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BMW E36 Battery Drain – ECU Relay Always On

March 29, 2011

This answer is applicable for many BMWs in addition to the one listed below.

car year: 1994
car model: 318i
Over the span of 10 hours the new battery installed will be in a state of discharge (11.2v) and unable to start the car. After some initial diagnosing, when the main relay (white) is pulled, the problem (discharge) is solved. I can leave the car overnight, plug in the relay in the morning, and the car starts, no problem. When I use a test light in the socket where the relay plugs into, I get power from contact 30 and 86 as expected from the wiring diagram ELE-74 in the Bentley Manual. The strange thing is I also get a “ground” from terminal 85 which is fed by the ECM(?). Is this correct? Should this relay be “hot” all the time? Which contact should the “signal” come from to close the circuit? Thanks for any insight-

You are correct, terminals 30 and 86 in the Main/ECU relay socket should have power at all times. Terminal 85 goes to the ECU, which then grounds this input in order to “turn the relay on”.  At rest, this wire should not be grounded as this will energize the relay.

So … if the main (ECU) relay is supposed to “turn-on” the ECU …. what tells the ECU to ground the input from the main relay?   When input from the ignition switch (green wire, terminal 56 on the ECU harness plug) is sent to the ECU, this energizes the run/start circuit and through this, the relay terminal 85 gets grounded (via brown wire to terminal 27 in the ECU harness plug).  This fully turns on the ECU.

If your terminal 85 is grounded all the time, you either have a short in the wire between the relay terminal 85 and the ECU terminal 27 or the ECU is faulty … or, the ignition switch is not “turning off” and the green wire to terminal 56 at the ECU is energized.

1)  Unplug the ECU harness plug and check for continuity from both the relay socket terminal 85 and the ECU plug terminal 27, to ground.  There should be no continuity (should be open circuit).  If there is continuity, the wire is shorted someplace.

2)  If above test shows ok, as it should, check pin 27 on the ECU, to ground (with the harness plug removed, and the ECU mounted in place or grounded by a jumper wire to the case).  There should be no continuity.  If there is continuity, the ECU is faulty as it is internally grounding.

3)  If above test is OK, check for power from terminal 56 in the ECU harness plug, to ground, with the ignition switch turned off.  There should be no power.  If there is, the ignition switch is faulty.

Let us know what you find.

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Categories → 3 Series 92-98 - E36

  1. Rob Brooks permalink

    With testing complete, wire from ECU to pin 85 on relay, ok. Pin 56 at ECU to ign switch is tested ok. Pin 27 on ECU does not show any short to chassis (ECU), however with harness and ECU installed testing pin 27 shows 8.4v at rest, which seems enough to trigger relay. With car running, it tested .78v. Does this indicate ECU issues? If replacing with new unit, will it need to be programed?

    Thanks Otto!

    • Rob,
      Hang-on, let me get the wiring diagrams back out …..

      Ok …. with everything installed, as normal, if you test from the brown wire at ECU pin-27 to ground, you would have voltage, since you are effectively grounding the wire and completing the circuit through the coil side of the relay. Therefore, this would be proper.

      If the brown wire from Relay pin-85 to ECU pin-27 is not grounding to the chassis (or another wire) someplace, then it certainly sounds like the ECU is at fault …. grounding pin-27 all the time.

      How about, cutting the brown wire (that goes from pin-85 on the relay, to ECU pin-27) near the ECU harness plug, or pulling the pin from the ECU harness plug? This would isolate the ECU from the circuit, but keep the wire in the circuit (in the potential case of the wire grounding …. even though your testing did not show that it was grounding). At this point, if all is well (the relay is not “turned on” and the battery is not drained), it looks likely that the ECU is indeed at fault.

      Let us know what you find and we can test a car here to verify the findings, prior to purchasing a new ECU …. if that’s where the tesing points.

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