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MG MGB Technical - Puzzling Ignition Problem

Folks,

Elkectrics are not my first love, so any help woudl be gratefully received!

Problem is this.

Ignition light came on when I needed to be somewhere by a certain time. So I kept going, hoping i would make it. I did, and still the car is running, some 300 miles later, si I don't think it's an alternator problem!!!

I have power to the bulb holder, through the white wire, and also through to the alternator. Now, if I disconnect at the Alternator, and put MultiMate into the circuit and earth it, the bulb does not light.!?!?!? So I am stumped!

Anyone come across this before?

E
JE Richardson

Put a voltmeter across the battery with the engine running at around 2000 rpm. You should see 13.5 to 14.5 volts. Lower than this indicates that the charging system is not up to snuff.
John H

As alluded to, I'm 99.999999999999999999% certain it's not an alternator problem, else would not have got 300 miles. Yes across the terminals I have circa 14.2v

MY lack of knowledge is that I don't understand what the circuit involving that light. e.g What stops happening to cause that light to illuminate.

Development now is that the light has a discenable to pulse when I indicate, so am guessing it's some sort of earth..but as the earth is on the brown and white side of the lamp holder does this indicate the problem is that somehow the power is being put in to the circuit on the white side of the lamp holder...Sorry, thinking out loud here!

Thoughts?

Eric
JE Richardson

The bulb is connected between the ignition white wires and the alternator via the brown & yellow wire.

With the ignition on and the engine not yet started the 12v from the ignition wires flows to earth through the alternator and the bulb lights. When the engine is started the alternator starts producing voltage on the brown & yellow wire so the bulb sees the same voltage on both sides and thus goes out.

If there is a difference in voltage across the bulb because of problems in the alternator or in the ignition circuit then the bulb glows dimly and sometimes varies in brightness depending on engine speed.

I would check the condition of the fusebox - a bad connection between terminals or the fuses can cause this sort of problem.
Chris at Octarine Services

The light is a balance bridge between Alt voltage and IGN voltage. With key ON and engine stopped, current flows from IGN to alt through the bulb, to ground in the alt. With the alt charging, there is system voltage at both ends of the circuit: ie, the former ground is now a voltage source, so no current through the bulb since both sides are at system voltage. When the IGN voltage source is removed but the Alt source is still present, ie key turned OFF but engine still turning, the light lights again. Now the current is flowing the other direction from Alt through the bulb to ground, which is provided by all the stuff the Ign circuit normally powers - TS, instruments, coil, etc. In fact, if the bulb is not correct, it is possible for the car to keep running with the key OFF, power being supplied by the Alt!
You are looking for a bad connection in the White circuit from the keyswitch, a very common trouble. This is causing a big voltage drop between the Switch and the rest of the circuit, with the take off for the alt indicator light after the bad connection. This lets the alt feed current into the rest of the circuit, while "seeing" reduced voltage from the keyswitch. The pulsing when using the turn indicators is a reflection of the variable voltage caused by the load from the flasher, also fed through the same circuit with the bad connection.
The problem is almost certainly a bad connector sleeve in the W wires where the switch harness joins the main harness. There are almost certainly many other such, in other places. I have found that due to age related stress cracking,the connector sleeves have a maximum life of about 30 years, which they all have gotten to by now. A drop this great will generally cause the connectors to get warm or hot to touch, especially with max. load on the circuit, so feel around a bit with everything ON and engine running.
FRM
FR Millmore

A bright glow of the bulb indicates a problem in the alternator, even though it may still be producing the correct voltage. A dim glow indicates that there is a voltage imbalance between the white and the brown/yellow is previously indicated. Unlikely to be caused by the fusebox itself as this comes after the tapping for the warning light, it can be the ignition switch, or the brown and white connections to it.

Whatever a 'Multimate' is, if it is switched to a voltage range it will prevent the light from lighting. If switched to a current range it may prevent it from lighting if the problem is in the alternator. But generally using multi-meters on current ranges is a bad idea, you can cause dfamage if anything should short out. Voltage ranges are much safer and can tell you all you need to know.

Measure the voltage with respect to a common earth/ground on the brown at the alt, the brown/yellow at the alt, the battery, the fusebox brown, and the fusebox white. In an ideal world they should all measure the same, at about 14.5v. If the brown at the alt is correct but the brown/yellow is higher or lower the problem is in the alt. If both those are correct but the battery is lower then it is a bad connection between the alternator and the solenoid. If the alternator and the battery are the same and correct but the brown at the fusebox is low it is a bad connection between ignition switch and solenoid. If the fusebox brown is good but the white is low the ignition switch or connections to/from it are at fault, check the multi-plugs by the steering column.

