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

 

A common problem on these ecu's (L1, L6 and L8) is the ground path on pin 10 failing. Usually when one starts the engine without proper grounding.
The track on the board from pin 10 to a diode will melt if to much power goes through it. Also the diode needs to be replaced.
The diode is very near pin 10 and is mounted in the direction from connector to front of the ecu.
The track can be repaired by soldering a wire. (From memory - it's been more than 20 years since I did this - on the back of the board.)

 

That's exactly the kind of info I was hoping for - I'll check that out later today, thanks very much!

 

Just had the ECU apart, and the track from Pin 10 is definitely broken. Time to put the fine tip on the soldering iron and run a piece of wire...

Thanks again for the info, I'll update the thread later if it works!

Saphire 4X4 Cosworth Pin 10 ECU broken track

 

One quick repair later, and it's all working as it should. Thanks again for the info!

 

Great info from oldford there!

 

Did you replace the diode also?
Have you checked all ground straps to your engine?
Wouldn't want this to happen again.

 

Hi bud, yes I replaced the diode (it actually tested fine, but better safe than sorry!) and all of the earth straps are good. I have a couple of suspicions as to why it may have happened, neither of which will be repeated lol

Thanks again for the info, it's taken us a long time of tracing wires and figuring things (not to mention reading an awful lot of old threads and looking at wiring diagrams!) out before being able to actually ask the right question!

 

Fantastic fix there. Bet that was a sign of relief when it worked!

 

It certainly was. We actually fired it up today for the first time since we rebuilt the engine too - something of a momentous occasion as it's been off the road since 2011!