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MG MGB Technical - No red ignition light

I have replaced the battery on my 78gt the old battery looked like it had grey sludge on top of the cells now I have no red ignition light I have checked the bulb that works car starts and runs everything works but I have no red light any ideas thanks for any help.

Dennis. A red light means an out of balance condition between the battery and the alternator. Check the battery voltage at the battery terminals, then at the clamps and make a note of this. The two voltage readings should be the same. if the reading at the clamps is lower than at the terminals, the connections need to be cleaned.

Then, start the engine and read the system voltage. I generally take this reading at the front of the fuse box, lowest terminal. This reading should be 13.5 to 14.5 volts. If you have a reading within this range, the alternator is functioning correctly. If you are reading battery voltage, the alternator is not functioning and the engine is running off of the battery only with the battery not being recharged.

If the battery voltage is good (about 12.5 volts fully charged) and the system voltage (engine running) is within specifications, the wires to the bulb need to be checked to make sure they are connected properly at each end.

Les Bengtson

Dennis, my car had these symptoms recently. I found that the plug/socket connection at the alternator was loose. It was ok again after I nipped up some of the spade connectors.
Brian Shaw

Thanks Les and Brian after I have done a few jobs around the house i will have a look around check all the connections check the volts at the battery keep looking for any loose wires etc

I also found the yellow brown on the alt to be pushed out of the back of the connector block as the internal clip was gone. I have to push the connections on with a screwdriver if I disconnect the alternator
B Anderson

Whilst the red light glowing when it shouldn't means there is an imbalance between the battery and alternator, when it doesn't come on when the ignition is on but the engine stopped that is a more straightforward disconnection somewhere between the white at the ignition switch (pre-77) or white/brown from the ignition relay (77 and later) and the brown/yellow at the alternator. You may also find that the alternator doesn't start charging until the engine is revved to about 3000 rpm.

In the latter case it could mean the ignition relay isn't operating, which for 78 and later would mean things like washers, wipers, fuel gauge don't work either. If they are working then there is a brown/white wire, one of four in the double bullet connector by the brake servo, which feeds the warning lamp, as well as the brown/yellow in the alternator plug. Also the bulb could have failed, but this is very rare indeed.

What's more concerning is why this has happened if all you did was replace the battery. If you inadvertently connected the battery terminals round the wrong way, even momentarily, the alternator could well have been damaged.
Paul Hunt

When i tried to start the car with the old battery I realised i had no red light or any electrics because the battery was flat I have replaced the old battery cleaned and checked bullet and spade connectors changed the two relays under the bonnet I have 12.50 volts at the battery engine switched off with the engine running 12.30 volts bottom fuse under bonnet 12.30 volts if I drop the steering colum I will try and look at the wiring under the dash

I think you shoulhave more than 12.3 with engine running. You need to be at about 1500 rpm, the alt should be putting something out at that speed.
Any way you don't need to drop the steering column. just reach under and undo the knurled nuts on the tacho clamp and it should push out. Usually loads of room :)
B Anderson

I managed to get the bulb holder out of the dash I moved the bulb about in the holder and it worked I pinched the holder and got a tight fit how did the bulb wiggle free the same time the battery went flat just to confuse us now the car is fixed i can finish all the jobs around the house and garden

Dennis. Please read my original posting again, paying specific attention to my comments about proper system voltage. Then, read Paul Hunt's posting. Between the two you should be able to figure out the problem you are currently experiencing.

Les Bengtson

Keep it simple. If a light bulb is not working, replace the light bulb before you start analyzing the entire electrical system. After thirty to forty years the light bulb probably will fail.

LS Sheldon

12.3v is battery voltage, i.e. the alternator is not charging. Rev to 3k and if it is still the same voltage then check the alternator plug is pushed fully in.

If it is, then remove it, turn on the ignition, and connect an earth to the brown/yellow wire in the plug (NOT the brown!!!) and see if the warning light comes on. If it does then the alternator is faulty.

If the voltage rises to 14v or so after being revved to 3k then the alternator is probably OK and there is a break in the warning light circuit, the secondary function of which is to act as a 'pump primer' to the alternator.

I don't know why you would consider dropping the column to investigate this, that could create all sorts of problems for the alignment of the column UJ.
Paul Hunt

This thread was discussed between 20/06/2013 and 23/06/2013

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