New horn installed, now electrical problems
 

I have a 31 that runs great.
I installed a new horn. Now the car won't start.

Key off, 6 volts to both sides of the starter fuse. Turn the key on, voltage there gone; won't start. What did I fry? Fuse is fine.

When key is turned off, voltage reappears but horn won't sound. Disconnected the horn. With key off and horn button pushed 6 volts get to the horn wires. With key in on position, no voltage to horn wires.

Any description of the method to solve this is appreciated. I am good with reading wiring diagrams and using voltmeters. But I was hoping it was something simple like a new ignition switch...

Re: New horn installed, now electrical problems
 

Sounds like you might have introduced a short circuit (connection to ground) somewhere in the path the electricity takes when the key is switched to on. Think back through the steps you took when you installed the new horn – what did you change?

Re: New horn installed, now electrical problems
 

If you take the horn off, going to the electric connections to the case, do you have continuity? It is possible to introduce a bad part.

Re: New horn installed, now electrical problems
 

Make sure the two wires on the cut out are not touching the cutout should be an insulated washer on the screw that goes to the cutout and the two wires.

Re: New horn installed, now electrical problems
 

Wiring diagram for stock car here: https://www.mafca.com/downloads/Semi...ex%20Janke.pdf Common change are to the ignition switch and addition of turn indicators. Draw out your wiring system on a sheet of paper by tracing the voltage with ignition off and on and with lights off and on. Keep this for reference. Designate the colors of the wires.

Start disconnecting wires to search for your fault. Label wires if you have a hard time keeping track of where they go. Check voltage and for a short to ground using the ohm scale.

Re: New horn installed, now electrical problems
 

Starter, lights and horn all work with key off. Horn connects to hot battery, button provides ground to the other side.

Re: New horn installed, now electrical problems
 

On your Avatar, it appears you have a non original horn. Has the wiring been modified to work with a non original horn. The original horn is not using a chassis ground. Check to see if the yellow wire has 6 volts to chassis ground, and that there is continuity between blue wire with the tracer when the horn button is pressed.

Re: New horn installed, now electrical problems
 

Good catch, I'll check that!

Re: New horn installed, now electrical problems
 

How is the ignition switch wired? It can open and close either the breaker side of the coil or the power side. Is there anything else connected to the switch?

If things go south when ignition is operational then there may be a problem in that circuit such as a bad coil. Direct shorts to ground will blow the fuse.

Re: New horn installed, now electrical problems
 

Well after checking all the voltage where it should be, and looking for possible short locations, I thought I might disconnect and charge the battery overnight. It needed it. The engine turned over and I replaced the newly shrink sleeved horn wires and cover. Everything works fine.

It could be that the ignition key needs to see full cranking amps (not just 6 volts) to send power to lights/horn/starter/etc.

Re: New horn installed, now electrical problems
 

The lights, horn and starter should work with the ignition switch on or off.

Re: New horn installed, now electrical problems
 

A severely discharged battery will read 6V without load. As soon as load is applied V drops through the floor.

Re: New horn installed, now electrical problems
 

The voltage of the battery should be 2.1-volts per cell for a total of 6.3. The electrolyte should be in the green range on a Hydrometer in all three cells unless it is a sealed AGM battery which has no way to check that. Headlamps should come on bright and they shouldn't dim completely out if a person hits the starter. These are the best ways to check a battery in situ.