View Full Version : Quick Tech, How To build a relay harness

05-26-02, 07:32 AM
This subject comes up often for those that want to add special fog lamps, cooling fans, or electric fuel pumps to cars that did not have them before. I will take a few moments with some photos and the aid of my wonderful animal assistants (see photos) to describe a quick inexpensive, yet professional way to achieve the goal.

In this first pic, is an example of a relay assmebly with weather pack as can be found in virtually any G.M. car after 1985. I prefer the oval bodied relays as they serve dual purpose as normally open, and normally closed (more on that later). You can get the relay in virtually any auto parts store for about $7. These relays are used for fuel pumps, cooling fans, lights, and horns. The one in this photo was pulled from a 92 Pontiac Grand Am. FYI, you can get this exact one from a parts counter for the cooling fan on a 1990 Corvette. The junk yard one will come with the harness/weather pack which is what you are after for a clean neat install.

Please note that the gauge of the wire on the weather pack will usually indicate the purpose of the relay. Small gauge such as in this picture usually indicates a fuel pump relay, or small cooling fan. larger gauge almost always indicates cooling fan assembly. I suggest you match your junk yard slection with your purpose. You can get just the weather pack as well from your G.M. dealer as a harness repair kit. Be patient, I have learned that not all parts men were created equal, so stay engaged with the part location process, as many will just give up looking. Again, I would suggest sticking with a 1990 vette cooling fan relay weather pack harness repair kit for larger diameter wires on a generic purpose, if a bone yard is not available and you want new wires.

05-26-02, 07:34 AM
Here it is pulled seperate. You will note how the wire harness end has a rubber seal to keep moisture out. These relays can be mounted in the fender well, under the hood, or in the case of a fuel pump under the car in the back where they may be exposed to water. Moderate amounts won't damage them as they are sealed.

05-26-02, 07:46 AM
This is the cool part. They come with a schematic description printed on top. These are the best relays because they can do things two ways. You can create a circuit that has power all the time until you switch it, or so that it only pulls power through when you switch it.

You will rarely ever use the normally closed function, but it is there if you need it by simply re-configuring the harness.

Note in this picture.

You would run power from the battery to terminal E. For a normally open relay that you only want to supply power to the accesory when say the key is on, or the engine is at a specific temp, you would then connect to terminal A. In other words, run the wire out of the harness to the cooling fan 12+, or fog lamp 12+ side.

If you wanted a normally closed relay, you would run the wire out of terminal C to the accesory. You can also identify the type of circuit you cut the relay from in the junkyard by observing this schematic and observing the wireharness plugged into it. There will only be a wire coming out of A or C depending on the type of purpose it was being used, but you can use it anyway you please.

Now, the "coil" side is where you apply a signal to make things happen. Note terminal D & F. For a cooling fan that you only want to run when the key is on, run a 12+ ignition on wire to terminal D, and run F to a constant chassis ground. This will only engage the coil and the relay when the key is on. You can use vitrually any key on power, as the amps drawn to pull this contact are minimal.

On the flip side, you can run 12+ ignition on to D, then run a toggle switch to ground and back to F so that you can only turn the item on when the key is on. If you have a temp sending unit that grounds at a specific temp, you would run that wire into F so that the cooling fan will ony turn on when, the key is on, and the sender has hit temp and made ground to pull the coil.

Another trick is for you a/c fan. You can send terminal F to a constant chassis ground. Hook terminal D to the same 12+ wire that plugs on to your a/c clutch. Now if you have E and A wired right, you can cycle a seperate fan only when the a/c compressor is engaged.

Is this of any help yet?

Same for the electric fuel pump. Hook the pump through E and A, D to key on power, and F to ground. That way you don't have to physically toggle the fuel pump on and off.

Finally, same for the fog lights. E to Battery, A to the light, D to 12+ ignition on (if you don't want them to burn when the car is off, or D to 12+ battery all the time power if you don't care), then F to a switch that is connected to ground.

05-26-02, 07:49 AM
This picture shows the bottom side of the relay. This is where the terminals are labeled A, D, E, F, etc. You simply pull the weather pack plug out, look up inside the relay, and you can see the corresponding terminal to the weather pack wire.

05-26-02, 07:51 AM
Finally, here is a look at the weather pack harness. Notice it has more terminal holes than it has actual wires. Like I said before, this allows you the opportunity to configure the harness to the relay to suit your needs. With a ice pick type screw driver, you can trigger the plasic release and slide the wire connectors in and out of the plug holes as needed.

05-26-02, 07:56 AM
Finally, why use a relay? Well, it allows you to make an electrical connection of a high amp draw circuit without having to physically switch the current each time you want it to do something. It isolates high amp circuits and wires under the hood where they belong instead of trying to run wires through a toggle switch all the way under the dash.

They are cool, since they automatically do the switching based upon a given signal, be it a supplied ground, or 12+ ignition. Just extra FYI, you computerized cars work off of a series of ground signals. Your computer will turn the fuel pump, cooling fan, etc on and off by getting a signal from a sensor, sending it through the computer, then the computer determines when something should happen, and sends a ground out to the relay to trigger the event. In most cases, the 12+ ignition side is key on only as a safe way of making sure that faulty grounds don't turn things on while a car is left unattended.

One other note. You can run ground or 12+ through the relay as desired. It really does not matter, as long as you provide the signal and ground to the coil side, it will connect the main circuit regardless if it is pulling 12+ or 12-

I hope this helps those of you considering doing you own wiring, or helps you at least understand how a relay works, and why they are in the system.

05-26-02, 12:22 PM
If only the magazines were this good. A thousand "thank-yous," Chris. It's stuff like this that makes me come back to the CAC almost everyday. :upthumbs