Another alt light question

drola · 01-18-2015, 08:20 AM

I'm helping a buddy with an alternator issue and after some searching i have not found an answer to my dilema.
Car is a '79 sc 3.0 with external regulator which went bad and car was charging at 18 volts. Fortunately it was discovered in time to not do much damage. I replaced the regulator. But then there was no charge coming out of alternator at all. Once i removed the alt i saw that the diode packs were toast. Needless to say the alternator needed to be replaced.
I bought a rebuilt bosch unit. Installed it and now charges well at 13.9-14.1 volts so all is well there. However the alt light on dash stays on when engine is not running and key not in ignition. Once engine starts it turns off but as soon as engine turns off the light comes back on and stays on until battery is disconnected.
From my research i know this is a diode issue. Since the alternator was just redone and diode pack inside looks fine, is there another diode behind the dash that may have burned from the overcharge? I remember once i parted out an '85 coupe and there was another diode back there that went to the light but i can't find it on this car. The only other thing i can think of is a defective alternator.
Any help is appreciated.

wwest · wwest's Garage

Try the old VR just long enough to test...

There is a relay in the VR that should only activate with key on.....

New alt. internal VR?

steely · 01-18-2015, 09:33 AM

Quote:

Originally Posted by drola

From my research i know this is a diode issue. Since the alternator was just redone and diode pack inside looks fine, is there another diode behind the dash that may have burned from the overcharge? I remember once i parted out an '85 coupe and there was another diode back there that went to the light but i can't find it on this car. The only other thing i can think of is a defective alternator.
Any help is appreciated.

Just a note - I don't believe diodes were used in the instrumentation panel (if at all) for the indicator ckt pre-84.

drola · 01-18-2015, 12:22 PM

Tried the old and two new vr's with same result. New alt is same as old one without internal vr.

clarkd · 01-18-2015, 01:17 PM

I'll throw this out as a possibility.

At least one of the diodes in your trio connected to the battery has failed in a "closed" position.

This should not affect charging, but will let current flow into the alt and back out the field diodes when the battery is connected and the car is not running with the alt charging.

steely · 01-18-2015, 03:43 PM

I am wondering if the sw is ok.
There shouldn't be +12 on the light in the first place if I am not mistaken (that is if the key is out and the light is on as you say).

juanbenae · juanbenae's Garage

ive thinned out a 78's gauge cluster extensively with expectations the 79 would be the same no diode behind the gauge in the trunk area.

patkeefe · 01-18-2015, 05:54 PM

There is a good discussion of the alternator light here...Alternator D- ????

Plus, I think there are a few Early_S_Man postings on the subject

rick-l · 01-19-2015, 05:52 AM

Quote:

Originally Posted by drola

The only other thing i can think of is a defective alternator.

Sounds like one of the power diodes that go to the battery have shorted. Unfortunately you have to take it out of the car to check.

Does the alternator come with a warranty?

Targalid · 01-19-2015, 09:26 AM

You should be able to test this easily. The battery should be drained through the alternator as discussed above and this should be detectable with a meter at the battery with the ignition switch off. I had this happen recently. Two legs of three in the alternator were charging and one was discharging simultaneously. The car would charge while running. The voltage was low but above 13 volts so the battery was holding. However once the car was shut down, the battery drained fairly quickly, within two days or so. You should be able to see the discharge with a meter at the battery if one of the alternator legs allows current to flow backward and drain the battery.

drola · 01-19-2015, 07:00 PM

Quote:

Originally Posted by rick-l

Sounds like one of the power diodes that go to the battery have shorted. Unfortunately you have to take it out of the car to check.

Does the alternator come with a warranty?

Are the power diodes in the back of the alternator or elsewhere? Yes, it has warrantee so my worst case scenario is removing the alt and taking it back

drola · 01-19-2015, 07:02 PM

Quote:

Originally Posted by Targalid

You should be able to test this easily. The battery should be drained through the alternator as discussed above and this should be detectable with a meter at the battery with the ignition switch off. I had this happen recently. Two legs of three in the alternator were charging and one was discharging simultaneously. The car would charge while running. The voltage was low but above 13 volts so the battery was holding. However once the car was shut down, the battery drained fairly quickly, within two days or so. You should be able to see the discharge with a meter at the battery if one of the alternator legs allows current to flow backward and drain the battery.

Is there a specific way to check for battery drain or just check it periodically and see if voltage drops over time, say a few days or something like that?

rick-l · 01-19-2015, 07:52 PM

This is the only way I could think of where the alt light would be on key off engine off but I don't know how to test it without removing the alternator (and then the diodes.).

manbridge 74 · 01-19-2015, 08:01 PM

Poor man's way is to watch for a spark at the batt when you lightly touch the negative cable to neg batt post. All doors and hood light, anything left on when key is off must not be pulling current. Any bit of spark is generally to much.

If one has a meter... well, youtube battery drain check. I'm on a flippin iPhone here.

ivanuf · 08:45 AM

Was there a final fix or finding on this topic? I have the same problem. After I shut the engine off the Oil and Alternator light on the dash light up. I went to the engine bay
and the blue wire from the alternator reads 1V after the engine is off and 14.2V when the engine is running. Also why both lights stay on after engine is off? Do they share a
common cable? What is the relation that will make both of them stay on after the engine is off?

rick-l · 05-12-2016, 09:50 AM

Do you read the 1V with the wire disconnected (T14?) and which side of the connection reads 1 V, the alternator side or the instrument panel side?

If the alternator were back feeding the alt light (K2) it could light up the oil light (K3) and power various other instruments.

not sure this is the right diagram.