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How Do I Repair My Car?
/ How Do I Check
a Car Relay?
How to Check an Electrical Relay and Control Circuit
READ COMPLETELY BEFORE BEGINNING:
Your car is constructed with many electrical relays. These control relays are
used like a switch that utilizes an electrical trigger signal to activate. Once
a relay is activated it will connect an electrical supply to a particular accessory.
These accessories can range from the main
ECM (computer control) to a fuel pump. Most control relays work on the same
principle even though their terminal configuration can vary slightly. There are
two tests that need to be considered when dealing with a relay problem. Is the problem
with the actual relay or the power, ground or trigger circuits that activate the
relay. In this car repair guide we will test the relay contacts, operation and activation
circuits. A relay is more prone to failure when it is warm. A rely can be considered into two separate halves. The first or
primary half of the relay utilizes an electromagnet to close the secondary electrical
circuit inside the relay. 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 the power to a particular accessory like a fuel pump or
ignition system. So, to re-cap, if you activate the primary side of the relay
(electromagnet) it closes the contacts (switch) to supply power to operate an accessory.
The relay primary side activation can be done manually, example: headlight switch
on the dash activates the headlight control relay to supply power to the headlights.
Or the relay primary side activation can be done automatically, example: When the
ECM (computer) commands the fuel pump to turn "on" it will send an electrical signal
to activate the fuel pump relay. The secondary side of the relay is the "switch"
and does exactly as it is described; turn "off "and "on" power to an accessory.
The guide below covers the most popular wiring configuration. Most relays are located in your car's fuse or power distribution center, to find a particular system
relay for your car consult your owner's manual or a
car repair manual.
Typical Car System Relay
Relay Terminal Identification
- Terminals 86 and 85 are the primary half of the relay that 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 half of the relay that is the "switch"
that transfer's 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.
Testing and Checking a Control Relay Guide:
Tools needed to perform these tests:
Test Light or a
Multi meter, a small piece of wire and a small automotive bulb and
socket. (A car side marker bulb and socket work great because it has small wires
attached)
- Step 1 - Anytime you have a problem with electronically
controlled components such as a control relay for an
engine,
transmission,
ABS
brake, or
SRS (supplemental
restraint system, Air Bag) inspect all fuses using a
test light
and check the under hood power distribution center and under dash
fuse panels. A fuse supplies power to operate the relay and to the controlled
component. If all fuses test ok continue to the next step.
How do I check
my car fuses?
- Step 2 - To check the relay operation have a helper turn
the ignition key to the "on" position and then to the "crank" position while
your fingers are on the relay in question. Either when the key is moved to the
"on" position or when the starter engages your fingers should feel a click beneath your
fingers. If not, remove the relay and inspect the connections, if corroded or
over heated repair and reassemble with a new relay to recheck the operation,
if the system is still not functioning properly proceed to next step.
- Step 3 - Connect a
test light or
multi meter to ground (black lead). Turn the ignition key to the ''on' position
(engine off) and remove the relay. Using the probe test all terminal sockets
in the relay connector, two out of the four should have power. If power only
exits at one terminal or no power exists re-check related system fuses. If all
related fuses are ok and you still have no power a
wiring diagram from a car repair manual is needed to trace the wiring back
to the power source to be repaired. If ok proceed to next step.
- Step 4 - With the ignition key in the "off" position
take a small piece of wire (14 to 20 gauge) and strip it on either end.
Insert each end of the wire into the 87 and 30 relay block terminals and
turn the ignition key to the "on" position. The relay is now bypassed so the
accessory that it controls should now be on. Example: if you are testing the
radiator cooling fan relay, the cooling fan should be operating. If the
accessory the relay controls are not operating, in this case the radiator cooling fan, use a test light and
check for power at the cooling fan motor harness. If no power exists there is
a short in the wire or connection between the relay power supply and the cooling
fan motor. If power is present at the cooling fan motor, test the ground wire
at the motor harness with the test light still grounded. If the test light doesn't
illuminate the accessory or fan motor has failed and replacement is required.
If power is present at the fan motor ground wire, the "ground" has failed and
repair is needed. If all tests "ok" proceeded to next step.
- Step 5 - Connect the test light lead to the positive battery
terminal, this is done to test the relay "trigger" circuit. With the key in
the "on" position (engine off) use the test probe and insert it into the either
the 85 or 86 relay block terminal (whichever one did not have power). Next,
command the relay to operate, this operation will vary depending on the relay
being tested. Example: If you are testing a relay that can be manually controlled
like a headlight relay, simply turn the headlight switch to the on position,
the test light should illuminate. If you are testing a relay that is automatically
controlled like fuel pump relay, crank the engine over and observe the test
light, it should illuminate, if not consult a
wiring diagram from a car repair manual to trace the wiring to the source.
(note: control relay power and ground configurations may vary depending on application,
if you are unsure of the configuration of a relay consult a
wiring diagram from a car repair manual)
- Step 6 - This test will help diagnosis intermitted failures
which are common for relays and relay control circuits. Remove the relay in
question, take a small wire strand about two inches long and insert it into
the relay connector for the power or ground being tested. Next reinstall the
relay while keeping the wire strand inserted and clear of any other terminals
or grounds. With the wire strand secured in the relay terminal attach one wire
of a small automotive bulb and socket. (A car side marker bulb and
socket work great because it has small wires attached). The remaining wire
of the bulb and socket attach to power or ground depending on your test.
Example: if you are testing the relay ground trigger circuit, attach the
remaining bulb wire to ground and vice versa. The bulb will now illuminate when the relay is
use, and go out if a circuit fails. (Note: install the small automotive bulb
in an area you can see while driving, taped to the hood or dash works well)
Common Problems:
- As the relay heats up 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. To test for
this problem attach two test lights to terminal 30 and 87. Attach the test
lead to the both terminals, a small piece of wire works well for this.
Reinstall the relay and monitor the test lights while in operation. If one
of the test light bulb goes out while in operation replace the relay. If
both test lights go out trace the main power supply to that relay, consult a
wiring diagram from a car repair manual and repair as the short circuit
as needed.
- If too much amperage is being 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 cause
the control relay to "stick". This condition will run down the battery in a
short time until corrected.
- Moister can get inside a relay hindering the relay operation.
- While testing for power, ground is contacted causing the fuse for that
circuit to fail.
WARNING! Always have the vehicle under inspection on level
ground, in park with the emergency brake on. Always wear protective eyewear, gloves
and necessary clothing before inspection or work begins. Never crank an engine over
when anyone is near the battery or engine. Always have an operational fire extinguisher
close by, obey all first aid instructions in the event of an injury. Never stand
in front or behind a vehicle when starting or running. When engine is cranked over
keep hands and clothing away from rotating components.
How Do I Repair My Car?
/ How Do I Check
a Car Relay?
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