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Discussion Starter #1
I was wondering could the resistance of one horn keep them both from working, here is what I've diagnosed and replaced over the past few years during my restoration.

The turn signal switch & steering wheel is new, I'm getting 12+ volts (14+ engine running) at the steering contacts and at both horn wire connectors 482 (disconnected). The driver side horn (RH) properly blows without the passenger side (LH) connected, I can swap the horns at the connectors and the one moved from the driver side (RH) will blow (weak) but the passenger (LH) horn moved over will not. I also used a jumper wire from the driver horn connector to the passenger side to eliminate the harness, same result. However a direct connection to the battery both horns blows loud!!! which tells me the they will blow with direct 12 volts applied.

With both horns plugged up and button pressed, the voltage at the horns drops to around 4 volts. Is there a resistance measurement I can check on the horns, do you think I have a bad horn and BTW they were replaced (aftermarket) a few years ago and they never blew like they should.

Please advise, thanks JCS
 

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It is not just volts that makes them work, but amps, and there is a LOT of amps straight from the battery to overcome resistance, and a lot less through the car's wiring and chassis to ground. Have you pulled the horns to be sure there is no dirt dauber or wasp nest in it? Also be sure the ground for the weak horn is clean, maybe unpainted where it bolts on. Horns are just a small stack of vibrating discs, and the circular (snail-like) shell just directs all the sound in one direction. Something may be between the discs, look for an adjustment screw, perhaps an air-blow or small oil port, and it may be oiled where the screw comes out with a light spray of electrical contact cleaner or a drop of WD-40, and some love-taps with a rubber mallet while blowing it sometimes frees them up. Then again, it could be just a bad horn. Try the simple and free stuff. Good luck!
 

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I added a relay for the horns when I did my headlights and it made a huge difference. If you want them to really blast like they do when you connect them straight to the battery, that's the way to go.

First though I'd make sure the ground point on the car and the horns are clean and bright and see how that works.
 

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Ditto, the horns can have some debris in them. Putting a relay in is a good idea too. That's on my short list. Horns have low resistance which means they'll draw a lot of amps. Add into the resistance of the wiring harness, that's going to reduce the amount of amps the horns need to work correctly.
 

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Discussion Starter #6
Thanks forum members for your replies, the horns definitely work if direct power is applied and I'm certainly considering installing a Relay in the circuit. Is there a relay recommended for the 67 mustang that works with the horn & headlights. If so I will get it before I pickup where I left off troubleshooting.

Thanks JCS
 

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JCS - the passenger's side horn is the RH, and driver's side is LH using Ford's standard.

If we look at the information provided by Ford, shown below, the horns are protected by a 15 amp circuit breaker located in the headlamp switch. There is always margin involved, but we can safely say that the horns take a maximum of 7.5 amps each. With quality Ford parts, no interposing relay is required. It sounds like the 7200 harness assembly (including the horn switch) and the horns are aftermarket. It was not stated if the headlamp switch is original or not. Since the OP stated that the voltage was down to 4 at the horns (good, quality information by the way) we at least know that the headlamp switch circuit breaker is not tripping when power is being applied to both horns.

This comes down to two basic questions:

1. Is one or both of the aftermarket horns drawing too much current? Keep in mind that the 15 amp circuit breaker should limit this, provided that it is not out of tolerance or failed. In other words, with a functional circuit breaker, I would expect that way too much current would trip the breaker, and we would not even have 4 volts.

2. Are the horns good, but there a defective component, connection, or wire that is making up most of the voltage drop?

Do you have a basic multi-meter that can measure up to 10 amps of current (a typical value that they are fuse protected at)? If so, disconnect both horns, and then run one horn circuit through the ammeter and report back on current flow, for each horn. If you blow the 10 amp fuse in the meter, you found your problem and will need to replace one or both horns. If you don't, I would recommend getting or borrowing one. They are readily available. If you've never used one please read the instructions :)

If the total current requirement for both horns is less than 15 amps, then we need to look for the location of the voltage drop. If I can't get the volt meter probe to get a connection to a terminal, I will use a sturdy needle and insert through the wire insulation so that I can measure the voltage at specific locations. I would start out in the interior, at the low end of the steering column, and measure the 460 (yellow) wire which represents power out of the headlamp switch and into the horn switch. If you have low voltage here (when attempting to blow the horn) the problem is most likely in the headlamp switch, or the 14401 connector on the headlamp switch. You could measure the voltage on wire 25 (black-orange) when trying to blow the horns to make sure that you have a solid 12 volts into the switch. If you suspect the switch, even if you go ahead and replace it, you should carefully inspect the connector, cleaning it up if needed. It would be a shame to replace the headlamp switch only to have the same problem exist.

