After installing the cut off switch and giving it a go, all seemed okay... until I went to try and start the car again. Now what follows is a classic bit of mechanical deception, aided by some user ineptitude! But all sorted in the end, and now I know much more about alternators and E30s.
So before I'd disconnected the donor battery, and all ran fine until I killed the power to make sure the switch worked. The following day I jumped started in the usual manner, disconnected the batter an everything died. Very strange... So I tried jump starting again, nothing not even a click. What the hell? Turns out the negative connection I'm using on the engine block needs a bit of finnesse, but still when I got the M20 engine running she just died every time I disonnected the Fiesta's donor battery. So off to the E30 Zone I went for help.
First off people suggested the battery, I wasn't so sure I wanted to check the alternator in case I'd wired things up wrong and blown it when using the cut off. This switch is actually designed to send current back to the battery via earth when used rather than frying your alternator, but maybe ironically it hadn't worked? Using my new multimeter - which is also fantastic help for confirming where wires go! - I checked the volts at the battery and found 11.8 volts - pretty rubbish.
I asked about my bodge it electrics job joining the battery feed from the switch to the car, apparently this wasn't a bad effort and wouldn't have been the cause. Bottom wire needed turning upside down, and the left and right holes sealing. This was a B&Q junction box and if it passed the "someone who knows what they are on about" test I'd also invest in 2 cable clamps. So that had been okayed.
I was advised to take the alternator out and take out the alternator regulator for a look at the bushes - scary stuff to me! I didn't have a clue what most of this stuff was nevermind how to take it out. But after plenty of advice later I gave it a go, posted some pics and the alternator was deemed fine from pics alone. The parting comment was "sorry, can't see anything wrong, if you're battery light is definately on then try a new alternator". Battery light? No, that would involve the dash clocks being in which they aren't at the moment. The whole dash had come out and was only in temporarily to make sure my revised switches and electrics worked.
Out of curiosity I had a quick trawl for battery light and the alternator, turns out if your battery warning light is blown, a circuit isn't complete and the car can't tell the alternator when the battery needs charging. Ahhh, so no clocks at all would probably do the same trick I guess! So back to the forum! Turns out I'd probably found my issue, and would let them know how I got on. I was tempted to get dressed at 11.30pm and go outside to give it a go. Glad I didn't!
Next day very excited, plugged in my dash clocks and put the alternator back in. Total volts from the battery: 14.4! Awesome! Ahh, the Fiesta is running, so it's alternator is charging both batteries, perhaps a false hope? Knocked off the Fiesta... 11.8 volts :( Bugger! It literally looked as if the alternator was buggered. Maybe the battery light was just an aside? Just having the good donor battery connected still only 11.8 volts. So off to ebay for an alternator then. I found a reputable seller - easy to do when you see good feedback on the forums and ebay - they have a reputation to protect. £13 and I decided I'd pick it up only up the road in Leicester.
Jim320i had some bits of advice which was handy as a sanity check. Also his alternator was a Bosch where as mine seemed unbranded and had a "reconditioned" sticker on. Now my battery always goes flat very easily - partly why it was a newish Halfords one fitted recently. So the alt had been worked pretty hard I guess, and a Bosch one in probably far better nick for £13 seemed like a good future investment. I'd also been advised to try a new alternator belt as I said mine seemed a bit loose when I refitted - a bit more on that later. I'd looked through the servicing history on the car, and there was plenty for the first 50k miles.... nothing for the last 50k sadly. So new belts it was to be, both under a tenner from Halfords. The cam belt is probably overdue a replacing so that will definately get done - all this will be recorded and I will have piece of mind.
So I get back with new alternator, stick it in, new belts, tension the alternator belt as much as I can and it's still more loose than was supposed to be acceptable. Was the engine skew-wiff? Did the alternator fixings get replace with ones the wrong length, and had that always been a problem? Very strange. Anyway, time to fire up very excited! Turned it over, did a volts test, 11.8 volts again....balls. Ah well, not the end of the world that's one thing ruled out. But very puzzled now.
I thought I'd have a look around the new-ish alternator, see how it was performing. To my utter horror I saw the new belt flailing around in the engine bay! Cut off switch got used straight away. What the hell? Is this the wrong sized belt? Is this always been the issue with the battery not charging properly? Surely something is amiss. I had a good look at where it fitted, and spotted another cog behind the one it was on. This cog, was bigger... I tried getting the belt on this cog, ahhh.... perhaps this was why it was too loose after my refit. To get it on I had to slacken the tensioner loads! Splendid! Got the belt on this "new" wheel, and funnily enough the belt tensioned to a perfect 1/2 inch travel, and I now felt very optimistic.
Jumpered up the battery, started her up and took a volts test. 14 volts!! I said something excitedly and loudly and immediately felt stupid. After a quick look round to see who heard, I apologised to my bemused neighbours and carried on sheepishly. But I was in a good mood, now for the acid test, and time for the ambilical cord to be cut! I gave it some revs from the engine throttle body, and for the first time in a month disconnected the negative battery lead to see the M20 carrying on under it's own power.
So... the old alternator is now an emergency spare, as are the belts. I have a better condition alternator, two new belts, and much more understanding of how these things work! It's all been a bit of a catalogue of comedic errors, but it's all a learning curve and I'm enjoying the challenge. You have so many variables, it's genuinely too easy to get distracted and assume one thing is definately the cause. I'd been certain I'd found the issue about 3 or 4 times! But many thanks to the E30 Zone and BrianMoooooore for his Encyclopedic knowledge! The E30 Zone is full of very helpful people and E30 ownership is much easier as a result.