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MG MGB Technical - Minor Ignition Light Problem

I have a minor problem with my ignition warning light which I've failed to cure after many attempts and its starting to give me bad dreams at night.
Everything seems to work as it should except that with the ignition turned on and the engine not running, the light doesn't glow very brightly. My worry is that should it come on while I am driving, it is not bright enough to notice, particularly in bright sunlight. To date I have replaced the speedo, the alternator and the warning light bulb.
If I run an earth lead from the alternator connector, directly to earth, the warning lights glows brightly. Running an earth wire from the alternator to earth makes no difference at all.
Is there anyone who can suggest a cure?

Geoff
G Waller

The warning light has no connection with the speedo other than being physically mounted to it - and then not in later models. It should have two wires (white and brown/yellow) and unlike e.g. instrument illumination lamps which only have one wire is electrically isolated from whatever it is plugged in to, i.e. it should work exactly the same when dangling on its wires.

If the warning light glows brightly with an earth connected to the brown/yellow in the alternator plug, but not when that plug is in the alternator, then either the spade is not making a good connection or there is a problem inside the alternator (replacement alternator noted). It cannot be anything else. Is the alternator the original model for your car? If not then different alternators have different connection arrangements and it is possible the brown/yellow is connected to the wrong terminal. With the plug in, and the ignition on but the engine not running, what voltage do you read on the brown/yellow at the alternator?
Paul Hunt 2

Hi Paul,
Thanks for your response. I have just measured the voltage from the Brown/Yellow wire at the alternator end and it gives a full 12v. This new alternator was purchased from a reliable MG 5 star supplier so it should be OK. By the way I replaced it because it was not charging adequately, not to cure this specific problem. I renewed the terminal on the brown/yellow when I changed the alternator.

Geoff
G Waller

The Lucas Alternator has a 3 phase rectifier (three coils) to supply the DC for battery charging. A single diode (one way device) is used in series with one of the coils to indicate if there is a back EMF (Voltage) from the coil ie the alternator is working. When it is NOT working the pure DC resistance (Plus a very small voltage drop) makes the “Charging” lamp light up.

As Paul says, if you have ‘proved’ the vehicle wiring by earthing the Brown/Yellow wire, you must have a faulty alternator.

If you measure the voltage at the Brown/Yellow wire at the alternator end, you should and have got 12V, as you are measuring no volt drop (no current flow) through the “Charging” lamp. The alternator when not rotating gives an effective earth, when it spins up it too gives 12V (actually more) and makes the “Charging” lamp go out. (same voltage on both sides of the lamp – no current flow)

I suggest that your diode in your alternator is not working correctly, or you have a high resistance joint in the associated coil.

Did the replacement alternator come as a complete unit or was any parts of your original item used (ie coil or rectifier module)?
MG Mike

Hi Mike,

Thanks for joining in. My new alternator was a complete unit except for the pully.

Geoff
G Waller

When measuring the voltage on the brown/yellow with the ignition on *and the plug out of the alternator* then you should see 12v and the lamp will not be lit. With it plugged in and the ignition on the light should be lit and you should measure very nearly 0v or an earth on the brown/yellow. If you measured 12v with the plug in and the ignition on, then the alternator or the connection to it is faulty.

When the alternator starts to charge it will suddenly rise to something above 12v, nominally 14v+ with minimal electrical load switched on. This voltage appears on both the brown/yellow and the rest of the cars electrics including the white at the ignition switch (which is where the other side of the warning lamp goes to) and as Mike says having the same voltage both side causes the warning lamp to go out. Thereafter it acts like a pair of scales. If either side gets higher or lower than the other, then a voltage will be developed across the bulb which will cause it to glow, proportionally to the difefrence in voltage. If the brown/yellow is at about 14v and the ignition white below that then usually that means there are bad connections between the alternator brown and the white. If the alternator brown/yellow drops, or goes above battery voltage, then usually that indicates a problem in the alternator.

Remember, replacement parts are no guarantee that they will be working when you get them.
Paul Hunt 2

Hi Paul,

Ran the tests you suggested. with the broen/yellow plugged into the alternator, I had a reading of 1.5v. started the engine and this went up to 14 volts. measured the voltage on the white lead at the warning light and that read 13v. Does this mean the problem is in the feed? I'll try to get some more readings on the supply side, at different points, to see if I can pinpoint the v drop. If its in the main loom I guess there's not much I can do.

many thanks for your help,

Geoff
G Waller

Mine shows just over 2v at the alternator, so your 1.5v shouldn't be a problem. However once running you do seem to be losing a volt between the alternator and the white feeding the warning light (you should really be seeing about 14v on both) so this will contribute to some loss of current and brightness. Paradoxically this problem, when more severe, actually contributes to the warning light glowing slightly with heavy electrical loads, although this is usually only noticed at night.

You could take some more voltage measurements at the solenoid, on the battery cable stud, and each brown on its spades or ring terminal. Of course in theory all these should be at the same potential, but particularly with spades and even with ring terminals you can get bad connections and hence volt-drops developing between them. Then also test at the brown at the fusebox, and the white. Again all these points should be athe same voltage, but anything more than 1/4v difference when under load (e.g. headlights on) does indicate bad connections. The voltage tests should help to isolate where.
Paul Hunt 2

I had the same symptoms (plus the light glowing dimly as Paul says)- turned out to be a dodgy ignition switch. You can bypass the switch with a (thick) jumper lead to find out.
Neil
Neil22

Hi Paul

I've taken some more voltage readings, as you suggested. I'm getting 13 volts everywhere, including the main feed into the fuse box. If I've understood the wiring diagram corrrectly, this points to the common connections on the starter solenoid being the culprit.

I cannot get back to my car untill next week-end. I'll try it then.

Geoff
G Waller

i am experiencing the same problem as you i have fitted a new altenator and a new altenator conction plug, and when the electric system is under load the ignition light glows but the readings from the alternator are showing 14+v.

i have heard that this is to do with the link wire that goes to the starter motor from the altenator.

i have not had chance to try this out myself but i ust thought i would give you the same chance i have. i will update you on any progress, if you get a solution please up date the log so i can give it a try too.

cheers matt.
matt linley

Matt - if both the brown/yellow and brown at the alternator show 14v when under load and the warning light is glowing the alt itself is probably OK. So check at the fusebox brown. If that is a volt or so lower then you will need to check at the solenoid, but while there it is probably works removing all the connections, cleaning up bolt-through tags with emery and reassembling with copper-grease or similar. For spade connections make sure the spade itself is clean and the connector is tight, giving the connector a gentle squeeze with a pair of pliers to tighten it up a bit if required. Do the same with the spades at the alternator.

If the brown at the fusebox is OK then check the brown and white at the ignition switch and the connectors by the steering column if you have them, as one of these connections or the switch itself will probably be showing the drop.

If you have higher wattage loads than standard i.e. uprated headlights but haven't upgraded the browns from the solenoid to the alternator and whatever feeds these high loads, you may need to.
Paul Hunt 2

This thread was discussed between 01/12/2007 and 19/12/2007

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