What’s this wire - series 3 1985 alternator

What’s the blue green wire thingy on the back of this original alternator? Do I need to transfer it to the new one or replace it?

I had the battery light on but still actually charging. Battery light sometimes glowing with ignition off.

That’s a capacitor, intended to minimize radio interference. Might as well move it over – or perhaps buy a new one.

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That means that there is a leak to ground through the alt, Martin. However, it may be that the capacitor, mentioned by Kirbert, is ‘leaking’ - remove and see what happens?

Frank
xj6 85 Sov Europe (UK/NZ)

It could be; elsewhere I’ve read it’s a bad diode that causes that light. Anyways I’d rather only have to put it back in once. There’s a capacitor that looks similar for old AC Delcos that seems readily available so I’m thinking I’ll pick up one of those to replace.

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That referring to the diodes inside the alternator, Martin - and is indeed an indication of defective diode(s). Which will also show up in a very high current in an external battery current leak test. Faulty diode(s) will feed the battery with alternating current - which won’t charge it…

In this case; the external capacitor, connected to ground as a radio noise suppressor, may leak - lighting the warning lamp. Disconnect for testing…

Frank
xj6 85 Sov Europe (UK/NZ)

Thanks for the help. I found an AC Delco one for $10 that seems fairly available and a good fit.

Got it in and all seems well - no more scary lights. I think the voltage had been running a bit low for a very long time. Never trouble starting but my windows definitely work better now! The volts gauge stays much closer to the 13.

Well… it worked fine for a couple days but now it’s back to…

Haven’t had a chance to properly investigate but:

When idling voltage gauge drops quite a bit (yes need to quantify with a real meter).

Driving/higher RPM the battery light comes on and voltage goes back up. Brightness of battery light is kinda proportional to RPM

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This may happen if battery voltage is very low, Martin - or battery connections are ‘imperfect’. The lamp will light up in response to voltage difference between the two lamp terminals. Ign ‘on’; 12V from battery - and ground through the alternator. Engine running; 12V from battery and 12V from alternator - no voltage difference, no light. Or lots of volts from alt; little from battery - lamp will glow. The actual charging from the alternator is best monitored with anything but the dash gauge…:slight_smile:

It’s all a bit odd - have you tried removing the installed capacitor? It may have failed thrugh overload…?

Frank
xj6 85 Sov Europe (UK/NZ)

I guess that capacitor is as good an explanation as any, I’ll see if I can try that. The connections all seemed like they should be ok, I cleaned it all pretty good and put lots of dielectric grease on everything.

At least I made it home without incident driving across Chicago at Friday rush hour, 95° Aircond trying it’s best with at least half hour of barely moving.

Makes sense about the voltage differential. Could it be caused by a wacky battery? It’s fairly new but it’s been allowed to flat a couple times which I know isn’t good for it.

The battery warning lamp is there for a reason, Marin - so if it is lit/glowing something is not quite right…:slight_smile:

As long as the battery keeps it’s charge and voltage, it is likely OK - but as a routine, you could check battery external current leak. Disconnect a battery clamp and measure current between the post and clamp - it should read either 20 or 35 mA. Higher currents implies something drains the battery…

Frank
xj6 85 Sov Europe (UK/NZ)