How To Bench Test A Ford Relay (Large Relay) How To Bench Test A Ford Relay (Large Relay)

Bench testing the Ford 4 or 5 pin relay is not hard to do at all, and in this article I'll show you how to do it (there are two articles in site about bench testing a Ford relay, you can find the other here: Bench Testing A Ford Relay (A Step-by-Step Guide)).

These relays (like the one in the photo in the image viewer) are used in the majority of Ford vehicles.

Contents of this tutorial at a glance:

  1. What Tools Do I Need To Test The Relay?
  2. Relay Basics.
  3. Relay Bench Test.

You can find this tutorial in Spanish here: Cómo Probar En Banco Un Relé De Ford (Relé Grande) (at:

For your cross reference information:

  1. Ford part #:
    1. F0AB-14B192-AA
  2. AutoZone part #:
    1. Duralast 19273
    2. Santech MT0532
  3. O'reilly part #:
    1. BWD R3177
  4. Other part #:

This relay is used for the PCM, fuel pump, fan motors, and a bunch of other stuff.

What Tools Do I Need To Test The Relay?

You need a few basic things and they are:

  1. Multimeter
    1. A digital or analog multimeter will work.
    2. If you need to buy one or are looking to upgrade, check out my recommendations here: Buying A Digital Multimeter for Automotive Diagnostic Testing.
  2. Jumper Wires
    1. You'll need two of them with alligator clips on both ends (you can make these yourself).

Relay Basics

This section isn't mandatory reading to test the relay, but I know a lot of folks dig some working theory (I do myself when researching anything on the Internet) and so this section is for you.

The core purpose of a relay is to control a high amount of current with a smaller (lower) current. How? You might ask. Well...

Every relay has two basic circuits and for the purpose of our discussion, we'll call them:

  1. A high current circuit.
  2. A low current circuit.

Both of these are completely independent from one another, in other words, voltage/current flowing thru' these circuits don't mix.

The high current circuit is the one that delivers the voltage (and thus current) to the component that needs to activate. This component could be a cooling fan motor, an A/C compressor, a fuel pump, you get the idea.

The low current circuit is the control circuit that activates or deactivates the relay. By activating I specifically mean allowing voltage to pass thru' the relay (internally) and on its way to the component that needs to activate (fuel pump, cooling fan relay, A/C compressor, etc.).

How To Bench Test A Ford Relay (Large Relay) How To Bench Test A Ford Relay (Large Relay) How To Bench Test A Ford Relay (Large Relay)

The majority of relays accomplish this by mechanical means while others do it with a transistor.

The relay we're talking about in this article is the mechanical type and works in the following way:

  1. The relay has the following core components inside
    1. An electromagnet
      1. This is a coil of wire wrapped around a soft iron core
      2. When energized with voltage, it becomes a magnet.
      3. Photo 2 of 3 (in the image viewer) shows the electromagnet (top arrow points to it).
    2. A movable armature.
      1. This is the part that completes the high current circuit by uniting the circuit that has voltage to the circuit that will deliver it to the component (fuel pump, cooling fan relay, etc.) that needs the voltage/current.
      2. Photo 3 of 3 (in the image viewer) shows the armature (middle arrow) and two contacts.
    3. A spring.
      1. Photo 2 of 3 (in the image viewer) shows the spring (bottom arrow).
    4. Several contacts.
  2. When the electromagnet gets energized (by having current flow thru' it), it moves the armature to close against another set of contacts.
  1. The armature acts as a switch and completes the circuit that will have the high current flowing thru' it.
  2. When the electromagnet gets de-energized, it loses it's magnetic power and thus let's go of the armature.
  3. The spring now acts on the armature to place it back into its normally open position
    1. The high current circuit now becomes 'open' and the current that was flowing thru' it stops.

OK, working theory lesson is over, let's get testing in the next page.