The V8 Owners Forum -

I have converted my Rover SD1 to wasted spark. Have set up a tacho feed as per https://wiki.autosportlabs.com/images/1 ... ircuit.jpg but with 4 coils of course.

I get an output like http://angelz.co.nz/wave.png

My tacho doesn't read anything from that.

Where have I gone wrong?

I had exactly the same problem with mine. But on a mate's car - exactly the same model - it worked just fine. I then tried a spare tach - still didn't work.

I then tried the transistor and relay coil circuit from the IDM feed on EDIS. That did work mostly - but the tach stopped working above about 5000 rpm.

Since I had a spare unit, I eventually found a universal tach driver circuit using an IC (replaces the electronics inside the tach head) which also allowed the RPM reading to be adjusted.

There are commercial adaptors available to drive this type of tach from wasted spark, but haven't tried one.

Converted to wasted spark how, and with what parts, what kit ? etc etc ?

stevieturbo wrote:Converted to wasted spark how, and with what parts, what kit ? etc etc ?

I used a 36-1 ford wheel and sensor. Mounted behind the front pulley. The wheel and sensor mount had to be customised.
The coils are GM once again customised from a v6 with an extra coil for the v8.
In the middle is a megasquirt 2.
The motor also runs 8 individual throttle bodies, ported heads, hot cam, adjustable pushrods, headers, twin exhausts.

The diode from each coil negative with wasted spark is designed to replicate the original feed from a single coil. There are some variations to that circuit - some have a zener feeding the tach - some just a plain diode.

MS2 can generate a dedicated tach output, but this a low voltage signal. The pulse at the negative of the coil can be 50 volts or so.

If you use an EDIS8 module, that also has a low volts tach output.

Sadly, I never looked at the pulse the SD1 tach got from the standard coil, so didn't know if any of the adaptor circuits generated an identical one.

The transistor switching an inductor seemed to me a much more sensible way to generate that high volts circuit, and did sort of work here. There are commercial adaptors that do just that, if you don't fancy building one. For a reason I don't know, the SD1 tach seems to be more picky that others of a similar design.