Tacho died (using diodes)

DeeZee · Post by **DeeZee** » Mon Aug 15, 2011 12:18 pm

The original dash instruments on the 1992 Ford Sierra were running off the single coil on the 1.8ltr CVH engine. To get the tacho to work with the new 2ltr 16 valve engine and MJ I used the diodes (1N4004) on the coils, with a zener (1.3w 15v). However it was super erratic, so I ditched the zener and all was well for a while.

After around 20 minutes use the tacho has ceased working again. Should I try replacing the diodes and put a different rated zener diode in there? I'm not sure what wattage the zener diode was meant to be, so I assumed 1.3w was plenty enough. Is there any chance my tacho has been cooked? Or do I need to protect the 1N4004 diodes from the current with resistors or a zener?

trevil · Post by **trevil** » Fri Sep 02, 2011 12:11 pm

I do not use zener on my tach, but I use resistors with each diode. Sees to work ok so far.

DeeZee · Post by **DeeZee** » Sat Sep 03, 2011 9:36 am

I have re done the circuit again. Popped on a 1N4007 on each coil, with a 100K ohm resistor in parallel with each. Again this worked fine for around 20 minutes (accumulated over a couple of runs), then the tacho just stopped working. I'll pulling my hair out (what little I have left!) trying to get something to run here, without failing.

NITROPIXIE · Post by **NITROPIXIE** » Mon Sep 12, 2011 7:07 pm

Try using pin 11 of the EDIS unit. This is a Clean Tach Output (CTO) of the EDIS and should not give you any problems.

regards

Ryan