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Thread: Fuel pump power problem

  1. #1
    Join Date
    Aug 2021
    Location
    Whitehall, Pa
    Posts
    55
    My Cars
    1984 733I

    Fuel pump power problem

    I did the swap to the single fuel pump and everything works like a charm. Perfect. The car runs better than ever after a full recent rebuild.

    Heres the problem. I can not get the fuel pump to run without jumping it at the relay. I have replaced the main relay and the fuel pump relay with multiple different ones from a bunch of different retailers at this point and I just can't figure out what the issue is.

    Hopefully someone here will be able to help me out, otherwise its just gonna have to live with the jumper installed until I do.

    Thanks in advance!

    Michael

  2. #2
    Join Date
    Jun 2010
    Location
    TX
    Posts
    4,487
    My Cars
    1983/84/85 745i
    Does yours have the seven pin AFM? The FP relay closes via a connection in there on that model.
    I make E23 parts.
    09/1983 745i (stolen spring '13 around Houston, TX Achatgruen on nutria buffalo. 8481080)
    10/1984 745i
    11/1984 745i
    11/1984 735i (10:1-265/6)
    Ford, MB, and GM round out the pack.



  3. #3
    Join Date
    Aug 2021
    Location
    Whitehall, Pa
    Posts
    55
    My Cars
    1984 733I
    My AFM has 5 pins only its a 1984 733i. At least thats how many pins are on the plug.
    Last edited by Feyrerm; 06-19-2022 at 09:02 PM.

  4. #4
    Join Date
    Jun 2010
    Location
    TX
    Posts
    4,487
    My Cars
    1983/84/85 745i

    Thumbs up

    Okay, let me do some brain storming. You jump 30 to 87 and pump runs, yes? So basically sending power right to it? FP relay on later cars closes with signal from the flexplate or flywheel via the 'reference sensors' fed back to the ECU.

    You need to see if the ground at the relay is lacking or the switched 12V from the ECU is lacking.
    I make E23 parts.
    09/1983 745i (stolen spring '13 around Houston, TX Achatgruen on nutria buffalo. 8481080)
    10/1984 745i
    11/1984 745i
    11/1984 735i (10:1-265/6)
    Ford, MB, and GM round out the pack.



  5. #5
    Join Date
    Aug 2021
    Location
    Whitehall, Pa
    Posts
    55
    My Cars
    1984 733I
    I may have some more info. So I have been slowly working to get all the systems back up and working on the car. today I replaced the fuse in the OBC control unit. It had lights for the first time ever and came up to a 12.00 reading and a green light next to the code button. A few attempts to get it to do anything else and all hell broke lose. the thing is flashing crazy numbers and the horn is beeping... Dam and I thought it would be easy. Long and short of it, doesn't the OBC send a signal to the main relay to kick on the fuel pump relay? Could this OBC thing be my entire issue? Also what is wrong with my OBC? lol

  6. #6
    Join Date
    Jun 2010
    Location
    TX
    Posts
    4,487
    My Cars
    1983/84/85 745i

    Thumbs up

    Get your head under dash on driver side and bypass the ignition kill (green) out of OBC. Lots of people hack it, no. One wire is green, the other is green with a red tracer. Simply reverse the wires and no issues, I posted pics on here forever ago. You can find them...I think it was under Corey's (CMG) no start thread.

    Green out should be the switched 12V trigger to close the main relay. Then outputs red wire and red/white.
    I make E23 parts.
    09/1983 745i (stolen spring '13 around Houston, TX Achatgruen on nutria buffalo. 8481080)
    10/1984 745i
    11/1984 745i
    11/1984 735i (10:1-265/6)
    Ford, MB, and GM round out the pack.



  7. #7
    Join Date
    Aug 2021
    Location
    Whitehall, Pa
    Posts
    55
    My Cars
    1984 733I
    Thanks Hit Man X! I'll give it a shot. I went and tried to clean the OBC boards and reflow some bad contacts. Failed. Now it just blows the fuse as soon as I plug int the battery. Ordered a replacement, Hopefully that will fix it. I would like to have it working as intended Fingers crossed.

    I know I talked to you in another thread about my block issues. I Replaced the block and bought new pistons. The thing is running like a top. It's awesome. Re-conned the HVAC vacuum units too. I've never done a full engine rebuild before. I really thought the dam thing may never start again. The resources here on this board were the safety net I really needed to get the job done. Thanks!

    You had a piston question. I still have the originals I can send you one if you want.

    The only non functional systems on the car are the OBC and the cruise control. The car has no cat. Someone removed it and put a pipe in there and that pipe leaks... so that has to be welded or repaired, but otherwise its perfect.

    I've spent a lot of time under the dash on the driver side. the only thing I've found that seemed odd was a wire in one of the relays hanging out. it was on the right most relay from the door. I am not sure what that relay is for but the wire was for the odd pin in that relay. there is like a small pin and thats the wire that was not in the loom. I plugged it back in. not sure if thats the cause of my OBS fuse issue lol could be I don't know what the relay is.

    I also installed a keyless entry system under there because the drivers door lock for the key doesn't work. Now i can just lock it with the fob and all the locks work including the trunk and gas door. that was a shock.

    Anyway hopefully the new (used) OBC will fix the issue. If you know what the relay is I described please let me know. Then I can look up what the wire is in the troubleshooting guide to figure out what it does.

    Thanks for your amazing help!

  8. #8
    Join Date
    Jun 2010
    Location
    TX
    Posts
    4,487
    My Cars
    1983/84/85 745i
    Contact Kris Rose, he does mainly MB VDO cruise stuff but I sent him an entire E23 setup to repair these. He did mine in the 10/84 and works fine. Well did last I drove it.

    I put keyless kits in trunk, but I enjoy them too. Saves lock wear for sure.

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