After three days of hitting it pretty hard on the Monza, today it ended up being a wash. About the only thing really accomplished was picking up some fuel parts from Progressive Performance and getting the wiring done for the fuel pump relay.
Funny thing about those relays, seems like everyone has trouble figuring out how to wire them but it is actually very simple. What is hard to understand sometimes is the reason you are using one in the first place. There are a number of places on a race car where it just makes sense to use one and in a few instances you either use one or you burn up an expensive piece of hardware – ask me how I know.
Here is the most common relay used on automotive circuits, also known as the Bosch relay. This picture is showing the 4 post unit but it can also be found in a 5 post unit – either can be used and for the majority of your installations you will only need to use 4 posts.
Let’s talk about connecting a relay for the operation of a fuel pump. First off, the pump is normally at the rear of the car and as fuel pumps have gotten stronger over the years, they also have drawn more amperage or power if that is a better idea. To operate the pump you would want to have a switch located near the driver so that the pump can be turned off or in the case of a street car, it could be connected to the hot or the on side of the ignition switch. Now, due to the amount of power this pump will take, it might mean running a 10 gauge wire from the front of the car to the rear. That’s a rather heavy wire and of course we want to stay as light as possible. But there is still a problem with having a wire that long, simply said the longer the wire the more resistance it has and resistance will keep the pump from operating as well as it was intended too. This is where a relay comes into the picture. By using a relay near the pump and battery, we can connect a heavy gauge but short length of wire to the pump for power, yet operate it with a much lighter gauge of wire. In this case we would normally use a 16 or 18 gauge wire from the switch.
Starting with the post at 9 o’clock on the upper picture which is marked as post #86, you make this connection to that switched power input. Moving clockwise, you would make a connection from post #87 to the pump – this should be a heavy gauge wire to reduce resistance. Post #85 is the ground. This can be a nominal size wire and can normally run from this post to a good ground, in most cases that includes the point where the relay is actually mounted to the car. Last is post #30 – this needs to be a heavy gauge wire directly from the positive side of the battery.
So a quick review:
#86 – Connect to the switch that will control the item (pump, fan, light, etc)
#87 – Connect to the item (pump, fan, light, etc)
#85 – Connect to a good ground point
#30 – Connect directly to a positive battery source
Another thing that you can do is to gang the relays. If you have a number of items to control, such as a radiator fan, an electric water pump and lights on the front of the car, group the relays together and share a common ground and positive battery source for them. There is certainly no reason to run multiple leads from the battery source for each of these components. This will reduce the amount of wire needed and keep things a bit cleaner under the hood.