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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:

1.  Car 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 dirty. 

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.

 

Here's a solution that will help prevent the internal mechanics and contacts of a relay from getting dirty.   Sealing the case is all you have to do using silicon seal glue.  Find the breather hole and carefully dap some glue over the hole.  Then around the edges of the case apply the same silicon seal glue to seal the case to the relay base, alleviating any dust from creeping into the relay box.  This preventative measure will help your relay last 5 to 10 times longer than normal in the sand.  It only takes a few minutes to seal your relays, it could save your weekend.

Some advice on wiring:

1.  This may be obvious, but when you are wiring up your car, or if you have bought a used car and you're unfamiliar with the relays, LABEL YOUR RELAYS.  That way you'll know which one controls the fuel pump, left fan, right fan, and each headlight.  Find them, identify each by removing them and determine what's not working on the car.

2.  When wiring your relays it's best to use a relay socket that allows you to quickly unplug and plug in a new relay.  Some car builders don't use sockets in favor of individual blade connectors.  This arrangement is just as reliable, but then you replace the relay you must remember each of the 4 wire connections and where each one plug onto, which is kind of a hassle in the middle of the desert.

3.  If you're building a new car, LOCATE THE RELAYS IN AN EASY ACCESS LOCATION.  You may not think this is important, but it is.  It really helps eliminate problems in the middle of the desert, trust me.