M50 lambda sensor relay / ecu earth question - help needed

[joeseth]

As per my other post, fitting a lambda sensor to my m50b25 e30.

The problem I’ve hit now is the lambda relay does not energise. There is power to it and the relay is good. But it does not seem to be earthing through the ecu, pin 38, as it should. Connection between relay and ecu is also good. Therefore I determin that either the ecu is dodgy or in is not earthed properly.

So.... checked all the ecu plug pins that are earths and all are good except pin 15 does not earth. The wiring diagram I’ve got doesn’t show what colour wire comes from pin 15 or where it should earth. Anyone know about this earth or if it’s even related to the lambda relay?

Complicated question I know sorry!

Cheers, Joe

[Brianmoooore]

Pin 15 is not an earth as such, although it is a brown wire. It is connected to the 'earthy' end of the HV winding of the ignition coils, and is connected to engine earth via a resistor near the coils. This results in a small voltage pulse being fed back to pin 15 every time a spark plug fires, allowing the ECU to detect a misfire.
O2 sensor relay is turned on by the ECU earthing a brown/green wire via pin 38, as you say, but the 02 heater is only operated when the ECU decides it needs to be.

[joeseth]

Ok so the Heating circuit and relay not being live may not be a problem? I would have thought it would be live when first starting up the engine, until the exhaust heats up. But then again would be running open loop then so may not use the lambda when cold. May just have to fit the lambda sensor and see what happens!

[Brianmoooore]

Connect a small bulb to between pin 87 of the relay and earth, and watch to see when and if it lights.

[joeseth]

Just in case anyone else has this problem, the ecu powers the lambda heating relay after about 8 mins of the engine running from cold. I assume this is correct and have stopped messing with it!