Motorcycle·

CB500T coil wires that end on a red-and-green box

On this 1976 CB500T both coils' black/white wires were found joined to a small box with a red wire and a green wire. The bike later ran on the kickstarter. That box was not confirmed as the repair.

Measure the black/white wire before you call the coil dead. On this 1976 CB500T both coils' black/white leads were found joined together on a small rectangular part bolted at the coil bracket. The part looked as if it went nowhere. It had a red wire and a green wire. At the same time, a meter from one black/white wire to ground read about 0 volts. Between the two black/white wires it read about 0.6 volts. The dash lights were on. The kill switch was on Run.

That is a coil with no supply. The points can still throw a spark of sorts. The plug wire on the dead side will not. A new coil wired into the same dead junction will not fire either.

What the meter said on this bike

ProbesReadingMeaningShop
One black/white feed to groundAbout 0 voltsThat coil is not poweredView One black/white feed to ground (affiliate)
Black/white to the other black/whiteAbout 0.6 voltsThe two feeds are tied together and still not fedView Black/white to the other black/white (affiliate)
Kill switch off RunZero, and the engine cannot runPut it back on Run before you condemn a wire—

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The right-bar harness is where those black/white wires actually travel. On this bike they drop down toward the carbs, which is easy to miss if you only look at the coil bracket. A box with a red lead and a green lead, both coil feeds landed on it, is not the picture in a stock diagram. Red and green are colors a rectifier uses. It is reasonable to suspect that box is the rectifier and that the coil wires were moved onto it. It is not proven. The notes never record unplugging it and trying again. The bike did later run on the kickstarter, after the coils and the fuses had also been worked, so this page will not name the box as the part that fixed it.

If the black/white wire has battery voltage to ground and the plug is still dead, the feed is innocent and the coil or the condenser is next. The coil that was pushing white crust out of its case is the coil guide. The condenser that would not plug in is 30250-369-004. Lights that died entirely after the coils were disturbed were a fuse, and they came back before this voltage reading meant anything.

Frequently asked questions

Where should the black/white wires at a CB500T's coils come from?
From the kill switch on the right bar, which drops down toward the carbs on this bike, not from a box that only has a red wire and a green wire. Both coil feeds being tied to that box is a place to stop and measure, not a place to assume the factory left them.
What does 0 volts on the black/white wire mean?
With the key on, the lamps lit, and the kill switch in Run, that wire should be the coil's supply. About 0 volts to ground means the coil is not being fed. On this bike the reading between the two black/white wires was about 0.6 volts, which is the same story. The points can still be capable of sparking from whatever energy is left. The plug wire will not.
Is the red-and-green box the rectifier?
It might be, and this bike's notes do not prove it. Red and green are the colors a rectifier uses, and a rectifier is not the coil supply. Call it a rectifier only after the wiring diagram says so. The bike ran later on the kickstarter, and the step of unplugging that box was not recorded, so the box is a suspect, not a closed case.

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