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Is it an Ignition Light or an Alternator Light?

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Cuttyblue Avatar
Cuttyblue Travis S
Corpus Christi, TX, USA   USA
Team MGEx,

First, thanks to all concerned for ideas on storing (or not storing, have Grandma drive it!) my B for a year while I work overseas - I need to PM a few more folks on that.

Anyway, I'm still dealing with a persistent rapidly flashing or glowing Ignition light. At least, that's what the owner's manual calls it... searching back through topics on this site, folks are calling it the Alternator light.

- After start, red light comes on steady as normal and properly extinguishes revving up a little past 1100rpm.
-... But then, I will often (not always, sometimes a steady light-free idle) notice the faintest flicker of the red light which almost always coincides with the tach jittering. Very mild effect at idle.
- Passing 2nd to 3rd gear and in 4th, red light begins to rapidly flash/flicker brightly... accelerating through 2nd/3rd, a slight hesitation in acceleration is heard and felt.
- Cruising at 55mph in 4th, sometimes a light glow, sometimes a rapid flicker... but synchronized with a rapid and violent fluctuation in the tach needle (although, if at steady cruising speed, no engine hesitation at all, steady engine).

Twice, repair shops said 1. "Oh we cleaned out your carbs and the light went out" - wrong. 2. "We tightened some loose wires out of your alternator and the light went out" - Wrong!
Should I be telling these guys "It's an alternator light! Rebuild my diodes/rectifiers/brushes!" Forum says I can do this myself as well if I can have my buddy help me solder...



First Time, Novice MGB owner ('72 BRG)

Issues that the MGExp community has helped me out with:
- Sun visors
- Under-dash trim panel adjustment
- Oil filter replacement problems

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tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
We call it an ignition light, but it is really a warning if the ignition is on and the alternator is not charging. Why don't we call it a charge warning? Don't know. It does bother/scare me how little most mechanics know about electrical and worse, how they assume we are so stupid to think a carb adjustment can solve a charging issue. Well, OK, if the idle was so low, and when it started it was never rev'd past 2K or so, the field would maybe not kick off. Maybe. Once. Find a new shop.

Fluctuation of the tach means you are losing electrical power somewhere. Bad connection. Of course you will loose power. If the connection from battery to ignition was stil good, you would not see the drop, so that implies it is between the charging/battery and the rest of the car. Do you have the schematics from Advance Wire? Critical! 72, I'll look at mine and see whare the bad connection may be. Don't forget the engine ground strap.

Could be a lot of things from belt slipping to bad contact somewhere in the circuit. Go through the wire diagram piece by piece. Don;t forget the contacts in the light socket it'self.



Cogito ergo sum periculoso

tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
Look at the fat brown wire connections on the ignition switch from , I guess, where they are all tied together on the starter. . Maybe a bad switch.



Cogito ergo sum periculoso

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riley1489 Avatar
riley1489 Gold Member Bruce H
Great White North, QC, Canada   CAN
1953 Jaguar XK120
1959 Riley 1.5 "King George"
1973 MG MGB
In reply to # 3717755 by Cuttyblue Twice, repair shops said 1. "Oh we cleaned out your carbs and the light went out" - wrong. 2. "We tightened some loose wires out of your alternator and the light went out" - Wrong!
Should I be telling these guys "It's an alternator light! Rebuild my diodes/rectifiers/brushes!" Forum says I can do this myself as well if I can have my buddy help me solder...

If you need to take your car to a 'garagiste' and then must tell them what to do, then something is seriously wrong, and the "repair shops " you use aren't worth a whole lot to you!
So;
Simple solution, is to replace the offending alternator. Three simple fasteners and a fan belt tightening and 1 hour later you are good to go.
Next simple solution is to remove the existing alternator and have it rebuilt by an automotive electrical shop.
Next and most fun, is do the rectifier replacement your self.

