What with Craig's prompting, I thought I'd better try and get this sorted. Has anyone got the Honda engine charge warning light working on Rover instruments yet? I can't get it working from the hondata circuit (relevant pin not there on a UK spec engine loom), nor by tapping directly into the white/blue alternator wire which I think carries the signal to the multiplexer.
I've sketched out a circuit I think should work to provide the warning light. Any comments?
The Zener provides a max 12V reference to the non-inverting op-amp input. This means without charge, the potential divider voltage will be less than this and hence the lamp lights. As voltage rises with the alternator running, the inverting input will rise above 12V and turn off the charge light. The nice thing about the variable resistor is I could fine tune the voltage the lamp goes off. As the honda alternator seems to run at about 14V I could set this to 13 and so get early warning of any problems. I'll have a rummage in my parts tray and see if I've got the bits to knock this up and maybe try it at the weekend.
Richard