Battery Charge Indicator Light

[Foster48x]

Does anyone know what might keep the Battery Charge Indicator Light (the small one on the speedo) on while the ignition is off on a 1980 FI. The light goes out after the car starts but whatever it is its causing the battery to drain while the ignition is off.

I did a recharge on the battery and it tested at 12.84v. The car doesnt have any problems starting. Not sure if it makes a difference but the center and radio consoles are out of the car with the heater blower cables unhooked.

Any ideas?

[mdrburchette]

That's weird. What year car do you have? On the early model cars, the exciter wire went to a battery charge relay. I guess you could check to see if there is a reading with the voltmeter at the wire coming from the alternator to the relay and from the relay to the light and come up with what is causing it.
If it's a later model with the internal regulator, you can check the exciter wire at the alternator.

[rlux4]

That's what I was thinking Denise. My guess is battery voltage is going through a closed contact in the alternator that should be open when the key is off. If that's the case, the light is reacting to partial battery voltage, instead of alternator output.
Ron

[baltobernie]

What about a faulty ignition switch? Pull the smallest wire from the alternator (the excitor wire) and see if the light goes out.

Any other circuits drawing power with the key Off? Can you see 12V at the coil, etc.

[mbouse]

we're talking an '80 Spider here, so i don't think it is a relay issue. any chance the exciter wire is catching a short somewhere? i'm suspecting a failing regulator. like Bernie says... pull the exciter wire from the alternator when the light is glowing and see what happens (engine and ignition off).

[So Cal Mark]

a bad diode in the alternator can cause that. The alt will still charge, but at reduced output

[Foster48x]

You all hit the nail on the head. I pulled the exciter wire off of the alternator and the indicator light went off.

So.... is my alternator toast or about to be toast?

[So Cal Mark]

sounds like it's time to get the alternator rebuilt

[Foster48x]

Mark would a going bad diode show up on a bench test at a shop?

[So Cal Mark]

yes, it will also show up when testing on the car. There is a diode trio, as each diode fails output will be cut by a third. AND, current can flow thru the diode the opposite way as you've discovered, keeping the light on and discharging the battery

[Foster48x]

Thanks Mark. I'll look into having it repaired or replaced.

[baltobernie]

Hey Foster, would you satisfy my curiosity and see if you have 12V present at the (disconnected) exciter wire, with the ignition Off?

[mbouse]

yeah, i thought we asked for this test earlier... but we'd surely love to hear the answer now before you have the alternator looked at by a mechanic.

[Foster48x]

New Alternator = No Battery Light when the ingnition is off. Thanks for the help.

baltobernie and mbouse, sorry I missed your posts. I purchased the new alternator this afternoon and wasn't able to test the old one.

Rick

[baltobernie]

All's well that ends well; Mark is usually right.

Upon reflection, I can see how a bad diode would allow voltage from the big wire to go backwards up the little wire, illuminating the charge lamp. Glad you got it fixed.