Any of these conditions will cause any of your circuits to fail.

Once you know how to test a fuel pump relay, you can test other automotive relays without much trouble.

You'll find relays with three, five or even six terminals, though. On three terminal relays, the case itself may serve as the ground connection.

Others with five or six terminals control more than one power circuit, and some internal switches are normally opened and others are normally closed.

Analyze the diagrams on the relay itself or the wiring diagrams that come in your vehicle repair manual to identify these connections, circuits and switch positions, if necessary, and proceed with your tests as described above.

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