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CB500 Alternator

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masonmart:
Having finished my Dresda CB500 and registered it I've at last had chance to run it. It ran like a dream but the battery flattened so no charge. It had been left outside for 40+years so my guess it is the alternator but not my area of expertise. A quick look around seems to show that they are rare or, if NOS, very expensive. Can anybody advise on possible solutions like aftermarket parts, rewinding or anything else I may be able to look at?

Many thanks.

robvangulik:
I would first recharge that battery and then measure what the voltage does with the bike running, before ordering possibly unnecessary parts. Alternators rarely go wrong, other than by throwing them on the street with force.
Measure first, buy later!

Lobo:
Hi Masonmart,

Don’t be too quick to bin your alternator; there’s no moving parts in it, and given a multitude of other components in the charging system the culprit likely lies elsewhere.

You can however, easily check it, accepting that if you hate electrics you may need some help... (and definitely a multimeter - which can be easily got for £10 upwards)

Before you junk it then, check the continuity of the various wires, as basically it’s made up of windings ... which are either broke, shorted, or likely fine.

1st up disconnect the alternator loom plug; which’ll have a GREEN, WHITE (= Field Coil), 3x YELLOWS (= Stator Coils) and possibly a couple of others ... which may be Oil Px & Neutral feeds (uncertain of the 500). Poke your multimeter probes into the various connectors on the Alternator side of the disconnected  plug...

The STATOR winding has the 3 YELLOW Wires. Measure the resistance (Ohms scale ♎️) between each of the Yellow wires. (No need to worry about which way ‘round the Red/ Black leads go). You should see a reading of 0.61-0.69 ohms for each measurement.

The FIELD COIL has two leads... the GREEN and a WHITE. (Again, same Ohms scale, and no need to worry about Red/Black leads), measure the resistance - should be 4.6 to 5 ohms.

It goes without saying that these bikes and their looms are rather old, and all bullet connectors must be clean & tight. You really need to check the connectors throughout the charging system... ie include the regulator, rectifier, fuse holder blah blah.

NOTE ‘500 owners! - help required with quoted resistances as I’ve used the 400F numbers which I assume to be the same. Please chip in if different....

Good luck,
Simon

(Alternator whole shown in Red box, Stator windings in Green box, and field coil in a Blue box.

Bryanj:
I have never seen a bad field coil and only stator coils physicaly damaged in accident.
95% of problems are caused by poor or high resistance connections and/or switches, 4% by broken wires and the othe 1% rectifier or regulator.
I did do a testing procedure which was in tricks and tips

AshimotoK0:
Main problem I have seen on 400 &500/550 stators (that run dry and  not in oil like Honda twins) is a breakdown of the insulating coating of the copper magnet wire probably due to corrosion of the stator steel laminations. The quick test for this is , with the three yellow output wires disconnected from the sub-loom there should infinite resistance (i.e open circuit) to the mag. frame steel  laminations. If there is any lowish resistance (say less than 100k Ohms), then the stator is usually toast. I had this on two 400/4 stators on the four 400/4's I owned 'back in the day' and when the bikes were less than 10 years old. They are very  complicated to rewind so a good used one is probably the cheapest repair option.

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