Hi Pete, Thanks for the update.
Can you do an extra test please.
* With the engine running and warmed up you mentioned you can see the IGN light glowing occassionally.
If you give the engine about 2,000 rpm, them turn on headlights + heater blower + heated rear window + wipers does the IGN light glow much brighter ? ) Just to give an idea of the problem.
* Do you have a voltmeter ? If so can you check the voltage across the battery in this state and also the voltage on the thick brown yellow wire at the alternator please ?
From your description the thin brown yellow and brown green being linked and going to the small connector on the alternator is O.K. This is the IGN light connection, provided it goes nowhere else.
I'd try and leave the ammeter in for now just for originality.
The thick brown/yellow wire going to the top large terminal on the alternator is also O.K. , but as you say a tidy-up of the wiring may help , perhaps using the large style 8mm? yellow spade crimp connectors. These have a much higher current rating.
* One thing that you may need is an additional thick brown wire from that middle connector on the alternator to also connect to your thick brown wire at the starter solenoid.
The reason is that on some alternators that is used as a remote sense wire that controls the output voltage.
The next bit is a technical explanation which you can skip if needed.
Start of speech !
There are at least 3 different variants of MGB alternator. But all do the same basic function.
Since you have an MGB
you are bound to have the Haynes manual.
Pages 190 to 201 show the evolution of the charging system.
Fig 10.26 shows the positive earth dynamo with the IGN light fed from the control box.
Fig 10.27 shows the early alternator (15ACR) with the separate electronic control box.( not your one).
Fig 10.28 shows the later version with no external control box but with the electronics now inside the alternator casing.
The IGN light wire also feeds a 2nd terminal to provide alternator field excitation, but the function is the same.
There are 2 brown wires. One thick on that carries the main current and a thin one that is the sense wire . This looks at the voltage on the end of the brown wire and feeds the voltage back to the alternator and adjusts the output voltage to be within range. Note that on this model if this brown sense wire is disconnected the alternator voltage will go very high. Again with only 3 terminals this is not yours.
10.29 shows the 71/72 model with no sense wire. This is known as a Local Sensed model, but is not as good as a remote sensed unit.
10.30 shows the 73/74 model and the sense wire is back in.
This is probably the one you have fitted. If that middle large terminal is not connected there is a built in safety resister in the electronics to stop the output voltage going too high. Much better to have it properly connected to the starter solenoid brown wire in your case.
10.33 shows the next major change where the output current capability is going up and up ... and here, instead of a remote voltage sense, the 2nd large terminal is used in parallel with the 1st terminal to supply the high current, so there are 2 thick wires up to the battery. Voltage sensing is then local as per early models.
So Lucas have tried to maintain an element of backwards compatibility.
Which is good.
End of speech.
So all you have to do to ensure compatibility is have 2 thick brown wires each going separately to the battery connection at the starter solenoid and the IGN warning light going to the small spade connector.
Let us know how you get on.
I'm sure we can fix this.
Tony.