Topic: Horn repair, doesn't work below 3000 RPM with headlight on

[Dunk]

My horn was not working consistently, or below 3000 RPM wasn't working at all with the headlight on. I figured it was because I ride this bike daily year round so it sees plenty of rain, occasional snow, and salty roads.

First I cleaned all electrical connections at battery, grounds, main harness. Added silicon grease to the open big connectors, seems like a good idea. Checked voltage at idle headlight off and on. Not good, slowly dropping below 12v. At 5000 RPM it's around 14.7v after giving it a few seconds to get there. Battery is an AGM at least a few years old, probably not helping matters. Solid state voltage regulator that's adjustable, but did not adjust it since it seems about where it should be at RPM.

So I removed the horn and heard rattling, rotated it a few times and some small gravel came out. I opened it up and found a bunch more that surely wasn't helping things. Cleaned the points with fine sandpaper, reassembled. It works better, now around 1400 RPM with headlight on but still flakey presumably due to low voltage. Will adjusting the screw help it work better at lower voltage?

Not looking forward to winter, heated gloves and such with such a marginal alternator.

[scottly]

Checked voltage at idle headlight off and on. Not good, slowly dropping below 12v.

The battery voltage with the headlight off drops below 12v?? The problem is with your charging system, not your horn. Check the voltage drop from the battery plus terminal to the black regulator terminal, with the key on, engine not running. The voltage should be less than 1v.

[Deltarider]

Heated gloves? Really? How much do they draw?

[Deltarider]

Checked voltage at idle headlight off and on. Not good, slowly dropping below 12v.

The battery voltage with the headlight off drops below 12v?? The problem is with your charging system, not your horn. Check the voltage drop from the battery plus terminal to the black regulator terminal, with the key on, engine not running. The voltage should be less than 1v.

Although I have never done this test, I have the gut feeling this is best done with a battery charger max. 1A connected to the battery. Is my gut feeling right? Kill switch better be off. Please report back.

[HondaMan]

Your 'heated gloves' comment spooks me: the CB550 only makes 1A 'extra' power (when new) when the lights are on (and the engine is turning 4000+ RPM). Your gloves probably pull several more amps than that.

This particular electrical system was designed before the USA suddenly demanded that all motorcycles sold here had to ride with the lights on all the time.

[Dunk]

This bike is a CB750, not sure how much better than 1A surplus it may have. I do have a field and stator coil from a CB750A I was considering swapping in, also considering the CycleX 20A field and stator coil.

[scottly]

Didn't we go through all this several years ago???
Found it. From 2019:

" I've put a couple hundred miles on the bike, much of it commuting with some amount of sitting in traffic and always running the headlight on. about 3 hours after getting home today the battery measured 12.645v. I'll call my charging problem solved. It was a combination on a bad 6.5 year old battery and some loose bullet connectors. Sometimes it's the simple things."

[Deltarider]

Dunk, I'd look for V-drop between battery plus terminal and the horn/headlight in the following steps.
First measure V-drop between battery plus terminal and horn/headlight. Take notes.
Then measure possible V-drop between battery plus terminal and fuse box battery side. V-drop is unlikely here, but who knows.
Measure V-drop between fuse box battery side and its 'consumer side'. If needed, clean contacts and fuse.
Any V-drop between fuse box 'consumer side' and IGN key switch?
Any V-drop between IGN key switch red contact and black contact?
Any V-drop between IGN key switch's black contact and plus side of consumers horn/headlight.
Personally I'd start with the NEG path which is often overlooked. Switch on the headlight and measure possible V-drop between headlight NEG side and battery minus terminal.
If the V-drop is in the NEG path, perform steps similar to those in the POS path.

[Dunk]

Scottly, maybe the battery is older than I thought. Time for new I guess. I already cleaned all the connections.

[scottly]

Usually a bad battery will still show a good voltage while the bike is running, but lacks any capacity to store the charge. I would still check the voltage drop going to the black reg wire with the key on, engine not running, and go from there.

[Dunk]

Voltage drop from battery positive to various locations. Measured with key on, ignition on. Voltage drop is about half as much with key on, ignition off.
   VR black   1.5v
   Engine harness big red   .5v
   Ignition switch   ~1v

I realized I did not disconnect and clean the ignition switch connector, so I did that. Then I cleaned the horn connection and ground terminals to the bar switches. Also cleaned and added silicon grease to the main harness to engine harness plug. That one I had to remove the airbox to get to, so I did a few coats of that black plastic restorer stuff to take care of the chalky faded color and also cut out the baffle from the rear lower airbox intake and blew out the air filter that looked clean but probably due for replacement. I have an older K&N style filter on the shelf I may clean up and use since I've read paper filters are restrictive on these, or just buy a new Uni filter.

I also tested the electric starter, which I probably haven't used in 5-10 years since last time I was curious. It started fine, so the battery even if old can't be that bad. Tested the rectifier too which seems fine, no bad diodes.

In any event, now the horn is working at idle with headlight on.

[scottly]

Are you still getting a 1.5v drop at the black reg wire? What about the ignition switch, did you measure 1 v drop at the red wire, the black wire, or both? If at the red wire, the most likely suspect is the fuse holder.
You mentioned that you had an automatic field and stator, which puts out something like 70 watts more than the standard charging system, IIRC. Since you have it, you might as well use it. If the stock system is marginal with the heated gloves, the extra power may be more than enough?