A single pole, double throw relay (SPDT) is a standard relay. It has 5 terminals on the bottom. Each terminal has a # by it. 85 and 86 are the control terminals, it doesn't matter which is wired to power or ground. When power goes to this circuit, it activates a magnet inside. 30 is the output terminal. 87 is the input when at rest (not activated) 87a is the input when activated. What ever is hooked to 87 or 87a will come out of 30. Think of it as the giant hand thrown switch used by hollywood to throw power to the electric chair, only activated by a magnet. If the relay is used to turn something off (interrupt) then power is hooked to 87, 30 goes to the accessory. When the relay is activated (12v to 86, ground to 85, pulls magnetic switch) the sweep will go from 87 to 87a (which has nothing connected to it), thereby breaking the circuit. If used to power something, (ex. driving lights) it will keep high current from going through the switch and melting it down. Wire 12v from switch to 86, ground to 85. 12v from battery (fused) to 87a, nothing to 87, 30 goes to the lights. Low current flows through the switch, activating the relay, sending high current to the lights. If used to power a trouble light, such as when the oil pressure switch kills the engine, wire 12v from the pressure switch to 86, ground to 85. 12v to 87, nothing to 87a, 30 sends 12v to the indicator light. When the pressure switch trips, 12v is cut from the sweep, which sends it from 87a (nothing) to 87 (12v), sending power to the lamp. Now you won't drive yourself crazy trying to troubleshoot the ignition and fuel, when you suddenly remember to check the oil. Once you wrap your mind around the concept, relays can be used for many things. Remember, they are your friends!
Additional info on relays Ol'Cheb was speaking of Bosch (and pin compatible) relays. Here is additional information I find helpful and a picture for Nate. http://www.classictruckshop.com/clubs/earlyburbs/projects/bosch/relay.htm
Russ, That is good the relay looks like a good way to go with the 5 prong relay, Thanks for posting it. Bill, Thanks for posting that site the diagrams on it are really helpfull for wiring those 5 prong relays.
The relay is the safest way to wire in high current draw accessories, or to make an emergency cutoff, or to make a low current output control high current items, like door locks and power windows controlled by an alarm.
Cube Relays FWIW , The # 30 terminal is supposed to be the power feed..... (DIN spec. don'cha know, # 10 Ga. RED wire) These are REALLY good and can be found for free on pretty much any German car in the scrapyard . They're all rated to continuous duty and the rated amperage is printed (older , aluminum ones) or stamped (newer , plastic ones) on the case , they're surprisingly robust for such a tiny little thing and make wonderful (& easy to hide) stater relays , we used to use them on the gutter lights on our sweepers , multiple air craft landing light bulbs never fazed them , not once . typically 50 Ampere rated . Some even have a fuse built in , mostly the wretched ATO typ but a fuse is a fuse I suppose . Can I begin extolling German engineering again now ? . -Nate (who's American GM truck is in little bits in the back yard but who's 27 year old , 365,000 miles Mercedes Diesel Sports Coupe is parked outside...) Sorry but YOU asked for it ! .
Not Really ' Harsh ' But *very* misleading ! . You see , I dipped into the front brakes on my '76 GMC C-2500 and being the goofball that I am , it took me two days of fiddling & hand polishing the lug nuts , washing the insides of the oily , filthy alloy wheels , so on & so forth before it was back in one piece . Of course , it now stops on a dime and the lack of wobble in the RF brake rotor prolly helps the handling a bit too.... It's not like this poor old Mercedes is any cherry , it's just my all time favorite automobile , also brought back from the dead . I found European bumpers for it just yesterday , I can hardly wait to get them all taken apart , polished and re - spray the brackets etc. before fitting to my beloved Diesel Sports Coupe .
