73 Calais - "GEN" light flashing

[Nashville Native]

While driving my '73 Calais sedan to work this morning, the "GEN" light started flashing. And then it went off. And then it started again. And went off again. It did that all the way to work. I won't have a chance to look at it until my lunch break, but aside from the belt and connections on the alternator, what else should I check?

[Scot Minesinger]

If alternator, belt, battery, and cables all check out good, then it seems like an intermittent short.

This happened to me on a 72 Eldorado and the "trans" fuse was gone along with other shorts.  One of the cures was replacing the trans wire leaving firewall because I could not find where the wire insulation was compromised.  Another fix was taping up connections at starter better.  These can be a pain to find, sorry you have to start your new year with this challenge.

[TJ Hopland]

Start with a good look at connections and other basics.  You may find something that looks questionable and it may be an easy fix.   

The light has one side connected to the side of the fuse box that comes on with the key.  I don't remember 73 specifically but its usually a fuse labeled gauge or inst.   The other side of the bulb goes to one of the small wires on the alternator.   Its the wire closest to the output terminal.   The farther from the output terminal is usually just a short wire that goes to the output terminal.    When the alternator is not running this terminal is basically a ground so the light lights.   The alternator senses the positive voltage coming through the lamp and that is the signal to turn on.  Once its turning and it 'sees' that voltage it starts outputting voltage and that terminal basically outputs the same voltage as the rest of the system so the light is seeing a + on both sides so no light.

So the somewhat normal circumstance you would see the light is the alternator output is less than the system voltage.   Common reasons would be belt slipping or broken or the diodes have failed internally making the output weak.  Maybe at idle after starting would not be strong enough to charge or under heavy loads like heater blower or trying to recover the battery from starting it would have issues keeping up.   

If the light happened to burn out or the gauge fuse blew the terminal on the alternator will not ever 'see' the + signal and won't turn on.   If the fuse happened to blow while it was running the alternator would back feed other things on the circuit which would make the light glow and perhaps other gauges would be reading strangely.    A common problem in the old days when people did alternator swaps was they would just tie that wire to the coil.   Problem there is when you turned the key off it would still feed the coil so basically engine would not shut off.     

[dplotkin]

While there is some truth to what you each have said, it doesn't need to be or sound that complicated.

The light illuminates when there is no output.

If this is the internally regulated alternator (I think the 1st year) it is likely worn brushes/slip rings. If so, very soon the light will remain on. Back in the day the tech would replace the brushes, bearings, clean up the slip rings and replace the voltage regulator which may have included the brushes, I forget, and down the road you went. It could be one or more diodes. I doubt it.

Look up the auto-electric guy in your market. Take it to him. He can fix it in 30 minutes or less. You can also buy rebuilt units and off-shore clones. Rebuilding the one you have is always the best option, you know what you have and what you will get.

Dan

["Cadillac Kid" Greg Surfas 15364]

Dan has got it I believe.  Over the years I have had the flashing "Generator" light syndrome and after checking all the belts, wiring throughout the system to no avail, I (the first time) just waited until the light was on steadily and replaced the diode trio,, the regulator and the brushes. Good for (it has been now) 5+ years.  The second time that happened I just pulled the alternator, checked the brushes and diode trio (they checked out okay) and replaced the voltage regulator. Problem solved.  Since the voltage regulator is difficult to check by the novice it often does not get replaced during "re-builds" and often leads to warantee returns.
Greg Surfas

[Nashville Native]

Thanks everyone.

On my lunch break, I went out to the garage with a multi-meter and checked. At idle, the reading was 14.20 volts at the battery. The light was not flashing, just showing the faint glow it has had since I bought the car (almost not visible while driving on a sunny day, but visible at night or in the garage). Drove it home tonight and the light never flashed, just had the familiar faint glow.

There is a shop down the street from the museum, and they do all of our alternator, generators, starters, etc. They did both the alternator and starter on my Corvair last year. I'll pull the alternator later this week (antique tags on the Caddy, so I have to drive my "normal" car tomorrow) and have them check it out. If they find nothing, and/or the problem comes back, I'll start going through the wiring and connections and post an update.

Again, thanks to everyone.

[jwjohnson86]

If you have an Advance Auto Parts store nearby take the removed alternator in there and have them bench test it.  This confirmed that my alternator was dead and I just purchased a new one, which immediately solved the illuminated "GEN" light problem I had (it came on and remained on until I could deal with the issue).  Had the correct unit for the 1970 in stock that day, installation and reconnection solved the problem and all the lighting was significantly brighter too, everything was no longer being powered solely by a drain on the battery.  I did not touch the voltage regulator at this time. 

[TJ Hopland]

I just remembered I had similar issues with a similar era Cad.   It may not be your issue but may be worth checking. 

A 73 has a connector in the starter harness.    Its about a foot away from the starter so its in a place where it can get crap from the road and or tangle with hot exhaust.   At some point they got rid of this connector so I'm guessing it was a possible problem area.  Among other things its got the main power feed wires in it so its a good place to check when having electrical oddities.  One of the ground wires is in the same area so check that while you are looking down there.

And here I go again with 'sounding complicated'.  Its a 6 pin connector with 5 wires in it.  It does not have any sort of seals in it.  You don't need special tools to get the terminals out, just a small flat screwdriver to jam in and hit the release tab.   With the terminals out of the body you can inspect and clean the terminals.    3 of the wires are the fusible link feeds from the big battery terminal.  One is to the coil and the last one is the one that engages the starter.    The 3 fusible link wires all start on one large ring terminal at the starter.  One of these feeds the headlight switch, one feeds the half of the fuse box that is always on, and the last one feeds the rest of the fuse box via the key for the stuff that comes on with the key.   

