Easy step by step procedure to test an automotive relay, this information pertains
to all relay controlled circuits.
Difficulty Scale: 4 of 10
- A relay is used to control (switch) a
high amperage electrical circuit with a low amperage one, for example a radiator
fan can pull up to 25 amps when in use, which would burn the computer circuit that
controls it, a relay is used to bridge this circuit to prevent electrical damage.
Step 2 - Before beginning tests, use a test light
and check all fuses and replace any that have failed. -
Power Distribution Center (PDC) - Relay Cover
Step 3 - Many vehicles supply relay location and
identification information on the lid of the PDC. If this information is not available,
check the owners manual or
Step 4 - Once the relay has been identified, gently
grasp or touch the relay in question, have a helper turn the ignition key to the
on position, then crank the engine over, the relay should click in one of the ignition
switch positions. If so the trigger circuit of the relay electrical system is working,
if not continue to next step. ( Note: If the relay clicks and the circuit is still
not working there is a good chance the contacts inside the relay have shorted.)
Feel Relay Action
Step 5 - Next, remove the relay for inspection,
grasp the relay and pull outward while slightly wiggling the relay housing, note
the orientation of the relay, it must be installed the correct direction.
Step 6 - Once the relay has been removed, inspect
the relay terminals for signs of extreme heat or corrosion.
Removing Electrical Relay
Step 7 - The relay is mounted in four electrical
terminals housed in plastic and when overheated (due to overload or resistance caused
by a poor connection) can distort and melt.
Inspecting Relay Terminals
Step 8 - Use as small metal scribe or tool to scrape
clean any corrosion to ensure a good connection once the new relay is installed.
Inspect Electrical Terminals
Step 9 - Most relay's describe the internal circuit
by an illustration on the side of the relay.
Relay Terminal Identification
Clean Electrical Terminals
- Terminals 86 and 85 are the primary side of the relay, which utilizes an
electromagnet to close (connect) the secondary electrical circuit inside the
relay. This electromagnet is activated by a simple power (+) and ground (-)
much like a light bulb circuit.
- Terminals 87 and 30 are the secondary side of the relay which acts as the
"switch" that connects electrical current from one terminal to the other.
- Terminal 87a is not widely used and does not need to be connected for the
relay to operate. 87a can be used for many different things such as relay activation
monitoring or connecting a separate circuit that uses power when the relay is
not in use.
Step 10 - Each terminal is identified at the relay
Relay Terminal Identification
Step 11 - To test the trigger or primary side of
the relay set up a test light by connecting a scribe to a test light clamp, being
illustrated in the picture below while testing the tail light socket.
Relay Terminal Identification Base
Step 12 - Once the test light is set up connect
each end to terminals 86 and 85. Now start the vehicle and operate the accessory
switch, the test light should illuminate, if not the switch or circuit ground has
shorted. ( Note: if the circuit is computer controlled a delay could be programmed
into the operation of the accessory, additionally if a cooling fan is being tested
the engine must reach operating temperature before the computer will trigger the
circuit.) Use the test light grounded to check for power, and then switch the test
light lead to the power side of the battery to check for circuit ground.
Scribe in Test Light Clamp
Step 13 - Next, use a piece of wire automotive
wire (20 to 16 gauge) and strip both ends exposing the copper wire. Turn the ignition
to the "ON" position and jump terminals 87 and 30 the relay is now jumped and the
accessory should activate, if so the relay has failed. If no power is observed at
either 87 or 30, the fusible link or maxi fuse has shorted. Example: Testing the
radiator cooling fan relay, the cooling fan should be operating.
Testing Circuit 86 and 85
Step 14 - When replacing a relay be sure to match
up the terminal location from the old relay to to the new unit.
Testing Circuit 87 and 30
Step 15 - When installing the new relay be sure
the orientation is correct or the relay will not work.
Step 16 - Once proper relay operation has resumed,
reinstall the relay (PDC) cover.
New Relay Installed
Reinstall Relay Cover
When a particular two wire accessory is not operating, use a grounded test light
to check for power at the wiring harness (either wire). If there is no power, the
electrical system must be tested starting with the fuse then the relay. If power
is present use the test light connected to battery power to check the ground circuit.
if these tests check out, the accessory is bad and needs to be replaced.
Some relay's can differ from this configuration but follow the same principle.
To confirm the wiring configuration consult a wiring diagram from
- To test for intermitted relay failures
which are common, remove the relay in question, take a small wire strand, about
two inches long and insert it into the relay connector terminal 87 or 30.
- Next, reinstall the relay while keeping
the wire strand inserted and clear of any other terminals or metal (ground).
- With the wire strand secured in the relay
terminal, attach the wire to a small automotive bulb and socket and ground. ( Note:
A side marker bulb and socket work great because of its small size, make the wires
long enough so the bulb can be seen while driving.)
- Temporarily mount the small bulb in a
visible area to be seen while driving, masking taped to the hood or dash works well.
- This test can be used to test all aspects
of the wiring circuitry by moving the bulb ground to power. The bulb will now illuminate
when the relay is use, and will go out to signal a failure.
A relay is switch that utilizes an electrical trigger signal to activate. Once
activated the relay connects an electrical supply to a particular accessory. These
accessories can range from the main computer PCM (powertrian control module), radiator
fan, fuel pump, door locks etc. There are two tests that should be considered when
dealing with a relay problem, is the problem with the relay itself or is the problem
a power or ground issue. A relay is prone to failure when used for a long periods
of time (hot) or when the amperage of the accessory has increased beyond its designed
A rely should be considered as two separate halves, the primary side which utilizes
an electromagnet to close the secondary electrical circuit. 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 or ignition system.
In short, when the primary side of the relay (electromagnet) is activated, it
closes the contacts (switch) to supply power to operate the accessory.
Tools and Supplies Needed
- Test light
- Small piece of automotive wire
- Small automotive bulb and socket
- When a relay warm up as in normal operation, the electrical contacts inside
the relay can short circuit causing the electrical flow to stop, when the relay
contacts cool it will resume the flow of electricity.
- When excessive amperage has been drawn through a relay circuit it can cause
the relay contacts to "stick" not allowing the power to be shut off to the accessory.
Example: When an ABS system motor ages it will draw excessive amperage causing
the control relay to "stick". This condition will run down the battery until
- Moister can get inside a relay hindering the relay operation.
- When testing relay circuits for power, ground is accidentally contacted
causing the fuse to fail.
Article first published 2016-02-04