If the 460 wire has good voltage, measure 482 (blue-yellow) which represents power out of the horn switch. Assuming that there is still 4 volts at the horn, if you have good (say greater than 11) volts on 482 then we move on. If not, you could have a marginalized wire in the steering column (pinched, some failed strands), or a poor quality reproduction. Pull the steering wheel and, with the volt meter, determine which of the two horn switches is hot all the time. Then figure out where to push to blow the horn. Measure voltage on each of the horn switch terminals to see what you have. If you have 12 coming in and 4-ish going out, you found a horn switch problem.

If 482 is good, the only remaining item is the 14401 to 14290 connector. You can use the needle and check the 482 voltage on each side of the connector. However, I would pull, inspect, and clean first before sticking the needle through the insulation. I try not to do unless necessary.

740455
 

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A horn is a solenoid with the pin hooked up to a diaphragm and a contact. The diaphragm contacts the 'pitch' adjusting screw. At rest, the diaphragm is touching the pitch screw. This completes the circuit. The solenoid is essentially a dead short as it is a very low Ohm coil. Lots of current goes through it making a magnetic field which pulls the diaphragm away from the screw opening the circuit. Current stops and the diaphragm relaxes starting the cycle again. Any extra resistance lowers the current appreciably.
1. Turn the adjusting screw back and forth a quarter of a turn to get the corrosion off of it.
2. Check the wiring for additional resistance. Clean all connectors in that circuit.
Usually it is 50+ YO wiring that is to blame. The relay mode gets around that.
 

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In my experience, 50 year old wiring is sometimes, but not usually, the problem. Quite often the problem is due to aftermarket components that do not meet the OEM specifications. In the case of halogen headlamps, they draw much more power than the originals, and the headlamp circuit breaker will trip causing lights to drop out, and then come back for a spell before cycling again. Many times, however, the aftermarket stuff is just junk, to put it bluntly. Even when it looks to be an accurate reproduction.

"Any extra resistance lowers the current appreciably" - that is true. But to continue along that train of thought, when the current is lowered what happens to the input voltage? It should increase. Another way to look at extra resistance is to go to the extreme. The most you can do with extra resistance is infinite resistance. In other words, pull the wire. Can't get more than that. When the OP did that, he had 12 volts at the horn connector. But with it hooked up, he had 4. The problem we are looking at here is not additional resistance in the horn.

That said, the horn could be mechanically stuck and shunting current to ground and never modulating. But if that is the case, I'm surprised that the circuit breaker is not opening up to protect the wiring. This is why I think that measuring the amps to each horn is vital to moving forward with a systems approach to finding the root cause. Until we have that data, we will continue to keep throwing things at the wall.

The relay might be needed when there is a collection of marginal aftermarket parts. The other option is to find the root cause, and get good aftermarket or good original parts. Millions of these cars ran for decades with no relays, so I'm not quick to jump ship.
 

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Discussion Starter #10
Thanks members for your quick replies and to answer your questions. I forgot to mention I did replace the headlight switch with one from mustangs unlimited during the restoration and I have a Fluke digital multimeter with a 10a fused circuit, I just need to understand 67gta289 instructions on testing the horns with the multimeter and ammeter.

You've certainly lead me in the right direction, I will test the voltage running from the headlight switch through the turn signal switch, after reading some instructions on performing the amperage test on my horns and report back. This certainly helps because I wanted to know the problem before modifying it by adding a relay.

Thanks again JCS
 

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Discussion Starter #11
BTW, 67gta289 you are correct, my 30+ years of IT troubleshooting have always forced me to find the root cause or it will haunt you again. And I would much rather locate the root cause before choosing to add a relay. And like always I will report back my findings as I have over the past years of my project. Please check out my before and after pics, the car sat in my father-in-laws So. Cali backyard before he shipped to me here in 2010 to GA. It's been a fun project that I didn't want to take on, but my wife and boys talked me into it.

thanks again
 

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I will go back to the resistance here for a second. The battery is 12V nominal. With no current the 12V appears at the terminal when disconnected from the horn as it should because the current is zero. When the horn is connected, 4V is read at the horn. The battery is still 12V, not 4V. That means 8V is getting dropped from the battery to the horn across the wiring and connectors in between the battery and horn.The horns work when connected directly to the battery. The 4V across the horn coil resistance is not enough current to activate the horn. It is the current doing the work here, not the voltage. So if the horns drew 4A each, that would be 2 Ohms in the wiring back to the battery. Resistance of copper 20AWG is approx 0.011 Ohms per foot. For 2 Ohms, that's 182 feet. Pretty sure that harness is significantly less length than that. So the resistance is coming from corrosion in the wiring, bad contact arrangement in the steering column, or corrosion on the connectors. Them be the choices. Got a Grant steering wheel? Those horn contacts are notoriously [email protected]
Step 1 is clean all the contacts and the ground connection (back of horn mount and radiator support) so they are nice and shiny. Check the horn contact in the steering wheel for corrosion or arcing damage. Check any connector in that circuit and make sure both ends are nice and shiny. You have to pull the wheel to check the horn contact ring for corrosion or damage. Check the wires for fraying at the connectors. Fewer strands because some are broken mean very high resistance (relatively).
2. Measure the resistance from the radiator support to battery -. This should be 0. If not, check all the ground cable connections.
3. Start measuring the voltage at every junction with the horn activated. Look for where the voltage drops the most. This is one of the culprits. There may be more than one. Keep going until the horns start working.
 

