Join Date: Mar 2002
Location: Austin, TX, USA
I've been reluctant to answer because I don't have the wiring diagrams in front of me to tell you precisely how to wire it up on the workbench. But here is some general info about relays that may help.
A relay is made of an electromagnetic coil and a switch. When a small current is applied to the coil, it pulls the switch contactor in to complete a high current circuit. Some relays have multiple poles. You'll see designations like DPDT for double pole double throw or SPDT for single pole double throw. Double throw means there is a normally closed contact (NC) and a normally open contact (NO). A horn relay is a SPST type or single pole single throw - the most basic type.
An SPST relay generally will have 4 connections. Two are for the coil and the other two are for the contactor. But I believe the horn relay only has three connections. That is because one lead of the coil shares a terminal with the B+ line of the contactor. I think there may be another lead used for a key in the ignition buzzer. Not all cars use it.
The B+ line is the bus-bar on the relay. It servers a dual purpose of providing vehicle power in addition to providing voltage for the horn. One of the small terminals connects to the horn button. The horn button provides ground for the other lead of the relay coil to energize the magnetic field. The other terminal connects to the horn. When the relay coil is energized, the relay switch closes connecting the bus-bar to the horn wire.
To test the relay, start by identifying the terminals. I use an ohm meter. Either digital or analog will do. We know the bus bar is B+. Between the bus-bar and one of the other terminals you should be able to measure a resistance. I'll guess and say it will be between 10 and 100 ohms. This is the relay coil. Provide 12 volts to the bus-bar and ground the relay coil lead. You should hear it click. Next measure voltage on the horn wire terminal with a volt meter. You should see 12 volts appear when the relay is energized. But this isn't a conclusive test.
What really counts on a relay is that the relay contacts can handle their rated current. So in place of the volt meter install a load of some sort. A horn is one option. A simpler option would be a 12volt lamp. I have a brake lamp bulb that I have soldered wires to for this purpose. It is a fairly powerful lamp that will draw at least 1Amp.
Using the lamp or other load, energize the relay. You should see the lamp come on at full brightness. You can measure the voltage accross the contactor by putting a volt meter lead on the horn terminal and the other lead on the buss bar. You shouldn't see more than a couple of millivolts drop. If you see more than that or if the lamp is dim, you probably have a relay with burned contacts which has too high of a contact resistance to work properly. Of course if the bulb doesn't come on at all or flickers when the relay is tapped with a screwdriver, the contacts are probably toast.
Hope that gets you going. Search the archives for 'horn relay'. Somewhere in there is a web link to an informative article that has been posted here a few times. Also, you can search for 'schematic' or 'wiring diagram'. There have been several vehicle schematics posted that may help as well. It would be a good idea to note the wire colors before removing the relay. The schematic doesn't really show how the terminals are connected, but the color codes should get you in the ball park. Oh and by all means (DISCONNECT THE BATTERY FIRST!). That bus bar is can provide enough current to weld with.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI