Horn Wiring

rdorman · #1 (**permalink**) 04-02-2005, 02:32 PM

One of the things I struggle most with is wiring. I am wiring in some Hella horns and NEED HELP.

Looking at the wiring diagram for my car, it shows two wires. One is wire 21 black that runs from the fuse box to the front of the chassis (it says to horns). The other is black 18 that runs from the fuse box to the steering column (it says that this one goes to the (-) on the horn relay). The horn relay is marked terminals 30, to 12 volt; terminal 87 to the horns; terminal 85 to another 12 volt and terminal 86 to the horn button (one from finishline for dash mount) and then to ground I am assuming. 85 and 86 active the relay while 30 and 87 supply the power to the horns.

Question is, how to wire. Considering the physical location of all of this, what is the best way to connect what? Will a relay work if you connect power in reverse? For instance, if you apply power to 86 and ground 85 will that work?

Thanks
Rick

strictlypersonl · #2 (**permalink**) 04-02-2005, 03:52 PM

This is an ERA schematic for the fans, but the horns are similar. Your horn switch substitutes for the switch/thermoswitch. Run a fused power wire to 30 and 85 on the relay. I don't think the coil polarity is critical, but why take chances.

Thomas Gayer · #3 (**permalink**) 04-02-2005, 03:57 PM

Sure, you can trigger the low current side of the relay with hot or ground. you are just completing the circuit.
remember, all a relay is , is a low current circuit that turn on a hight current circuit. so the high current wont be on or honking unless both the low ground and hot are complete.

Jack21 · #4 (**permalink**) 04-02-2005, 05:09 PM

I'm a reasonably decent automotive electrician so I'll give this a try. Petek is another good electrician.

Relays are used in several locations on your car, and the horn is one. A relay takes a small control voltage to activate a relay. The power side of the relay takes battery power and operates a device.

It helps to draw a picture of this, then refer to it during assembly.

The control side of the horn is called "complete the ground". The control side of the relay is always hot (term 85). From the BAT lead on the fusebox. Term 86 completes the ground through the horn button. The horn grounds through the steering column, which is grounded through the frame. You can reverse these two leads and it will still work.

Your black 18? Does it (a) have +12v, or does it (b) show continuity with the frame when the horn button is depressed? By your description, it goes to the horn button. So term 86 goes to black 18. And term 85 must be connected to an always hot power source.

On the power side, term 30 goes to a good power lead, and term 87 (black 21) powers the horns. The power side is the same source used for high amperage devices like fan, lights, MSD, etc.

The relay has no internal, or case grounds, but the horn does. If you bolt the horn to fiberglass, you need a ground wire from the horn case to the frame.

rdorman · #5 (**permalink**) 04-02-2005, 08:50 PM

Thanks guys! I did draw up a diagram based on the directions and it looks just like the one Bob posted.

Jack21, you raise a question. For the Black 18 the diagram shows it coming from the fuse block to the steering column and says "18 black to (-) on horn relay coil". If I connect that to term 86 then wouldn't term 85 be connected to ground? Of course with the horn button some where in that curcuit.

Thanks again
Rick

rdorman · #6 (**permalink**) 04-02-2005, 09:08 PM

How about this.......

The fuse box is on the firewall. Wire 21 already runs to the front for the fans, 18 already runs into the interior, currently to the steering column so it would have to be moved to the dash where the button would be.

With that in mind..........I am thinking mount the relay at the fuse block, run 18 to the switch, from the switch to 85, from 86 to ground. This would complete the 'energizer' circuit. From there, cut into black 21 running it from the fuse box to terminal 30 and then connect 87 to the horns and then on to ground.

Of course, like drawn above, take wire 21 and hook it to 30, jump to 85, 86 to switch(button) and ground and just forget 18 since it goes to the column and obviously was meant for a column mounted button (mine is in the dash). Of course 87 would go to the horns but with the relay right at the fuse box I could cut into wire 21 to go to 30 (from fuse box) and then from 87 on to the horns.

I suppose either would work fine.

Thoughts?

Thanks
Rick

trularin · #7 (**permalink**) 04-03-2005, 07:28 AM

Rick, I used the Hella parts and figured I might mention something.

They are sensitive to ground. You must have a good ground.

Mine was simple, I powered one side of the relay and passed the other side to the horn button on the column. Then I powered the contact in the relay. the other side of the relay I ran to the horns. Then one side is grounded and the other side is powered when the button is pressed.

One more thing, make sure you bolt them down well.