B



Check your ego Amigo!

simenon Toni Kavcic
Kocevje, Slovenia   SVN
1966 MG MGB
Ignition light is nothing else as indicator of current direction. As longa as current is going from alternator (dynamo) to battery is light off. Current always flows from higher to lover potential. When RPMs drop and generator can’t generate voltage higher as is battery voltage so current starts to run from battery and feeds engine/lights/radio,… Typical battery voltage is from 11 to 13 volts, generator gives 14 to 16 V.
If your RPM counter (tacho) jumps at same time as ign.light goes on then you have problem with alternator not bad wire connection. Tacho basically counts spikes generated by ignition coil. Same spikes could be generated by other sources. I would suggest to start with brushes on alternator, then rectifier. But as Bruce suggested maybe would be easiest to source one used 55+ A alternator (https://www.mgexp.com/article/delco.html) and make quick check.
And light never lies so don’t expect that problem will go away – it will grow.

MGBGM1977 Avatar
MGBGM1977 Anthony Piper
Mossy Head, FL, USA   USA
I guess I am old. I have always called it a "generator light". When I started driving, many cars still had the "Gen" label on the dash.

ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
1971 MG MGB
The manual calls it an ignition lamp. To be precise, the lamp shows whether field current is being supplied from the battery instead of from the alternator's own output. If it is flickering or glowing even a tiny bit, the alternator is not working properly.

One common cause is corrosion in the crimp on the spade terminals inside the connector. They can be physically solid but electrically poor. If there is any doubt, disassemble the connector and inspect and solder/recrimp. After that, the symptom you describe is probably due to a failing diode in the rectifier pack. Easily replaced (but you have to solder three wires).

The rectifier diodes are difficult to test on the bench. An ohmmeter will often show that the diodes are fine - but the test is made at about 1v and a fraction of an amp. Often the diodes will misbehave when they get warm and are exposed to higher voltages and currents.

Occasionally you will see this condition if the brushes are near end of life and make intermittent contact. Also check the slip ring when you replace the brushes - they can wear right through.



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
If you have power from the battery through the ignition switch to the ignition, it won't drop out. Light may come on, but the tach should not flutter and the car should keep running. If it was just the alternator, again, you would run on battery until it slowly dies.

I believe it is a poor electrical connection or faulty ignition switch.



Cogito ergo sum periculoso

ingoldsb Avatar
ingoldsb Silver Member Terry Ingoldsby
Calgary, AB, Canada   CAN
1971 MG MGB
Quote: but the tach should not flutter and the car should keep running.

The tach is pretty sensitive to pulses (from any source). The fact that his tach is fluttering suggests that the alternator is gaining and losing activation. I agree with you that a poor connection is the most likely cause, although some of the other things I suggested are also possible.



Terry Ingoldsby
terry.ingoldsby@DCExperts.com

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Fred Winterburn Avatar
Ripley, ON, Canada   CAN
That's how I always referred to it, IE a generator light and that is how it was marked on many old cars. This subject came up a few months ago and I believe it was Terry Ingoldsby that corrected me when I stated that the term ignition light was a misnomer. The name 'ignition light' is historical. On my Morgan for example, there is no generator light as it has an ammeter instead. It has an ignition light which is on constantly once the ignition key is turned on, thus indicating power to the ignition system and the light stays on regardless of whether the generator is charging or not. The gen light or ignition light on an MGB is dual function in that it indicates that there is power to the ignition circuit when the key is first turned on, but then functions as a 'lack of charging' light when the engine is running. Fred .
In reply to # 3717829 by MGBGM1977 I guess I am old. I have always called it a "generator light". When I started driving, many cars still had the "Gen" label on the dash.



'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.



Edited 1 time(s). Last edit at 2018-04-16 06:47 PM by Fred Winterburn.

MGBGM1977 Avatar
MGBGM1977 Anthony Piper
Mossy Head, FL, USA   USA
1960 Chevrolet C10 instrument panel.