However the most common and likely cause is bad connections at the solenoid. With the battery ground strap disconnected remove the battery cable and all browns from the solenoid, if all bolt-up terminals clean them to bright metal and reassemble with copper grease, Waxoyl or similar. If the browns are spades then clean the spades on the solenoid and make sure the connectors on the wires are a tight fit, pinching them up slightly with a pair of pliers if loose, and again reassemble with copper grease or Waxoyl.
Paul Hunt 2

FRM's explanation of the function of the ignition warning light is a good and simple one. However, I've often wanted to change the warning lights on my dashboard/fascia from bulbs to LEDs. The bi-directional nature of the current flow through the ignition warning light would seem to preclude the use of a LED.

Any thoughts would be welcome.

However, just as a mischievious thought, perhaps a bi-coloured LED could be used to indicate either power from the battery or from the alternator.

Again any thoughts would be welcome.

Peteer
P L Hills

PL, you can make whatever your heart desires to indicate operation of your car, even if only you understands the function. The ignition light was put there to indicate a problem in the system. Some people are able to understand and repair the system, but others are made a bit dizzy by the wonderful explanations given above. They should take their cars to someone with the ability to isolate the problem and return the system to normal.

Kudos to Paul and FR for their detailed replies.
Tom

If you can get 2-colour LEDs that vary according to current direction they should be OK ... at least from the point of making it glow. Whether it would pass enough current when the dropper resistor is included to prime the alternator is another issue. You could put a bypass resistor in parallel with the LED to overcome that, and if you wired 4 diodes as a full-wave rectifier you could use a standard LED.

Hardly seems worth all the bother, really, apart from bragging points. Has anyone ever had an ignition warning light fail?
Paul Hunt 2

Paul-
Well yes! The light fails when one of the gizmos in the alt whizzy box goes away. With that particular fault, the light blows when the voltage gets too high as rpm increases. When the bulb blows, the alt quits charging, and the lights go out, the engine stops, and you walk home. Unless you have cleverly laid on a supply of spare bulbs. I have previously posted the more acceptable technical description, and it is in the manuals. Just having some fun!
FRM
FR Millmore

"MY lack of knowledge is that I don't understand what the circuit involving that light."

This might help:

http://www.vtr.org/maintain/alternator-overview.shtml
Dan Masters

PL, I am doing a custom dash in my car and have replaced all the bulbs with LEDs except the ignition light. I did think of doing something with bi-colour LEDs or similar but in the end I decided the normal bulb works well and can show you a lot. I am not sure an LED will be able to show very small differences in voltage (i.e. those times where the bulb glows dimly). To preserve the look of all LEDs though I took a 10mm red LED and chopped the legs off it. That is in a little housing with the normal bulb behind it. So from the fornt it looks like an LED like all the rest but it is back lit by the normal bulb.

Simon
Simon Jansen

Simple failure of the bulb will not force you to walk home. You may have to rev the engine to 3k or so to get it to start charging, but then it will charge normally down to the usual 600rpm or so. If the warning light bulb has failed then having any number of spare bulbs (like in the main-beam warning and each of the gauges) will not help if iit is the alternator that has failed and that has (rarely) blown the bulb. And even if it *has* failed you still have the capacity of the battery to work through before the engine dies. Whatever method of indication is used that will always be the case. Unless one carries a spare alternator :o)
Paul Hunt 2

Paul-
My experience was this . My Austin America developed the fault listed in the book as "Live side output diode open circuit - Light goes out at 850rpm, then glows progressively brighter as speed increases." At 3500 or so the light was approaching the normal no charge brightness, from there on up it gets brighter, and blows out around 5000 or so, very bright. At that point the headlights start getting dimmer, very clearly a "no charge" condition, regardless of engine speed or exitation tricks - which I know/knew about. If you are out in the boondocks at night, a couple of hundred miles from anything open, as you could easily have been twenty five years ago in this country, you will in fact wind up with a flattery. Replacing the warning bulb makes it all work again, but you have to remember to not get frisky with the loud pedal.
At the time I stole bulbs from other places, but soon got a supply of new ones, since I don't like not having instruments either. I had no money to replace a still functional alternator and had not yet gotten tired of paying others to badly fix my dead alts. I confirmed the charge/no charge conditions with instrumentation, but kept driving the car for a couple of years, replacing bulbs. I have confirmed that this was not unique to my AA, and can tell you that it seems to be typical of this failure, per experience with a number of identically misbehaving alts.
I have never bothered to figure out what associated damage in the regulator circuit causes this, since delving into the actual workings of "gizmos in the alt whizzy box" -a sealed device, is a waste of time.
I can tell you that I had a similar experience on my Royal Enfield. That runs on a magneto, so I wasn't concerned about the bad battery in my impoverished student state. As engine speed increased, lights got brighter, very welcome with the 6V electrics, but around 6000 the headlight bulb would blow, then all other lights that were lit. Inconvenient at night, and I wound up in a pile with the bike due to hitting a kerb (out in the country!) as I tried to get off the road before being run over. Replacing the battery cured that.
FRM
FR Millmore

This thread was discussed between 13/02/2007 and 18/02/2007

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