Have a problem, The ecu relay seems to have stopped working, i've traced the problem back to pin 10 on ecu(its the negative path to the ecu relay). When i connect a negative straight to relay the car starts fine, it seems the ecu is not outputting a negative so to speak.
Why does the negative route through ecu in first place, is it for safety, engine cutout ? overboost protection ?
Why would it stop working ?

Cheers all

 

Can't comment on that specific case. But in a lot of cars it's just there purely so things cannot be powered up without the ecu being plugged in. No fancy control, just a direct earth path through the ecu to the relay.

On some more modern cars, it can be because the ecu can remain live for other things for a period after key off for example, so the ecu has a section that can allow this by controlling the power relay. I doubt that's the case with this old ecu though.

So you could check for grounds at all the other relevant pins. Generally inside an ecu, all power grounds will be linked so you could bell test them to see if this is the case.
As well as checking chassis/battery grounds.

ECU sensor grounds should not be linked to power grounds.

 

Found the problem

 

This is a common issue, I've repaired them before by bridging the connections you have identified with an external piece of wire.

The relay as you say is grounded via pin 10, the only plausible reason I can think of is an immobiliser, which the ECU's don't have/support anyways.

I was told the cause of this issue is people starting them without the battery earths connected, not sure what truth there is in this.

It's not overboost/hard cut protection, as this is achieved by cutting fuel injection.

Martin

 

I dug a bit deeper, the relay in question must of got wet/damp at some stage as was heavily corroded inside, thus (i assume) in effect caused a short circuit/ low resistance which blew the track.