One time I had issues there it looked like one of the pins was just loose and had been getting hot.  It had not yet damaged the connector or wire so I was able to replace that one pair of terminals.   The other one was a headlight issues.  On that one the connector had melted into the exhaust, shorted, and blown the fusible link for the headlights.   It was melted pretty bad so the others were not far from failing.  On that one I ended up eliminating the connector. 

So a possible way this could be the issue is like I explained how the light works is that normally its seeing a + from the key and fuse box side and its also seeing a + from the alternator terminal.   If both are equal (and they should be) the light does not light.  The normal fault would be that the alternator terminal is lower than the fuse box side so the light lights but the reverse could also be true, the fuse box side could be lower due to a connection issue at that connector I mentioned so that could cause the light to light too. 

[Nashville Native]

And here I go again with 'sounding complicated'.

Hahaha! Don't worry, I followed you just fine. Thanks for the tip, I'll check that as well.

[bcroe]

Thanks everyone.

On my lunch break, I went out to the garage with a multi-meter and checked. At idle, the reading was 14.20 volts at the battery. The light was not flashing, just showing the faint glow it has had since I bought the car (almost not visible while driving on a sunny day, but visible at night or in the garage). Drove it home tonight and the light never flashed, just had the familiar faint glow.

There is a shop down the street from the museum, and they do all of our alternator, generators, starters, etc. They did both the alternator and starter on my Corvair last year. I'll pull the alternator later this week (antique tags on the Caddy, so I have to drive my "normal" car tomorrow) and have them check it out. If they find nothing, and/or the problem comes back, I'll start going through the wiring and connections and post an update. 

14.2V means the alternator is charging.  The rules are quite different
for external regulator and internal that I will discuss.  Above advice is
good, your "familiar faint glow" is a marginal wiring situation which
can be cured.  It happens because voltage is highest at the alternator,
and somewhat lower in other wiring such where the other end of the
bulb connects.  The quick fix is put a 1A diode in series with the bulb
wire, since the starting and running currents are in opposite directions. 
Diode arrow points to the alt; if the bulb doesn't light at key on, you
have it reversed.  The real fix is check voltage drops along wires in
the distribution system, and reduce the worst ones with cleaned
connections and/or bigger wire.  good luck, Bruce Roe

[Nashville Native]

So a funny thing happened on the way to the storage building today (had to shuffle some cars at the house and the Caddy takes up a lot of room, so...). The GEN light didn't flash on the way over, UNTIL I turned on the turn signal. It flashed opposite the turn signal indicator light on the dash. When the signal canceled, the GEN light went out. Thinking back to Sunday morning, I seem to remember that the GEN light started flashing after I used the turn signal for a lane change, then shut off after another lane change.

Curiouser and curiouser...

[bcroe]

The GEN light didn't flash on the way over, UNTIL I turned on the turn signal. It flashed opposite the turn signal indicator light on the dash. When the signal canceled, the GEN light went out. Thinking back to Sunday morning, I seem to remember that the GEN light started flashing after I used the turn signal for a lane change, then shut off after another lane change.   

If you have a blown main rectifier diode, the alt may still keep up
under light loads.  Draw more, and there is a pulsing drop in voltage
each cycle the bad diode doesn't take its turn.  I'm now inclined to
think its a diode instead of brushes.  A scope will show the waveform,
or remove the diodes & check each with a meter.  Bruce Roe

[dplotkin]

If you have a blown main rectifier diode, the alt may still keep up
under light loads.  Draw more, and there is a pulsing drop in voltage
each cycle the bad diode doesn't take its turn.  I'm now inclined to
think its a diode instead of brushes.  A scope will show the waveform,
or remove the diodes & check each with a meter.  Bruce Roe

I agree with you. But a simpler way to test this is to set your DVM to AC volts and measure across the battery, engine running. You should see fractional or no AC voltage. Anything more indicates one or more diodes failing. As suggested earlier-have a shop rebuild it - replace brushes, all the diodes, the regulator - and be done with it. It was common in the day for these to crap out, why should it be any different today?

Dan

[bcroe]

I agree with you. But a simpler way to test this is to set your DVM to AC volts and measure across the battery, engine running. You should see fractional or no AC voltage. Anything more indicates one or more diodes failing. As suggested earlier-have a shop rebuild it - replace brushes, all the diodes, the regulator - and be done with it. It was common in the day for these to crap out, why should it be any different today? Dan 

The AC meter should work, if that is what is available.  Some load (lights)
ought to bring it up.  If you can't fix it yourself, a shop rebuild might be
needed.  Here I have tried to identify the fault and replace only that.  Saves
a lot of cost, and I tend to trust the original parts a lot more than "new"
stuff from unaccountable sources.  I could tell about "new" torque
converters or ball joints or engines or diodes or a lot of other stuff soon
failing in my decades of car maintenance.  Not to mention badly done
repairs, like this rebuilt 76 HEI for EFI that will make the engine idle
badly because the injection triggers are reversed.  And I got tired of
seeing shops throwing away gobs of good parts (that I paid for).  I DO
replace maintenance parts (alternator front bearing, brushes) on a
regular schedule, hopefully before they fail on the road.  Bruce Roe

[Glen]

If the GEN light was turned on under load, wouldn’t it be turned on with the brake lights or the headlights also? 
Another possibility is a wiring problem. 
Question: does the turn signal indicator go on with the outside lights or opposite of the outside lights?   

[TJ Hopland]

Headlights have their own feed wire from the battery cable at the starter so they should have minimal impact other than to bring the whole system down a bit.    Stuff that would be most likely to just effect the light would be things that come on with the key.   Biggest load I can think of there would be windows and the heater blower.

Might not be a bad idea to disconnect the battery and carefully inspect and clean all the contacts and fuses in the fuse box.  Maybe you got a little corrosion down there?