The problem was with the alternator charging light. The battery appeared to be charging but no light to tell you it was. It was compounded by various bits wired wrongly, from day one I suspect. At one point I observed one of the wires between the alternator and charging lamp on the dash were not connected to each other and we could not identify where they were going (even worse). Matt had approached the problem methodically enough, sorting out this basic wiring problem behind the dash and even buying a replacement alternator. Needless to say he eventually got so frustrated he took up my offer of help hence this blog. I tend to bog everything on these cars as they are so interesting (not to the wife of course). You never know the information might be useful to another enthusiast.
I did an investigation by constructing the basic charging circuit including battery, alternator and lamp on my work bench. I turned the alternator using a Dewalt 300W drill driver and a socket (to simulate the engine). Using this method revealed both alternators faulty. The problem was the lamp cct output produced by the internal electronics gave no pull down to earth for the indicator lamp.
We used the documents on the Westfield Word site for this alternator. The diagram on one section appears to be wrong. I suspect this could be the source of the problem. On a Denso alternator pin (1) is always switched live and pin (3) is always the lamp cct. The only pin that varies it's function is pin (2) which acts as an external voltage reference point on some models. In Matts case of course this inverted wiring caused both alternators to fail.
When checking out the alternators I found one of them was still partially working. It was giving 2mA constant current when idle and 0mA when wound up to speed by my Dewalt. This was good news to me as this can easily be converted to a correctly working charging lamp using a transistor and one resistor. In other words the transistor in the alternator had failed in such a way that the internal electronics was 90% still working save the pull down function.
I wired up a new pull down cct using the case of an old 3 pin Ford bleeper module and inserted the transistor as shown below, which I also had in my scrap box (I have some wonderful stuff in my scrap boxes).This resolved the problem entirely.
Just in case Matt needs to keep a record of the module wiring I have reproduced it here.
I have to thank Matt for a really interesting days fault finding on one of my favourite cars.
