what is it?

Mr RB said:

Do you know what blew the original diode, and how did it fail, ie shorted? burnt? cracked open? etc etc.

Click to expand...

The regulator/rectifier that turns the alternator 45-60v AC to 13-14.5 DC failed, which in turn took out the tacho (30 Ohm resistor and zener).

I would love to find the fault, although i would have to purchase a power supply. Would you recommend any in particular that wont cost a fortune?

Even if it was the micro, how would i go about getting a replacement. I think someone already tried to look for the micro part and came up with nothing. Are there any alternatives i could use?

That's a nasty fault. If it is a 18v zener, and the alternator took out the 30 ohm resistor and 18 ohm zener it was probably exposed to 20+ volts at significant current, and the most likely part that may have been damaged after the zener is the micro.

If this is a valuable or collectable tacho you could contact one of the many speedo/gauge repairers and tell them the history. They may have a source for the chip or a small replacement module they can insert instead of the stock chip.