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)

Use the real meter to measure voltage.

Invest in a carbon pile load tester. this will reveal battery health. A good parts house may do that for you, in hopes of selling a battery!!! Most will be hest in the accessment. the battery must be fully charged…

Well, after sitting a couple days the battery went flat and wouldn’t take a charge. New battery today. Curiosly the [Battery light on with ignition off] came back when I connected it up, though that went away after I started and ran it for a while. Voltage seems ok, drops slightly with engine off; raises slightly with revs. Battery light is pretty steady now (though it flickers a bit)

Could it be as simple as a bad connection of the sense wire? Again I was pretty confident in the connection when I wired it up.

Don’t some alternators excite through the bulb? That would mean yes it can be that simple :slightly_smiling_face:
Belt under tension and all connections tight it shouldn’t flicker at any time unless the idle is very low, then mine would not turn off until I gunned the engine.

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Fair enough, Martin, but if your battery discharged quickly; do an external battery current leak test. Disconnect a battery clamp and measure current between the clamp and the battery post - it should show 25 or 35 mA with ign ‘off’ and doors etc closed. If the current is higher; something is drawing that current - and the battery will continue to discharge…

Needless to say; the batt warning should be unlit with ign ‘off’, lit with ign ‘on’ and unlit with engine running - anything else is plain wrong. The bulb is grounded in through the alternator (black/brown wire) - and powered from ign key (white wire)…

Lit with engine running; the *black/brown * is grounded detached from alt - or wrongly connected to it (or the alt is not charging). Should be connected to FLD or IND - or some marking specific to the alternator fitted. Lit with ign ‘off’; wrong connection at ign key - or a short…

Frank
xj6 85 Sov Europe (UK/NZ)

That’s a pretty fair description of how mine works too. I don’t know about ‘exciting through the bulb’ but if my idle drops too low I have to rev the engine to get it to charge again.

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In principle; alternators are ‘self-exciting’, Robert - due to remanent magnetism within…

The exciting through the warning bulb is likely to just speed up starting the charging, but the current is limited by the resistance in the bulb. The alt would not stop charging, or needing exciting, as idle drops, but if the alt deliver less current than being used; charging voltage will drop - and increasing rpms will then increase alt output…

It’s all about voltage differences between battery and alt - if the difference is large enough, the bulb will glow/light up whichever’s voltage is ‘guilty’.

Voltages must be monitored by a proper multimeter, the dash gauge is not precise enough - and too slow reacting…

Frank
xj6 85 Sov Europe (UK/NZ)

I certainly was not aware of the above at all! I had always assumed that if the light was on, it was the alternator at fault.

Bob

The purpose of the lamp is of course to indicate if the alt fails to charge, Bob, but there are other faults that may also light the lamp - or indeed keep it unlit…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)

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OK, finally got a chance to do some checking…

New Alternator (Autozone reman, even still has BL logo)
New Battery (Die Hard Gold)

worked OK for a couple days then:

Voltage at Battery + varies slightly with RPMs, so I guess that means it’s charging OK.

When disconnected, Brown/Black wire shows slightly less than battery voltage over ground, lights BATTERY light when grounded

Tab where brown/black wire goes on ALT shows approximately 0v over ground whether running or not.

Battery light goes out when Brown/Black wire is disconnected.

Can it be anything but a defective alt? I knew $100 was too good to be true!

Can’t think of anything wrong with the rest of the car that could have caused it.

Edit: also tried disconnecting the little external capacitor; no change.

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In principle; the brown/black tab should indeed show ground with the engine standing - it is what lights the warning lamp, by battery voltage. With the engine running, the tab should show alternator voltage - which counters the battery voltage at the lamp, extinguishing it…

Either you are using the wrong tab, or the tab is, wrongly, not giving alt voltage. Back to the battery; with the engine running; voltage between the two battery posts should be at least 1 - 2V higher than engine stationary - and indeed markedly rise as rpms rise from idle. ‘Slightly’ is not really convincing…

However, battery state will affect alt voltage; a well charged battery should show some 12,8V - and at some 2000 rpms should then show some 14V, give or take, between battery posts. It is very important that a multimeter is used for voltage checking - the dash gauge is fairly useless for such testing purposes…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)