MG Car Repair: Electrics - No life at all from MGB, 6 volt batteries, mazda mx5

Question
QUESTION: Hi, I have a barn find MGBGT 1970. It's been standing since around 1985, but
is actually in pretty good condition.

It had two six volt batteries, with very corroded cables. I have replaced the
batteries with a Mazda MX5 battery, redone the earth to the body, and put a
new live cable from the battery to the starter motor as it was before.

I have also replaced the fuel pump as it was in pieces!

Well, when I turn the key, absolutely nothing happens at all, I mean not a
banana, nothing, sometimes there's a little thud from behind, but very rarely.

I have no idea where to start, so any clues? Otherwise I'm thinking of ripping
our all the wiring and redoing it all. I'd just love to at least see the ignition
light come on or the lights go on, or the wipers move, or something happen!!

Thanks,

Tim

ANSWER: Hi Tim.  You have got the right idea.  I regularly convert MGBs from twin 6-volt batteries to a single 12-volt.  

Have a look at the connection where you bolted the battery cable to the starter motor.  There should be 3 more wires connected to the same terminal.  My guess is that one of them is not connected.  Two of these wires go to the alternator, and the third goes to a junction hidden inside the wiring loom, after which it goes to fuse number 4 (the bottom one) and to the starter switch.  I would check fuse 4 and the starter switch to see if there is any power getting through.  All these wires are Brown in colour, so they are easy to recognize.

---------- FOLLOW-UP ----------

QUESTION: Great, thanks! Now, out of interest, I checked the brown cable to the fuse box,
and there are two brown cables going into the same side, and then coming
out the other side there's a cable which goes ... nowhere at all! Where should
that go? I've checked the wiring diagram I have but it's not a great version, so
I'm guessing that might be causing a problem!

At some point in the not too distant future I will replace the wiring looms, but
funds are not available this month!

Thanks :)

Answer
Hi Tim.  Fuse number 4 is not very important.  It only powers the horns and the interior light.  The two Brown wires are (a) the power supply from the battery, and (b) the power supply to the starter relay which is mounted by the side of the fusebox.  

I would check the brown wire that goes to the ignition switch next.  My guess is that it has become unplugged from terminal number 1 on the switch.