[paulgato]

I think I managed to fix the alternator! And I now have a way to both switch it on and switch if off, although not in a regenerative-braking kind of a way.

This alternator problem was driving me nuts, I have to say. Having destroyed the original, working alternator - which I could disable simply by leaving the ignition feed wire disconnected, and re-enable simply by stopping and plugging the wire back in - I then got another 2nd hand alternator (Bosch) only to find that it self-excited at 1500 rpm, so couldn't be disabled. I hunted around, phoned around, became quite amazed at the lack of good information out there on which alternators self-excite and which don't, and eventually found a guy in an alternator and starter motor re-manufacturing company who was very certain that Bosch alternators sometime DO self-excite, but that the alternative brand for my car, Valeo, NEVER self-excite. They had a reconditioned Valeo on the shelf and I bought it. But, (Arrrgh!) the Valeo also self-excited, and not just at 1500rpm but at ANY engine speed at all, even at 800 rpm tickover. I sent it back and they refunded my money, but so frustrating!

So, really, this was driving me bananas. I read on ecomodders somewhere that a guy had experimented with simply leaving all wires disconnected from his alternator, and his alternator died within weeks. Apparently doing that (with a self-exciting alternator) will drive the voltage up in the alternator to 40v+ as it tries to charge an apparently completely flat battery (wires disconnected = "looks like a flat battery that needs every ounce of power we can produce guys!").

And then I had a stroke of luck - just a couple of days ago. I hadn't been able to identify the original alternator as it was obviously a reconditioned, after-market one and had no numbers or maker's name on it. However, on closer examination, I realised that it looked very similar to the used Bosch alternator I had bought as a first replacement. And in fact, the voltage regulators were interchangeable. So I did some testing, swapped the regulators over, and tada! the old one seemed to be working fine with a different regulator in it. So it looks like the only thing I damaged by shorting one of the wires to ground was the regulator.

(Now I did my testing as far as I could, and I did apply a good dose of logic to the issue, but I STILL don't understand what is going on. As far as I can understand, the symptoms I was getting indicate that at least one main output diode in the rectifier MUST be shot, and yet my substitution results show a completely working alternator - with no symptoms of blown diodes - when I put a good regulator in. So I conclude that despite reading up on alternators online as best I could, I still have not come to understand how they work - at least not the Bosch ones. Not a comfortable state of affairs for my engineering ego, but I can accept it. I now have a working alternator, and the 'repair' only cost me £50 for a used alternator, out of which I was able to steal a good regulator. But man! that was driving me crackers. I have literally lost count of the number of times I have now changed the alternator. I think my neighbours were beginning to think I had some kind of alternator fetish going on. I could quite literally and not metaphorically do it blindfolded now, and it is not a particularly easy job on that Golf TDi.)

So all is good. And a bonus is that on testing to check the old alternator still didn't self excite and would remain OFF with the ignition wire disconnected, I discovered, ...that it actually DOES self-excite, but only at engine speeds over 4000 rpm, which is why I had never noticed it doing so before.

This means that I have no need of a switch and wiring to control it. I can simply leave the ignition wire disconnected permanently, and if battery voltage does ever get dangerously low while driving - to the point where I am nervous about being able to re-start the engine if I switch it off - all I have to do to switch the alternator on is to blip the throttle and rev briefly to 4000 rpm. In normal eco-driving I would never rev that high, so I would never inadvertently switch the alternator on. (The red-line is just 4,500 rpm.) That is just nicely serendipitous. I was due a bit of luck like that.

So, I'm back in business, driving alternator-free full-time again. Just driving around locally the VW dash display is pegged at the maximum it can show, which is 99.9 mpg overall average. I know that display is somewhere between 6% and 10% optimistic, so, well, I'm getting something over 90mpg overall average in reality I guess. That's mostly driving around on short local journies for work with tools in the back, so I'm happy enough with that for the time being. The hot weather helps of course.

I'm also very close to having the 2nd battery and charger, wiring, etc. all finally installed, but that's for another post. I'm currently running the system with, ...but I'll split that into another post, as this one is long enough already...