Relays are commonly used for fans, fuel
pumps, and headlights to handle the high current levels of these
circuits. Fans can use over 25amps and headlights can draw 10amps.
These heavy currents are too much for switches or computers to handle,
hence the need for relays.
If a relay fails to work properly you can have a mess on your hands. Here
are some common problems:
was running fine a minute ago, but now it won't start.
Diagnosis: Have someone turn on the ignition
while you listen or touch the fuel pump to insure it's coming on for a
few seconds when the ignition is initially turned on. If the fuel
pump doesn't make a humming noise when the ignition is turned on then chances are 90% the
fuel pump relay is
Fix: If you know which relay controls the fuel
pump, take a small open end wrench and slightly bang the relay housing
while someone turns the ignition switch on and off several times.
Once you hear the fuel pump start running again......you're temporarily fixed.
Replace the relay asap.
2. Car is overheating out of the blue.
Diagnosis: Check to make sure all fans are
running when above 180F. If one of the fans (or both) isn't
running the problem
is most likely a relay.
Fix: Same as above, bang on the relay that
controls the fans as someone
switches the ignition on and off. Replace the relay(s) asap.
3. One of your headlights quit working.
Diagnosis: Swap bulbs with the good headlight.
If the failed light housing continues to fail, your relay is bad.
Fix, same as above, bang on the relay that controls
the headlights. Replace
the relay asap.