Bit of a long winded and technical first post, so my apologies for that!

So I'm currently working on my mate's 1990 4x4 Sierra Saphire Cosworth and I'm reaching the limit of my knowledge, hoping one of you fine folk can assist me somehow.

The long and short is that the car won't run as the ECU isn't getting power.

The 'power' relay that powers the ECU and switches the fuel pump relay is getting 12v from the ignition as it should. The blue/yellow wire from the relay goes to pin 10 on the ECU (which appears correct according to all the wiring diagrams that I've seen). As I understand it, this should somehow ground/earth the relay, allowing it to switch the relay and thus send power via the red wire to pin 20 on the ECU (and via a black/blue wire to switch the fuel pump relay.)

This grounding/earthing of the relay isn't happening. I can bypass this by grounding the relay with a 'standalone' wire, the ECU then powers up and we have spark etc.

So I suppose the question (for now at least) is: How does the ECU ground the relay via pin 10? What is the path to ground, as it isn't a direct connection (I assume)? Does it go through another component/sensor? I've read some sort of reference to the CPS, but that appears fine, reading 670 ohms between pins 3/4 on the ECU.

Any ideas? Or am I miles wide of the mark with what I've figured out above? lol

I guess it is unlikely, but does anyone have a schematic of the insides of the L8 ECU?

Cheers, Martin