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So I really don't know what this part is. I know I've hunted through the Clymer in the past to find something that looks similar without any luck. It cam from above the engine, hanging from the frame (before I broke the small band that holds it to the frame). It has three wires running from it to the wire harness that runs up and down the frame. Anyone have an answer for me? I'm really more curious than anything.

Are there fuses in there? My kz550 has 2 fuse boxes.

No, it's a solid piece except for the rubbery band that held it to the frame. It seems suspiciously resistor like, but I have no idea. Oh, sorry - it's from my 1976 KZ900A4.

you have a wiring diagram? Maybe you can track it thru the wire colors?

That's the brake lamp failure sensor switch.

I have a brake lamp failure sensor switch? Wow - who would've thought such a thing existed on a motorcycle in 1976.

They used that as early as 1972 or 73 on pretty much anything 500cc and up.

As known, the "dashboard" indicator lamp is the bottom indicator lamp above the ignition switch on the panel between the tach and speedo. The label underneath it says "Stop Lamp" in small white letters.

With ignition switched ON, the "dashboard" indicator lamp is normally off until it glows steadily during braking. But if "flashing" while riding along without braking, it's warning in advance that the rear brake light has failed and will not function upon subsequent braking.

The bike's rear light bulb is a dual filament bulb, meaning it has two separate filaments. One filament is the "brake" light supposed to burn only during braking. The other filament is the "tail" or "running" light supposed to burn whenever the headlight is on (or when ignition switch is turned counter-clockwise to "parking light" position). The tail light has no influence on the dashboard "Stop Lamp" indicator. And the dashboard "Stop Lamp" indicator gives no information regarding operation of the tail light.

Good Luck!

The brake light bulb's filament (when not energized) looks like a resistance of about 8 Ohms to ground. That stop light sensor circuit is a 555 timer running to flash the dashboard "stop lamp" indicator bulb whenever this 8 Ohm resistance is not there (which means the stop light bulb filament is burned out).

I designed a circuit to replace the one in mine which died in case anybody ever wants to build one.

Not that it matters particularly, but a bulb would measure like 0.8 ohms, not 8 ohms I would think as a bulb is really pretty much a short circuit.

A bulb measures low resistance until it is powered on, then the heat on the filament causes it's resistance to rise to it's operating value which is usually much higher than when it is off. You have to calculate the operating resistance based on voltage and current. The brake warning unit uses the bulb just as a coninuity to ground. While the bulb is off, it acts as a short to ground, as far as the brake warning unit is concerned.