Two diodes and one zener

[sierra2dr]

Well,this is my interpretation of getting a tacho working on my Sierra.
Here's the two diodes wired up

the wires on the bottom end of the diodes are leading to the coilpack plug wiring green/red and yellow/green

the wire on the top end of the diodes lead to the zener

and I've cut out a bit wire insulator from the green wire leading from the tacho.
So,that's with the zener,and now I've tried it without the zener

and it still doesn't work,am I going wrong somewhere? thanks

[brentp]

Without seeing everything at once it's a bit hard to tell if that's correct.

Now, if you connect your tach to only one coil pack, do you see an RPM reading? if so, it should show 1/2 actual RPMs. This would be a good first test before implementing the diode circuit.

[sierra2dr]

Now, if you connect your tach to only one coil pack, do you see an RPM reading? if so, it should show 1/2 actual RPMs. This would be a good first test before implementing the diode circuit.

thanks Brent,I shall give that a bash