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Whoops, 4 amps each is 1 Ohm for 8V drop.
 

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Millions of these cars ran for decades with no relays, so I'm not quick to jump ship.
Ya, and many had fires from the horn wiring. Spend an hour installing a horn relay, and you'll have a better sounding horn and much less chance of a fire.
 

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Ya, and many had fires from the horn wiring. Spend an hour installing a horn relay, and you'll have a better sounding horn and much less chance of a fire.
And to avoid fires and a short taking out the headlights, consider converting from switching the positive, to switching a ground.
A good friend of ours wrote up the procedure .

Here's a simple instruction that is good for 65/66 and other than maybe the wire #, will work through 73:

1. Cut yellow wire (#460) that runs from the HL switch to the steering column somewhere near midpoint.
2. Tape off the end coming from the HL switch so it can NEVER touch chassis ground.
3. Install a ring terminal on the end from the column and connect it to ground.
4. Install a Bosch type automotive relay in a convenient location near the left main lighting harness.
5. Cut the blue horn wire (in the left under-hood harness) near the new relay.
6. Connect the end coming from the firewall to pin 85 of the relay.
7. Connect the end going to the horns to pin 87 of the relay.
8. Connect a short jumper (wire) from pin 86 to pin 30 of the relay.
9. Connect a FUSED hot source from the solenoid, battery, voltage regulator, etc. to pin 30 of the relay (second wire on pin 30, plan ahead) THIS WIRE MUST BE FUSED (20A or so) AS CLOSE TO
THE SOURCE AS POSSIBLE.

Now when you mash the horn button you are applying ground to the relay coil and the horn honks. If the horn circuit shorts to ground the horn will honk but the lights will be unaffected. Simply pulling the horn fuse (Installed in step 9) or removing the relay will solve the annoying problem until you can make a permanent fix.
 

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Thanks members for your quick replies and to answer your questions. I forgot to mention I did replace the headlight switch with one from mustangs unlimited during the restoration and I have a Fluke digital multimeter with a 10a fused circuit, I just need to understand 67gta289 instructions on testing the horns with the multimeter and ammeter.

You've certainly lead me in the right direction, I will test the voltage running from the headlight switch through the turn signal switch, after reading some instructions on performing the amperage test on my horns and report back. This certainly helps because I wanted to know the problem before modifying it by adding a relay.

Thanks again JCS
There are a myriad of different multimeters out there, so I'll have to leave this somewhat generic. Basically you will need to:

1. Change the dial to select amps. In the picture below you can see that I selected the milliamp/amp setting, and not the microamp (ua) setting.
2. Change one of the meter leads from volts/ohms to amps. In the picture you will see that I plugged the red lead into the "A" terminal, which is good for 10 amps, and not the milliamp/microamp terminal.
3. Use the meter as your jumper from the battery to each horn, one at a time. Connect to the horn first, battery second.
4. Test your known good horn first and document the current value.
5. When you test your suspect horn, you might very well blow the fuse in the meter. So before you do it, take a look at the fuse and make sure that you can get a replacement. Note that the volt-ohm function will work with a blown fuse.
 

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Discussion Starter #17
Thanks again, I checked out some videos last night on using my Fluke 75 to test the amperage and it is simple. I was unclear if 67gta289 wanted me to test the amperage with the source power being the connector wire or jumper-ed from the battery, got it you want me to use the battery for power. And I will get spare fuse in case I blow it, BTW the fluke suppose to be able to handle 10+ amps under 30 seconds before blowing the fuse.

I will perform the amperage and voltage checks this afternoon and report back. Hopefully I will find my root cause and correct it, then I will follow 289vert instructions and install a Relay. 289vert, what is the part number for the Bosch relay I should get?

Thanks JCS
 

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Discussion Starter #19
Thanks 67gta289 for leading me in the right direction and 289vert for the instructions on adding a relay to the horn circuit. My multi-meter fuse was already bad so I began checking the voltage at the points 67gta289 suggested and the drop was caused by the springs in the (Scott Drake Repo) switch not making good contact. After using some sandpaper, tuner cleaner and an application of dielectric grease, I now have dual sounding horns again. But wait it gets better using 289vert instructions I picked up a 15 amp inline circuit breaker and installed a relay and wow what a difference. I need to read up on how to add a relay to the headlight circuit (although it doesn't get driven at night) before spring rolls around.

I accidentally lost the driver side fog light, it's not getting ground on the ground side of the connector, but I will deal with that this weekend.

Thanks again everyone, have a Happy New Year & Decade,

JCS
 

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Early Falcons and the 64 1/2 mustang, had a horn relay.
I wish I could claim the credit for the write up ... but I
can't take it.
Credit is due to a smart guy named Bullet Bob.
No fires for you now.
 
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