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Thread: Newly acquired Motronic 745i: Crank, no start. Bad DME?

  1. #1
    Join Date
    Dec 2017
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    Columbus, OH
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    18 GTI, 84 745i, 79 733i

    Newly acquired Motronic 745i: Crank, no start. Bad DME?

    Hi everyone! I recently acquired an 84 M106 Motronic 745i with a no start condition. I'm all but certain that it's the DME, but I wanted to get a second opinion in case I missed anything.

    Here's some background:

    Right after I got it, I immediately pulled the DME out of the car, cracked the case open, and re-flowed every major sketchy-looking joint with new solder. No burned traces or obvious damage, but some of the joints looked cracked and tired. Put it back in the car, cranked it for between 5-10 seconds, and it burst to life. Of course, it hasn't ran in 10 years, so it ran like crap considering the gas in the tank. It stalled and I saw that one of the rubber lines coming off of the fuel rail was gushing fuel. Not surprising. I replaced all the rubber fuel lines in the engine bay and the injector o-rings. I then dropped the gas tank and spent the last three weekends cleaning, re-sealing the inside, and painting the outside. Didn't try starting it again between the new lines and dropping the tank. When I put the tank back in, I replaced the rubber lines under the car, the filter, and both pumps. For the in-tank, I used the Airtex Vega pump; for the main, a Walbro 255lph.

    Tried to start the car with clean gas and it wouldn't fire. My immediate thought is that one of the pumps is bad. Checked both, both run when jumped to the battery. Swapped the old main pump in and same result. Checked the voltage, nothing on key on or crank. Checked the physical grounds in the back, no issues. Pumps get power and pressurize the fuel rail with gas when I jump 30 to 87 on the fuel pump relay. It appears that the injectors won't fire even with the relay bypassed and I don't think I'm getting spark either. I know the DME grounds the circuit, so I'm assuming it's not getting that signal.

    Here's some other checks I did:

    - Checked continuity on the speed and reference sensors. One is 964, the other is 965, well within spec. They also look practically new. Checked at both the sensors and at the DME 35-pin connector. Switched the plugs around just to see if it would run poorly, but nothing. Just in case and so I have a spare, I ordered an extra sensor today.

    - Checked grounds and power feeds at the DME connector, all responding correctly. Performed other 35-pin tests from http://www.rtsauto.com/ecu-pinout-an...cal-tests-e30/ pertaining to fuel and spark.

    - Grounded injectors at DME connector, 1-2-3 and 4-5-6 click when grounded on their respective pins.

    - Grounded the fuel pump relay from the DME connector pin; pumps run.

    - Jumped the main relay: 30 to both 87 and 87b. No change.

    - Checked voltage at main relay with ignition switched off and on. Results correct: 12V where there should be power.

    - Physically examined ignition rotor, looks fine.

    - New and fully charged battery.

    - Fuses not blown. Cleaned fuse box and main vehicle grounds near battery.

    - Swapped ignition coil out with the confirmed working one from my brother's E30, no change. Also tried another cylindrical one from an old Golf or Mk1 Cabriolet that I used to have. Coil positive pin is getting 12V, but still no spark.

    As far as I know, the only things I haven't checked for is the TDC trigger pin on the flexplate (to see if it broke off) and the CTS. I figure that because it was running before and hasn't moved since, it shouldn't be either of these things. Thinking about taking a look this weekend when it's dry and bright enough to crawl under there. Also, I didn't bypass the OBC box from under the dash, but by bypassing the main relay and checking the pin voltage, shouldn't this take it out of the equation anyway? I've tried to do my Motronic homework...the electronic troubleshooting manual has been a life saver.

    From what I can tell, it sounds like my "repaired" DME doesn't want to stay fixed, because it seems like it's not sending or receiving the correct signals for ignition and/or fuel. Any ideas would be entertained and are certainly appreciated. Also, if anyone is selling a DME, PLEASE let me know. If it didn't have the 4HP22EH automatic, I would've already started assembling a MegaSquirt kit, but I can't find any definite information on how to get the correct fuel load signals to the TCU so the trans doesn't go into limp mode. If anyone has any details on this, I'd appreciate that too. I would be fine with a 5-speed swap, but I'm not counting on seeing a full Getrag swap pop-up in the near future!

    Thanks in advance!

  2. #2
    Join Date
    Mar 2013
    Location
    USA
    Posts
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    My Cars
    E21/early e23/e34
    Hopefully hitmanX will chime in, he's the go to guy on these cars-
    Look a cple of posts under yours- There is a ECU for sale for the 745i, double check if for 102 or 106 motor. Sounds like you should go back to the point where the car was running, even poorly and look over everything you did just after that-
    Doesn't hurt to have a back up ECU regardless.
    Last edited by kojo96; 03-28-2018 at 05:05 PM.

  3. #3
    Join Date
    Dec 2017
    Location
    Columbus, OH
    Posts
    17
    My Cars
    18 GTI, 84 745i, 79 733i
    Thanks for the response. Yeah, it's got the correct part number for the M106. I commented on that thread a couple days ago, but the seller hasn't gotten back yet. We'll see. Everything else is as it was when it started.

  4. #4
    Join Date
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    I'll reach out to Steve, have him get on here.

  5. #5
    Join Date
    Jun 2015
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    Los Angeles
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    1981 Euro 732i
    Quote Originally Posted by kojo96 View Post
    I'll reach out to Steve, have him get on here.
    Thank you kind sir.

    BMWe23, you got PM.

  6. #6
    Join Date
    Dec 2017
    Location
    Columbus, OH
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    18 GTI, 84 745i, 79 733i
    Great, thank you so much!

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