Questions and Answers

GMC Jimmy Needs Horns Q. Hi Vince! I've got a question regarding the installation of aftermarket horns. Specifics: 1978 GMC Jimmy 400 V-8 150,000 miles Automatic transmission

The horn wasn't working so I replaced the horn relay. I'd like to install a separate horn switch but I'm at a loss as to how to wire it up. I tried to use the factory wiring but since the original horn button on the steering wheel isn't working I need to install the separate switch.

This is where I get lost. I've got the switch grounded proper and power running to the switch but I've had to disconnect it from the power as the horn blasts continually (not something my neighbors appreciate). All the wiring diagrams I have in the aftermarket books are worthless.

Any advice you could provide will be appreciated. I can't drive it on the street until I get it inspected and a horn is a requirement to obtain the safety inspection.

Thanks for any help or advice.
Regards,
Richard
Richardson, Texas

A. The horn circuit is one of the easiest circuits on the car! The factory horn is grounded where it bolts to the frame or body. There is a single wire from the horn that runs inside the car to a three pronged relay. Of the other two prongs of the relay, one runs to the horn fuse and the other runs up steering column through the turn signal switch to the horn contact plate in steering wheel. VERY simple.

Try to fix what you have. Most likely it's a bad contact inside the column under the cap. If you hook it up from scratch like the factory had it, a ground is all that is needed to make it work. If it's a bad horn, you will here the relay click with no horn sound. If you don't here the relay click, there is either a blown fuse or the ground circuit up the column is bad.

Taking a test light from ground you can probe every wire safely that is in the turn signal switch connector. Probing one of these wires should make the horn relay click. This would mean that the problem is in the column. You could fix it or just run your switch to this wire.