relay question

[daveshoe]

I have been able to figure out many terms used in TDR just by reading posts by others , but relay as it relates to electrical problems sort of has me baffleled. Is a relay sort of a fuse panel (what the fuse plugs into] ,a circuit breaker or what ? If someone could post pics or show me an example I would appreciate it.

Feelin a little stoopid and



Dave Shoemaker

 

[jwgary]

A relay is a switch that is electrically powered. It is usually used to allow a small signal to control a larger power draw or something of a different voltage. There is a coil inside the relay that acts like a magnet when electricity passes through it. When it does, the coil attracts an arm that has contacts on it. That throws the switch. The advantage of this is that you can have a small wire coming from something with a low power output that will turn on something else with a larger power output and only have to run the larger wire on that circuit. I hope that helps some. If you go to howstuffworks and do a search for relays. I am sure they will have some kind of animation or diagrams there.

 

[Steve St.Laurent]

A relay is simply an electric switch. For example you want to turn something on that uses a lot of current when something else turns on that is a low current device. Say you have a driving light that you want to turn on that needs 10 gauge wire, but you want it to turn on when you turn on a dome light that uses 22 gauge wire. A relay has four connectors on it (or possibly five - I'll get to that in a minute). Here's a diagram



    C

    |

A----B

    |

    D



A & B is the switch part of the relay, you run a wire from the +12v wire (22 ga) on the dome light to A and run a wire from B (22 ga) to ground. Then you run a wire (10 ga) from a high-amperage 12v source (like direct from the battery - warning you want a fuse on it) to C and then another wire (10 ga) to the +12v pin on your driving light (then of course one from the ground pin on the driving light to ground). When 12v runs across A & B the switch is closed between C & D and then your driving light will be on. If no voltage is applied to A - then the switch is open between C & D and your driving light will be off. So your only using the 22 gauge wire as a trigger for the 10 gauge wire.



That is a normally open relay. You can also get a normally closed relay where if voltage is applied to A then the switch is closed between C & D, and if no voltage is applied to A then the switch is open. You can also get relays that have 5 connectors - then can be setup as a normally open or normally closed. In this case you have the equivalent of a C1 and C2. If voltage is applied to A then the switch is closed between C1 and D - no voltage to A and the switch is closed between C2 and D.



Hope this didn't confuse you!

 

[daveshoe]

So the relay that 446 found to have bad contacts in the most recent 911 post would be found in the fuse panel like the one shown in ''How stuff works'' ? I assume you know which relay to check by using the service manual ?

BTW thanks for the explanations and quick response!

 

[RADdodge]

Very good explanations were given above, but they say a picture is worth a thousand words. Unfortunately, it takes a few to explain this one.



The relays are found under the hood in the fuse box (left side) or what Dodge calls the “Power Distribution Center” (PDC). Here is a diagram of how I wired a pusher pump using two relays. The control for the circuit is derived from the actions of the Engine Control Module (ECM). The power comes from Fuse D through the normally open contact (87A) of the starter relay. So when you engage the starter, the relay closes the contact to terminal 87 which will fire the starter solenoid and interrupt the pusher pump. Otherwise as long as the ECM is driving the lift pump, the pusher pump runs, but not from the power of the ECM. Clear as mud, right. Best I could do.



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