Alternator not charging/ignition light not on

[GeoffK]

There is only one wire from the ignition switch to the warning light so for the light to be off when the ignition is turned on and go on when the starter is engaged means that there must be fault in the switch. At both of these positions on the switch there will be no current from the alternator as it is not running so the only voltage must be on the switch side- the white wire. So for current to go along the same wire the switch must be controlling it- nothing to do with being wrongly wired. How does that sound?

[Charles Farran]

The ignition bulb serves a purpose in agitating the alternator into charging.
The bulb lights up when there is a difference in Voltage on each side of the bulb (or the ignition is totally off & hence any supply to he bulb is eliminated).
When the ignition switch is on the "on position" before the cranking position , on a negative earth car , a positive feed to one side of the bulb then flows through the bulb & earths through the alternator & hence a circuit is made & the bulb lights up. This circuit is interrupted when the engine is cranked given the huge drain on the supply by the starter motor. As soon as the engine fires & the cranking is stopped the alternator starts charging & the voltage created should match the electrical demands on the car's battery. Provided the matching is even then the bulb will not light up as there will be no potential difference between each side of the bulb. ( If you have some heavy electrical demands on the car which exceed the charging capacity of the alternator , which could arise for example by a loose / warn alternator / water pump belt or excessive additional electrical components all in use that the original set up was not designed for, then a potential difference will be created on each side of the bulb).

I had a similar intermittent fault to yours & when i had the alternator off (& totally disconnected) the car i checked whether there was continuity via the bulb terminal connection on the alternator to the case (effectively ground) . There was no continuity. I had the alternator repaired with a new rectifier etc. ( & brushes at the same time) & upon return to me i did the same continuity test & lo & behold there was continuity . i put it back on the car & everything worked as it should! ( Since replaced alternator some 3 years later when one of the bearings packed in - given that the old alternator had been on the car for some 40 years since i bought the car, i treated my self to a new one)!
As , in my example there could be no circuit created with the ignition switched on & the engine not running & therefore the ignition bulb would not light up - this could well be your problem provided you have checked that all the connections are good (& clean of corrosion) with regards to the wiring.
Cheers,
Charles

[Vic Butler]

If the ignition light works when the starter is operating it would appear that it's wired to the same terminal on the ignition switch that activates the starter solenoid. When the engine fires and the key is released the circuit to the solenoid is no longer active.and therefore the ignition light will go out. You need to check whether the alternator is charging with the engine running.
Unfortunately without the wiring diagram I can't say to which terminal on the ignition switch the ignition light feed should be connected.

[GeoffK]

Thanks for taking so much time with my little problem.
Ignoring the alternator, take a look at the mg- stuff website under the heading Ingition Switch Connections then search for 1973. The sketch shows the layout of the switch and which terminals are connected at each position of the switch.
What you say Vic is plausible as if the white wire is connected to terminal 1 it will only come on when the key is in that position. And if the alternator is not working there will be 0 volts on both sides of the bulb therefore no light when the ignition is turned on also when the engine is running. Will check the wiring when I can.
Thanks
.

[Vic Butler]

Geoff, when the ignition switch is turned to the start position, it doesn't switch off anything that's switched on at position 1. I watched when I started my Land Rover today and in the start position the ignition light and oil light are on until the oil pressure extinguishes the oil light and the key is released when the engine starts and the ignition light extinguishes. You need to check whether the ignition warning light is connected to the same circuit as the starter solenoid.
However the cut in speed of the alternator is about 1,500 rpm which means the engine has to be running at about 750rpm as the alternator runs about twice engine speed. My Land Rover engine will tickover slower than that and the ignition light stays on.

[Bumpa]

Vic my Dolomite shows the same thing. When starting, the charge light stays on until the engine revs have reached about 1500 rpm - a quick blip on the pedal does it. That kicks the alternator into life and all is well from then on.

According to the wiring diagram for a 72/73 model on Advance Autowire, the ignition switch terminals are as follows:
1 = white/red to starter relay
2 = brown live feed
3 = 3 or 4 white wires, one of which goes to the charge (ignition) lamp
5 = white/green to the wiper, heater fan, radio etc.

I wonder what happened to number 4?

[Vic Butler]

Thanks Mike. As I suspected Geoff's ignition switch is incorrectly wired so the ignition light only comes on when the key is turned to start. I have wiring diagrams for all MGB's but the manual is out on loan so the only one I have is for my 1977 BGT which is completely different to earlier cars.

[GeoffK]

I have now removed the ignition switch and had a look at the connections. Well! What a mess! Half of the wires were cut and reconnected with a piece of wire of the wrong colour.Only 5 wires connected instead of 6. Found the missing brown wire and reconnected it. All wires now connected to the right terminals as per recent comments and mg-stuff info. Refit the switch, key into ignition( fingers crossed), turn to ignition on position and nothing! No ignition light. Start the engine perfectly and no ignition light as the engine starts. No light on when engine is running.Retested the ignition light bulb and okay.Now totally confused. Slam the door and go inside for a coffee.

[Charles Farran]

Geoff,

Assuming all the wires are now connected to the right terminals of the ignition switch i am still suspicious of the integrity of the alternator itself.
Rick Astley's excellent MGB Electrical Systems book refers to possible causes for the ignition (charge) warning lamp not lighting: -

1) Bulb blown - you have confirmed the bulb itself is ok.
2) Brushes warn down
3) Regulator failure

2 & 3 are possibilities as the route to earth when the ignition is in the "on" position will be disrupted by a weakness in these areas.

You can check the integrity of you wiring from the warning light bulb to the alternator plug by removing it from the back of the alternator and attaching a test lead to the IND socket on the plug with the other end of the test lead to somewhere on the engine block / chassis. If you then turn on the ignition the bulb should light up. If it does you will then know the fault is within the alternator itself.

Astley also says elsewhere that a light on going out can be a loose of missing belt or a failure of one or more diodes in the rectifier pack (within the alternator).

Cheers
Charles

[Charles Farran]

My last sentence in the previous post should read "a light not going out" !