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78 B PaceCar Avatar
78 B PaceCar George Funderburke
Kennesaw, GA, USA   USA
Travis, I see a lot of good possibilities in your responses. I would probably go with the Alternator as the fault. Alternators can be intermittent and partially fail. If you checked the voltage when the light comes on, you will probably notice that the alternator isn't working. If the light stays on, the battery will eventually die and the car will stop running. The fluctuation in the tach is probably caused by the drop in voltage on the green wire to the tach. On a midget recently, I found a cold solder joint on the large brown connector to the alternator. The alternator would charge sometimes and other times it would turn the light on not charging. The movement of the engine caused the connector to make connections sometimes. In this case the alternator was okay. I recently had a problem on a 80 LE where you couldn't turn the car off without disconnecting the coil wire. You could unplug the ignition switch plug and the car would keep running. I checked voltage on white wire to the ignition relay with the ignition switch unplugged and discovered I had 8 volts to the relay causing it to remain energized. The white wire off the switch is tied to the alt warning light circuit at the inertia sw. You guessed it, unplug the alternator and the car would shut off like it should. In this case, bad alternator. The 72 B would have a little different wiring arrangement, but this take it to a bad warning light circuit in the alternator.

I would probably drive the car with a voltmeter temporarily connected to the battery. Watch for a voltage variation when the light comes on. Keep in mind, the white wire off the ignition switch feeds the third fuse, ignition coil, fuel pump, and overdrive circuit. This wire also feeds one side of the alt light for power. The other side of the light receives a ground from the alternator when it is not charging.

I hope this not to confusing. Good luck.

spikemichael Avatar
spikemichael Platinum Member Michael Caputo
Ocean Shores, WA, USA   USA
1973 MG MGB "Freebie"
1973 MG MGB
1974 MG MGB "Spike"
1979 MG MGB "MegaBeanie"    & more
There are five different wiring harnesses for '72.

Do you have the door buzzer?
Do you have side marker lights?
Do you have the seat belt buzzer?
Do you have the brake failure test switch and lamp?
Do you have an anti-run-on valve and switch?

Is your tach wire white with black stripe or just white?

If you have the white wire tach I would check the bullet connections where it connects the tach wire to the fuel pump wire and the ignition switch as well as the Alternator and the overdrive switch, it is a 5 or 6 way connection under the dash near the steering column. It just seems like a lot going on in one spot that could make your tach jumpy, your fuel pump sputter, your ignition light come on. The other thing might be the third fuse down on your fuse box, check connections on the front side where the whites go in (or come out depending on how you want to look at it... I won't judge).



Michael J. Caputo
'79 RBB and '73 CBB owner with extensive experience in 12v Audio System design and installation.
Vendor of Regalia and Promotional Products. Forum Member with a warped sense of humor.


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GMB3 Avatar
GMB3 Silver Member George Badger
Soquel, CA, USA   USA
1965 Jaguar E-Type Coupe "1QUICK E"
1968 MG MGB "1QUICK B"
It's supposed to come on when the ignition is turned on. Ergo "Ignition Light".

It's supposed to turn off when the charging system, gen or alt, is operational, ego Charge Indicator".

In reply to # 3717755 by Cuttyblue Team MGEx,

First, thanks to all concerned for ideas on storing (or not storing, have Grandma drive it!) my B for a year while I work overseas - I need to PM a few more folks on that.

Anyway, I'm still dealing with a persistent rapidly flashing or glowing Ignition light. At least, that's what the owner's manual calls it... searching back through topics on this site, folks are calling it the Alternator light.

- After start, red light comes on steady as normal and properly extinguishes revving up a little past 1100rpm.
-... But then, I will often (not always, sometimes a steady light-free idle) notice the faintest flicker of the red light which almost always coincides with the tach jittering. Very mild effect at idle.
- Passing 2nd to 3rd gear and in 4th, red light begins to rapidly flash/flicker brightly... accelerating through 2nd/3rd, a slight hesitation in acceleration is heard and felt.
- Cruising at 55mph in 4th, sometimes a light glow, sometimes a rapid flicker... but synchronized with a rapid and violent fluctuation in the tach needle (although, if at steady cruising speed, no engine hesitation at all, steady engine).

Twice, repair shops said 1. "Oh we cleaned out your carbs and the light went out" - wrong. 2. "We tightened some loose wires out of your alternator and the light went out" - Wrong!
Should I be telling these guys "It's an alternator light! Rebuild my diodes/rectifiers/brushes!" Forum says I can do this myself as well if I can have my buddy help me solder...



---
GEO

tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
I hope the OP has found ideas to fix his car between the whining about what the light is called. A couple votes for the alternator, mine for a bad connection.



Cogito ergo sum periculoso

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