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Electrical/Relay Questions


jerryasi
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Fog or Driving Lights?

 

A 4 pin relay can be thought of as a "remotely controlled" Single Pole Single Throw switch- The switch is open between (30) and (87) when the relay coil (85/86) is not powered, closed when power is applied.  A 5 pin relay is the Single Pole Double Throw equivalent- a common terminal (30) is connected to one  terminal (87a) when the relay coil is not powered, and to the other terminal (87)when it is powered.

 

The horn lead on your car is grounded when the horn button is pressed, so the horn relay has one terminal of the coil connected to +12 and the other to the horn lead from the steering wheel.  If your aux fan is turned on by a thermostat switch @ one of the water lines, that thermostat switch it will likely do the same thing- ground it's terminal when the water's hot and needs cooling.  If so, the fan relay needs to be wired the same way as the horn.  Headlights are normally done the other way 'round- the headlight switches provides +12 V when the light should be on.  So those relays have one coil lead connected to ground and the other to the headlight switch or dip switch on the column.  

 

Relays come with or without a suppressor diode wired across the coil.  Diodes care about polarity.  So, 85 = the ground lead, and 86 = the  +12 lead.  Always.

 

There's plenty of detail about the why and if of diodes online, but be assured that an auxiliary fan relay needs to have the diode.....

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