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Thread: Fuel pump relay connector measurement less than a volt

  1. #1

    Default Fuel pump relay connector measurement less than a volt

    Hello All,

    My son's car is a 1993 525i that's powered by a M50TU engine. The problem he's having is that it appears there's not enough voltage being supplied to the fuel pump. We thought that maybe the fuel pump was bad so he replaced it, but after installing it the engine still doesn't start. The troubleshooting that we've done are as follows: checked the fuel pump fuse, it wasn't blown; sprayed fuel directly into the throttle body, which does causes the engine to start; removed the new fuel pump from the tank and powered it directly, it worked fine; connected a jumper to the fuel pump relay terminals 87 and 30 and measured the voltage at the fuel pump's connectors 4 (-) and 5 (+), the resulting voltage measurement is less than a full voltage (0.09).

    If anyone has any ideas as to how I can resolved this problem, please let me know. Lastly, I also read some information on another thread that mentioned something about fuseable links so if anyone has pictures of the location of the fuseable links, please send them to me.

    Cea123

  2. #2
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    I would ask if you did measure also the voltage at the B+ post? Was it also that low? (Guess you verified the DC voltage setting to the tester, Not AC setting).

    I was left in the road once by the Fuel pump fuse, it was not blown but mechanically damaged so it looks fine but was opened (where the filament meets the crimped lead), a quick exchange with a similar fuse in the box will rule this out.

  3. #3

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    Hi Javier,

    No, I didn't measure the voltage at the B+ post. Where exactly is the B+, is it one of the fuel pump relay post and is it also suppose to measure 12 voltage? (Yes, I use DC voltage).

    I'll change out the current fuse as a good measure.

  4. #4
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    B+ post is a small square black plastic cap with a red + hiding the Battery + post in the engine compartment (Guess you also have your battery below rear seat, so this is a handy access to the Battery +). It will reveal the Battery voltage (though I'm sure you have it right as the car cranks Ok). Measuring B+ post after reading a funny lecture will rule out a tester issue.

    The Fuel pump fuse is after the Fuel pump relay contact, so jumping 30-87 does not feed the pump if there are fuse issues, no mater you can read 12V+ at pin 30 of Relay pump socket. If switching fuses does not solve the issue, you should test for voltage at the F23 (With 30-87 relay pins jumped, once verified 12V+ at relay pin 30) to verify voltage reaches the fuse, and if fuse is powered at both sides, you will have to rule out a wire issue from fuse to pin 5 in the pump/sender connector (measuring 12V + at pin 5 to ground). If Ok, be sure to confirm there is a good ground at pin 4 of the pump sender connector. Remember measuring no voltage between pin 5 and 4 does not necessarily means you are missing 12V feed, but also a bad ground at pin 4.

  5. #5

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    Javier,

    Here's what I found after checking the things you mentioned. After jumpering terminals 30 and 87 the voltage at terminal 30 is just above 3 volts, the voltage at F23 is 12v on one side (the side nearest the driver side fender) of the fuse terminal, the other side reads 1.77v, and the reading at the fuel pump's connector pins 4 (-) and 5 (+) were 0.04. Next I'll try to location where pin 4 ground leads to, which seems to require that I lift the carpet alongside the passenger doors. If you can tell me where the pin 4 ground terminates, I'd appreciate it greatly or a picture would be worth a thousands words.

  6. #6
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    The only way a fuse can read 12 volts at one side and 1.77 at the other is it is opened or somehow damaged, The fuse resistance is insignificant and the only way it could drop 10+ volts is that the current is extremely high which would lead definitely to a fuse blown. Also terminal 30 is directly connected to B+ post, so it could not have 3 volts unless wire damage between B+ post and relay socket pin 30, additionally, it feeds F23, so voltage can not climb from 3 to 12 in its way to fuse F23. Something is wrong with your readings (probably the way you are taking them).

    Please consider following procedures:

    Find a very good and solid ground connection in the vicinity of the Fuse box (that is also besides the B+ post, I usually use the shock strut nuts). Hook there the black lead of the meter in a way that you can steadily and repeatedly get a 12V + reading at B+ post (using red lead of course).

    Jump 30 to 87 in the pump relay and remove pump fuse F23. Take voltage readings at both posts of the fuse, one should read 12V+ and the other 0V. Note the one with 12V+ is the upstream terminal of the fuse (probably the fender side?), and will always read 12V+ volts as long as 30-87 is jumped.

    Replace the fuse and get the reading at the side it was 0V, if it is not now identical to the voltage at the upstream terminal of the fuse, then the fuse is gone and should be replaced.

    If you get 12V+ at both sides of the fuse, proceed to the trunk and:

    Find a good solid ground in the trunk area (an exposed metal of the body will do) and hook there your black lead. Take the voltage reading at terminal 5 of the pump/sender connector, should read 12V +, other wise there may be a broken wire from fuse F23 to terminal 5 at the connector (very unlikely).

    Set your tester to Ohms reading and check the resistance between terminal 4 of the pump/sender connector and the previously identified ground, it should read near 0 ohms. Otherwise, there may be a broken wire from terminal 4 to ground.

    Your readings, up to now, do not take the fault out of the fuse box. Discarding the 3V reading at terminal 30, which seems wrongly taken, the rest of your readings leads me to an open fuse.

    I'll be checking this thread more frequently so to be able to provide you quicker responses.
    Last edited by Javier; 03-19-2015 at 06:59 PM. Reason: Clarifiying

  7. #7

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    Javier, I'll redo the measures and let you know the results. In my last reply I intended to ask you whether or not the DME relay could somehow be faulty.

  8. #8
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    Once you bypass the Pump relay (30-87 jumper) you remove all control automation on the fuel pump, so if it does not run after bypassing, the culprit is not the DME relay or Pump relay, or the DME itself. The only remaining components are the fuse F23 and the pump itself, besides wiring. It is not usual to find wire issues, contact issues may be, but not wire issues. Overheated contacts in fuse, relay, connector sockets and ground connections may lead to high resistance current paths dropping the available voltage to values below operative requirements, but you would notice through visual inspection such damages. I have not seen this damages in the fuel pump circuit, but I do have seen fooling fuses (visual inspection of the filament presence is not enough to check a fuse Ok).

    Regarding your interest in locating the grounding point for the pump (Terminal 4), I'd suggest you to download from Shogun's web site http://twrite.org/shogunnew/topmenu.html => Reference Data => wiring diagrams => wiring diagram others => E34 directory of files => e34_93.pdf (Bookmark Shogun's web site, it is plenty of resources).

    In the downloaded pdf look after page 7100.0-02, figure 2 and identify location of X495 underside the rear seat Right hand side.

    Page 1210.13-02 shows the Fuel pump circuit, note that B tag leads to B+ post.
    Last edited by Javier; 03-19-2015 at 08:49 PM. Reason: Clerical

  9. #9
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    By the way, you can find in Walmart this funny tester for fuses. And can be used without removing the fuse if space permits.
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  10. #10

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    Thanks, Javier! I'll certainly invest in one of these!

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