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Thread: Relay delete

  1. #1
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    Relay delete

    Track only/trailered E36 M3.
    I'm down to 10 fuses, and can easily get to 6 or 7.

    But, 5 relays.
    ABS
    ABS pump
    O2 Sensor
    Engine Control Module
    Fuel Pump

    Looking at the ETM, the Fuel Pump relay is controlled by the DME. But for a track car (crash module removed, EWS removed, etc), why not power the fuel pump with 'Master ON'? (Keyed switch on steering column was removed and replaced with master ON and Start switches). So, the fuel pump would run 100% of the time with Master On.

    Hard to tell what the DME does with the O2 sensor relay and Engine Control Module relay. Would love to remove these also (and manually switch appropriately) if anyone has insight.

  2. #2
    Join Date
    Sep 2016
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    01 330Ci
    You'll still need a relay for the fuel pump primarily due to current draw. The idea of a relay being you don't have to run heavy gauge wire all the way to the switch. You can do it if you want, but you'll need to know the max amp draw of the fuel pump and the max amp rating of the switch, then run the appropriate gauge wire where it needs to go. Better to stick with the relay I think.

  3. #3
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    Yea, thanks, understood. Also, when the activation input of the relay is connected to a control unit, multiple events can switch the relay. Which is another reason the OEM configuration would do this (crash, theft protection, engine off, etc).
    The fuel pump(s) are a 15Amp ckt, with 1.5mm^2, which is about 16 gauge. Easy to switch and run from my switch panel back to the fuel pumps without a relay (fused, of course).

  4. #4
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    Tracking. Either way will work and it'll be nice to have full control. My setup is switched at the dash to a relay that turns on the pump in the fuel cell (well, will be once I install the parts).

  5. #5
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    I've pretty much decided to delete the fuel pump relay and wire the pump direct to a switch. 15A switches are cheap, and the wiring is already there. This is really enabled because of removing all the other control conditions the DME turns on/off the fuel pump, and that I'm moving some of the fuses to the passenger compartment.

    A little research on the O2 sensor shows that the heat for "most" 4 wire O2 sensors are full time. So I could delete that relay too. But...then there'd be an O2 sensor failure mode that the DME would no longer be sensing. So. Not worth it.

    In short, I'll be down to 6 fuses and 4 relays. I'm good with that.

  6. #6
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    '95m3(Avus)/996tt/4.8is
    15777311288124164571773466349830.jpg

    My setup, not sure its helpful but thought I would share.

  7. #7
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    Quote Originally Posted by olemiss540 View Post
    15777311288124164571773466349830.jpg

    My setup, not sure its helpful but thought I would share.
    Thanks. Interesting that you don't have an O2 sensor heat relay.
    O2relay.png


    My fan is about 15A startup current, and 6A run current. So I just wired it straight w/o relay.
    Last edited by aeronaut; 12-30-2019 at 06:37 PM.

  8. #8
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    How would the PCM activate the fan? Mine has a switch but is also auto activated by the PCM at temp via the relay. The PCM wiring is something like 22 ga....

  9. #9
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    '99 M3, '10 X5 35d
    Quote Originally Posted by aeronaut View Post
    I've pretty much decided to delete the fuel pump relay and wire the pump direct to a switch. 15A switches are cheap, and the wiring is already there. This is really enabled because of removing all the other control conditions the DME turns on/off the fuel pump, and that I'm moving some of the fuses to the passenger compartment.
    So you are terminating the connection of the pump to the DME? That isn't going to cause an internal issue with the DME? I'm not familiar with how the pump interacts with the DME but, based on what you're doing, I assume it's only acting as a source to activate the relay?

    If I am inferring this correctly, I might follow your lead. Would simplify another item for me.

  10. #10
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    Quote Originally Posted by olemiss540 View Post
    How would the PCM activate the fan? Mine has a switch but is also auto activated by the PCM at temp via the relay. The PCM wiring is something like 22 ga....
    Mine isn't. Manually switched only.

  11. #11
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    Quote Originally Posted by BlackHawkRacing View Post
    So you are terminating the connection of the pump to the DME? That isn't going to cause an internal issue with the DME? I'm not familiar with how the pump interacts with the DME but, based on what you're doing, I assume it's only acting as a source to activate the relay?

    If I am inferring this correctly, I might follow your lead. Would simplify another item for me.
    Good catch. The list of OBD1 codes includes "1261 Fuel Pump Relay Control". HA!! Well, it may not be worth it, but, I hate all those relays. I'll probably still remove, and just wire a resistor across the DME output & ground. I had to do this with the charcoal canister sensor/thing.

  12. #12
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    For the record, a little research and I've found that our O2 sensors need 700degC to operate properly, and mine is mounted fairly far down stream. And, if removing the relay, the DME wouldn't know if the heat element had failed and I'd get no code if failed. So I'll be keeping the O2 sensor heat relay.

  13. #13
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    A note for OBD2: OBD2 E36 M3's has the O2 relay integrated into the "ENGINE CONTROL MODULE RELAY" next to the fuel pump relay. It is not broken out separately like the OBDI cars.
    Last edited by BlackHawkRacing; 01-01-2020 at 06:44 PM.

  14. #14
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    Quote Originally Posted by BlackHawkRacing View Post
    A note for OBD2: OBD2 E36 M3's has the O2 relay integrated into the "ENGINE CONTROL MODULE RELAY" next to the fuel pump relay. It is not broken out separately like the OBDI cars.
    Hmmm...very interesting. Maybe I merge!

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