Fuel Pump Relay – Function, Failure and testing procedures.

Fuel Pump Relay - Function, Failure and testing procedures.
Fuel Pump Relay - Function, Failure and testing procedures.

So, the fuel pump relay, is activated whenever, the ignition is turned on.

Consequently, the primary function of the fuel pump relay, is to maintain a consistent voltage supply, to the fuel pump.

Therefore, when switched on, the fuel pump relay; will provide current to the fuel pump, so that it may function.

In addition, the fuel pump relay, is usually controlled by the ignition or powertrain control module (PCM). As a result, any faults with it, can cause fuel pump issues.

Fuel Pump Relay In Fuse Box
Fuel Pump Relay In Fuse Box

Because, it is part of the control system of a fuel pump, its failure can lead to start up problems. Consequently, if it fails, the engine management system, records this event.

In addition, a fuel pressure sensor, lets the computer know; if the fuel pump, has not created any pressure, during cranking operation.

Usually A Bad Or Failing Fuel Pump Relay; Will Produce A Few Symptoms That Can Alert The Driver Of Issues:

Engine Suddenly Stalls

One of the first symptoms of an issue with the relay is, an engine that suddenly stalls. Consequently, if the fuel pump relay has an issue, it will cut off power to the fuel pump. Also, if a relay overheats, the electrical contacts inside the relay can short circuit; causing the electrical flow to stop. However, when the relay contacts cool, it will resume the flow of electricity. As a result, producing intermittent starting problems.

Engine Not Starting

A faulty relay, can often prevent starting. But, this is only one of many other problems, that may be at fault.

No Noise From Relay Or Fuel Pump, When The Key Is Switched On

Most fuel pumps, will produce a low volume hum, when turned on. So, if the relay fails, it will cut power to the fuel pump, which will render it inoperable.

When Troubleshooting, Always Start With A Visual Inspection:

Burnt Fuel Pump Relay
Burnt Fuel Pump Relay
  • Pull out the relay and visually inspect, the terminals and socket; for signs of corrosion and overheating.
  • Because, corrosion and overheating can prevent, proper current flow and indicates problems.
  • Clean corroded terminals and socket with, electrical contact cleaner.
  • Next, check all fuses, before any repairs.

So, testing the relay of a fuel pump, is relatively simple. Fortunately, it does not have many parts. Consequently, the most common parts to burn out or break; are usually the contacts and the coil. So, when either of these begin to fail; the current to the electrical circuit, will also fail. This means your fuel pump will, cease to function properly.

How To Test, The Relay, The Quick Method:

  • The easiest way to test a suspect relay is to; swap the suspect relay with a good one.
  • You may find another relay in your car; with the same configuration, like a cooling fan relay.
  • Just make sure the other relay, has the same configuration.
  • Remove the suspect relay and install the good relay.
  • Next, check if your engine fires up.
  • If it does, install a new relay. Otherwise, the problem lies somewhere else.

Basic Relay Testing Procedures

So, a relay should be considered as, two separate halves. The primary side, which utilizes an electromagnet, to close the secondary electrical circuit. Consequently, this electromagnet is activated by; a simple power (+) and ground (-) much like a light bulb circuit. The second half of the relay is, the “switch” that controls power to a particular accessory; like a fuel pump.

Fuel Pump Relay Terminals
Fuel Pump Relay Terminals
  • Terminals 86 and 85, are the primary side of the relay. It utilizes an electromagnet, to close (connect) the secondary electrical circuit inside the relay. As a result, this electromagnet is activated by; a simple power (+) and ground (-) much like a light bulb circuit. “Note, This may also go to, a oil pressure switch”.
  • Terminals 87 and 30, are the secondary side of the relay. Consequently, it acts, as the “switch” that connects electrical current, from one terminal to the other.

Use Jumper wires to supply 12v power, to terminals 86 and 85. Next, use a test light on terminals 87 and 30, to see if you have power. Finally, if the test light goes on, the the problem lies somewhere else.

However, there are other things, that can cause issues:

  • Fuel Pump Circuit Breaker Reset.
  • Anti-Theft system issue.
  • Bad inertia fuel shut-off safety switch.
  • Bad fuel pump.


So, a fuel pump is a powerful component and it demands; plenty of current, from the car’s 12-volts power system. However, if the fuel pump receives too much current at once; the delicate wiring could become too hot. Leaving you stranded !!

Thank You !