Fuel Pump Relay: Essential Insights

As an Amazon associate, we earn from qualifying purchases.

Purpose

The fuel pump relay is an electromagnetic switch that allows electrical current to flow to an electric fuel pump, activating and powering the pump.

Location

In late-model vehicles, the fuel pump relay is located with other relays in the relay block. The relay block is inside a plastic box with a removable cover located in the engine compartment or, in some vehicles, under the rear seat.

A diagram on the underside of the cover identifies the location of each relay. In older vehicles, the fuel pump relay may be located under or behind the instrument panel or behind the “kick panel” adjacent to a front seat foot well.

Some late-model fuel injected vehicles do not use a fuel pump relay, but instead power the fuel pump through a fuel pump module or control unit.

Operation

A typical fuel pump relay has two circuits, the control circuit and the power circuit. There usually are four pins or blades protruding from the bottom of the relay that are numbered on the bottom of the relay case. These are terminals, two for the control circuit and two for the power circuit. A diagram of the internal circuits may be printed on the side of the case.

Electrical current to the control circuit passes through a coil of copper wire wound around an iron core, and then to ground. Current flow creates an electromagnetic field around the copper coil. The magnetic force pulls the arm of a switch in the power circuit closed, so that a contact on the arm touches a ground contact, closing the power circuit and permitting current flow to the fuel pump.

When current flow to the control circuit is cut off, the magnetic field collapses and a spring moves the arm back, opening the circuit and interrupting current flow to the fuel pump.

In some Toyota models, the fuel pump relay has five terminals, to allow the pump to be powered at two different speeds. The ECM/PCM directs power through the fuel pump relay control circuit only at start-up or when fuel pressure must be high, such as during heavy acceleration.

When there is no current in the control circuit, instead of opening the power circuit, the contact arm retracts to contact a second circuit that routes current to the fuel pump through a resistor. The resistor reduces current flow, powering the pump at low speed.

Drivability Symptoms

The typical symptoms of fuel pump relay failure are hard starting, not starting, or the engine suddenly dying. At first, these conditions may occur only when the engine is hot, disappearing after the engine cools. The symptoms will however worsen over time, until the vehicle will not start at all.

A defective relay can also cause the fuel pump to continue operating after the engine has been turned off, or to continue running with the key on, engine off, for more than a few seconds. In a system with a two speed fuel pump, a defective relay may cause fuel starvation under load.

On model year 1996 and newer automobiles and gasoline powered light trucks, which are equipped with On-Board Diagnostics, Generation 2 (OBD2), the instrument panel Malfunction Indicator Light (MIL) will illuminate.

Inspection, Test, and Diagnosis

To properly diagnose a potentially defective fuel pump relay, first determine the conditions under which the relay routes power to the fuel pump.

In some General Motors vehicles, for example, current is routed to the fuel pump through the oil pressure switch if the fuel pump relay fails. This causes hard starting when cold, because the fuel pump will receive power only after cranking has raised oil pressure sufficiently to close the switch.

The conditions under which the problem occurs can also assist in the diagnosis: as relays get old, the heat cycle to which the relay is repeatedly subjected causes cracks in the internal solder joints. These cracks then expand as the relay heats, opening the internal circuits and blocking current flow. As the relay cools, the solder contracts and the circuit is reestablished.

Four generic OBD2 Diagnostic Trouble Codes (DTCs) P0230 through P0233 indicate the fuel pump relay may be defective. These are Type “B” codes, which store Freeze Frame data and illuminate the MIL on the second consecutive ignition cycle during which the fault occurs.

In these vehicles, the Engine Control Module or Powertrain Control Module (ECM/PCM) monitors voltage at the control circuit and also, by a feedback circuit – a circuit which does not conduct current, but simply holds voltage – monitors voltage in the power circuit. DTC P0230 indicates the control circuit voltage is inconsistent with the voltage expected in that circuit (low when the circuit is closed, high when open).

When the fuel pump is powered, DTC P0231 is set if voltage on the power circuit (measured through the feedback circuit) is below expected battery voltage. DTC P0232 is set if the power circuit receives battery voltage when the fuel pump should not be powered. DTC P0233 indicates an intermittent fault in the power circuit. All of these codes may also be caused by defective wiring in the circuit.

To test a fuel pump relay, first listen to it. With a helper turning the ignition key, but not starting the engine, listen for an audible click from the relay when the key is moved from “off” to “on.” This should power the fuel pump to pressurize the fuel system before the key is turned to “start.”

An audible click confirms that current is flowing through the control circuit and activating the contacts in the power circuit, closing that circuit. No click may indicate a defective relay, but may also indicate a wiring fault in the control and/or the power circuit.

Remove the relay by pulling it directly upward and examine it for signs of corrosion or cracking. There are tools that are designed to assist removing relays, if tight space prevents getting an adequate grip, but do not pry on the relay or the relay block. If the relay passes visual inspection, test the relay with a Digital Multimeter (DMM) and jumper wires.

First, identify the terminals on the bottom of the relay for the control circuit and for the power circuit. Set the DMM to the ohm setting and measure power circuit continuity by touching the red DMM lead to one power circuit terminal and the black to the other power circuit terminal. This measures circuit resistance. The reading should be infinite, because the power circuit should be open when there is no current flowing in the control circuit. If it is not, the power circuit has an internal short at the contacts and the relay is defective.

If the relay passes this test, the next step is to power the control circuit by connecting a jumper wire between the battery positive terminal and one control circuit terminal and a second jumper wire from the second control circuit terminal to a good ground, such as the exhaust manifold.

With the positive connected, grounding the second jumper should cause a click from the relay, indicating current is flowing through the control circuit creating the magnetic field that closes the power circuit contacts. With the relay still jumpered, again test power circuit continuity with the DMM. This time, the reading should be zero, or close to it, indicating that current is flowing freely through the circuit. If it is not, there is excessive internal resistance in the power circuit and the relay should be replaced.

Testing a five terminal fuel pump relay follows the same procedure. However, the power circuit terminals leading to the fuel pump directly and through the resistor must be specifically identified, and individually tested. Without current to the control circuit, one power circuit output terminal should be infinity and the other zero; with current to the control circuit, the readings should be reversed.

Reprogramming

Replacement of a fuel pump relay does not require reprogramming.

Repair/Replacement

If the relay is defective, it should be replaced with an identical new relay. The relay should be firmly seated into the relay block.

Back to Blog Academy