Gen indicator light problem

[JohnD1956]

The generator indicator light on my 69 Electra is not lighting as it's supposed to, when the ignition is on and the engine is not started. I tried several bulbs including one known to be working so it's definitely not the bulb.

Coincidentally the fuel guage is malfunctioning too, and both of these circuits seem to share at least one wire in the shop manual.

The manual says that one possible cause is an open diode. I just don't understandwhere there may be a diode in this except in the voltage regulator or in the alternator. The system seems to be charging like it should, as the battery is holding a charge and the cranking is the same as it's been for over a year now.

There is a rebuilt starter in this for at least a year, and I believe the gen light worked at least up to this past week. But I can't swear to it.

Can anyone help explain this diode thing?

Thank you

[Guest brh]

I am no electrician but from what I understand a diode is like a check valve, for electricity it will only allow it to move in one direction. Regulators are no big deal to change but I don't think the problem is there. Again, could be wrong but what the heck would a diode have to do with the gas gage not working. You may have a ground issue with this circuit.

[JohnD1956]

I'm going to check that gas guage ground today, but the guage is not displaying the typical ground issue. What I've seen of a gas guage ground fault is that the guage will read full all the time( or is it empty? I can't remember) but my guage is working partially, in a very narrow band between a half tank and a bit over the 3/4 mark. For the longest time I thought it had a half tank, and since the car was rarely moved I did not suspect anything. Then I put a large quanity of gas in and the guage did not move. So I took out a loan and filled the tank and the guage moved to just over the 3/4 full position. So it seems to be working to some degree, just not all the way.

Meanwhile judging by the wiring diagram it seems the fuel guage shares a power source with the gen indicator light, which makes sense since the guage does not make any reading till the key is on.

ut oh, I'm sensing ignition switch failure here...something I never salvaged from the several cars I stripped.

[Guest brh]

Don't know about you but I hate electrical problems they can be a time consuming pain to find and its always something small and simple. Once drove a Pontiac for around 25,000 miles with an alternator light that glowed all the time. Never found the problem, charging system worked fine. Good luck.

[John Hopkins]

The diode is in the alternator. Generators produce DC voltage, like in a battery, alternators produce AC voltage, like your electrical outlet in the house. AC voltage looks like a wave when viewed by an oscilloscope, diodes stop reverse voltage, so 1 diode installed is half wave rectified, 2 diodes is full wave rectified. If 1 diodes opens up, you will still be able to keep your battery charged, but you are using only half the output from your alternator. Change or overhaul your alternator. Generators need voltage applied before they will produce voltage, alternators do not, which is why they are used. If you have a flat battery, and you push start your car, a generator will not recharge the battery, but an alternator will. Hope this helps.

[JohnD1956]

After reading the manual(quite a few times) I finally found that it was the voltage regulator. Pulled one out of stock (salvaged from another 69 Buick 12 years ago) and it solved the problem.

As it turns out a simple volt meter helped with diagnosis. With the battery disconnected I could see that there was a 7 volt draw with everything off. That disappeared when I pulled the harness on the voltage regulator. However, the book indicated that a defective diode could cause that draw so I first put on an old alternator and found I still had the 7 volt draw, and then I put on the voltage regulator and it stopped.

Things are looking up!

[Guest simplyconnected]

John, instrument panel diodes are used for two purposes:

To protect the gauge movement by shunting reverse voltage (so it doesn't peg in reverse).

To drop voltage. Your gauges may be using 6-volts, dropped from 12. It's a safety carry-over from the old days.

[Guest brh]

Glad to hear your problem is solved. Had my fair share of mystery draws, glad ya found it so quick.

[old-tank]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> Generators need voltage applied before they will produce voltage, alternators do not, which is why they are used. If you have a flat battery, and you push start your car, a generator will not recharge the battery, but an alternator will.</div></div>

Not that is matters for this discussion but you have the generator/alternator function theory reversed. A generator equipped car will run and can be push started (if fast enought) with the battery disconnected.

Willie

[John Hopkins]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: old-tank</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body"> Generators need voltage applied before they will produce voltage, alternators do not, which is why they are used. If you have a flat battery, and you push start your car, a generator will not recharge the battery, but an alternator will.</div></div>

Not that is matters for this discussion but you have the generator/alternator function theory reversed. A generator equipped car will run and can be push started (if fast enought) with the battery disconnected.</div></div>

I haven't owned a car with a generator, but I remember being told that generators are not self-exciting, alternators are. Is this information wrong?

[Guest simplyconnected]

John, this info is partially right. Residual magnetism - it's what is supposed to be hanging around in the iron, after a generator is shut off. With just a small amount of residual magnetism, the armature passes through, producing a very little voltage, then a little more, and a little more, until everything works normally.

A regulator opens and closes connection to the field, making the armature stop power output, then resume power output, hence, regulation. The regulation is done at the field. If there is no field-produced magnetism, all the rpm's in the world will produce NO power out of the armature. That's why, when we disconnect the battery or regulator, SOMETIMES the generator needs to be flashed; it's kind of a pulse to create magnetism for the armature to pass through.

Portable 115-volt generators sometimes lose their residual magnetism and need to be re-flashed. To do this, just plug in a drill motor, pull the trigger, and spin the chuck with your hand. The drill will immediately start spinning at full speed, indicating generator output is reinstated.

POINT--- All this could be avoided by putting a small permanent magnet in the generator.

How come starters don't need to be flashed? They have a SERIES FIELD, meaning, all the power going through the armature goes through the field as well. Generators have SHUNT FIELD coils (electrically separated from the armature, so it can be shut off).

[old-tank]

Flashing may establish/re-establish magnetism, but the main purpose is to establish polarity in the voltage regulator, ie negative=ground, positive="hot".

Examples: I once drove a 55 half way across the country with a broken battery cable. To start I held the stub of cable on the battery terminal and actuated the carb linkage (thanks vacuum start mr Buick) to start then left it on the fast idle cam until I got into the car to drive. As long as I kept the ammeter charging it ran. Driving at night was tricky and a few times the engine died when the charge dropped (repeat starting procedure). The only other issue was the headlights flickered in unison with the ammeter. Why not buy a new cable? Money enough only for gas and no tools and besides I was still moving...

Another example: I once rigged and old generator to an old reel mover engine and used it to power a couple of sealed beams for night fishing. No battery or regulator was used. The field was grounded and the engine speed regulated the output.

Now, disconnect the battery on a running engine with an alternator and see what happens.

Sorry for the thread hijack...

Willie