BMWLink E36X not powering on

[Giraffe2012]

When connecting my ECU I don't see any power, no blue LED's, not able to connect. When I jump the main relay pins 30 and 87 I will get the ECU to power up and I am able to connect. However I believe the ECU is supposed to trigger that relay at pin 85 on the main relay. All ECU grounds have ground and the power seems to have power. I'm starting to think it may be a bad ECU, any advice? This is in a 1995 E34 525i.

[Adamw]

Is there 12V on pin 56?

[Giraffe2012]

Yes, 12V at 56. All ignition pins were correct. 2 had 12v and 1 had ground. I would have to confirm the other pin numbers, but every ground had ground and every power had power.

[Adamw]

What is the voltage on Pin 56 and pin 27 with the ignition switch on?

[Giraffe2012]

12V at 56, 0V at 27. 27 also does not have continuity to ground.

[Adamw]

If the ECU was not grounding pin 27 to activate the main relay then you should see 12V on that pin.   You should see 0V when the ecu is grounding it to activate the relay.  

 

 

[Giraffe2012]

I see 500 ohms at that pin. Im thinking it isn't enough to ground the pin. I do think it is strange though that I see power when jumping the two pins as I am not sure what is getting power as a result to wake the ECU up. I wonder if I am misunderstanding the diagram and the ECU needs a signal at pin 27 either 12V or gnd?

[Adamw]

The main relay/pin 27 is controlled by a FET inside the ECU, you can think of a FET as a very efficient switch.  The FET is controlled by the ignition switch/Pin 56.  

When the ignition switch is off, the FET is effectively "open", giving a circuit roughly equivalent to below. With the FET open you have no current flow through the circuit so you should have 12V at point a, b, c & d.  If you measured voltage across the relay (c&d) you should have 0V (because same volatge on both sides).

 

 

When the ignition switch is on, the FET has a very low resistance to ground, effectively pin 27 is switched to ground, something like below.  If you measured voltages in this state you should have 0V at a, 0V at b, 12V at c, 12V at d.  If you measure across the relay (b&c) you should measure 12V.  The relay should be energised and the contacts pulled closed.  

 

 

[Giraffe2012]

Thanks for the explanation, I double checked, pin 27 is 12V with ignition on and off. Confirmed at the relay and back probing the ECU connector.

[Adamw]

If pin 27 is at 12V when pin 56 is at 12V then that would indicate the FET is fried/damaged.  It is pretty unusual since they have several protections and a huge current rating so it usually needs a pretty significant event to damage one - like the battery connected backwards or something.   It's not impossible that it just failed on its own but it was at least working when it when through its production testing after several hours of burn-in testing.   

Anyhow, start by contacting [email protected] to get it back for inspection.  The service team can usually determine the cause, if it doesnt appear to be related to a wiring issue then it will likely be a warranty repair.  If there is strong evidence that it was caused by a wiring issue then they will let you know if there is going to be a repair cost.   

[Giraffe2012]

Ok, thanks so much for your help. Much appreciated.