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512i BB Fuse Problem

Discussion in 'Technical Q&A' started by DJK, Dec 29, 2003.

  1. DJK

    DJK Rookie

    Dec 18, 2003
    10
    I have heard about the hot fuse problem on tne Boxer fuel pump but I have a different situation,everytime I start the engine the fuse on my
    right bank blows and the engine starts and runs on the left bank only.
    Has anyone had this problem or any suggestions on how to correct?
     
  2. henryk

    henryk Formula Junior

    Dec 9, 2003
    479
    Door County, WI
    You appear to have a short somewhere. Possibly in the fuel pump, or relay.

    Take an Ohm reading across the right bank fuel pump, with wires disconnected.......if it reads zero, then replace the fuel pump. If it is OK, then take an Ohm reading of the wires going to the pump......if it reads zero, then there is a short in the wiring, or relay.
     
  3. jselevan

    jselevan Formula 3

    Nov 2, 2003
    1,849
    Henry - would you please expand on your Ohm reading suggestions? Not sure I understand where you are measuring resistance between.

    If a fuel pump is not working (I assume that is what is meant by 1 bank only) it is more likely a bad pump (which is frequently a problem in Boxers). The fuse design, as bad as it is, can lead to an open circuit in one of the two pump fuses, which will result in a non-functioning pump. I experienced this when the fuse block got so hot that it melted, and the geometry of the copper fuse holder (clips) distorted so much that the fuse fell out.

    However, if the fuse is in, and making contact, then the symptom at the fuse block will be heat. A great deal of heat. It will burn your finger. If there is no current flowing, then it will be cool. However, it is easy to be fooled as heat from one fuse will be conducted via the copper bus on the back of the fuse block, making the other fuse feel hot even if it is not electrically conducting. The heat, of course, is not good, and a result of high contact resistance. I posted a fix on F-Chat, and could be found via the search engine.

    Good luck.

    Jim S.
     
  4. f355spider

    f355spider F1 World Champ
    Rossa Subscribed Owner

    May 29, 2001
    16,557
    USA
    Full Name:
    Hugh G. Rection
    DJK,
    How quickly does the fuse blow? Immediately, or after a few seconds..or a minute? If it takes a short while, another cause is a clogged fuel filter...the pump would be working awful hard against the resistance of a plugged filter.
     
  5. DJK

    DJK Rookie

    Dec 18, 2003
    10
    Thanks for the replys,here is the latest. I took ohm readings across the(+) and (-) on the pump and did not have a open indication,I did the same with the wires and no open readings.I then made a fused wire and connected the pump directly to the battery and it blew immediately.I assume its a bad pump,now I have another question,the pump number on JRV's post is Bosch 058 254 947 the number on my pump is Bosch 058 254 975 does anyone know what the difference is?
     
  6. henryk

    henryk Formula Junior

    Dec 9, 2003
    479
    Door County, WI
    Jim S.......in answer to your question:

    I have found that sensors, motors, have some resistance between the two (+ and -) connections. Since the fuse blew, I assumed a short; where the + and - are internally conncted. Therefore, an ohm reading of zero would indicate a short internally within the fuel pump........reason for fuse blowing.

    This is my understanding, in a very basic sense.
     
  7. jselevan

    jselevan Formula 3

    Nov 2, 2003
    1,849
    Henry - thank you for the clarification. However, measuring the resistance across the terminals of an electric pump may be misleading. In a typical electric motor, the resistance across the windings will be very close to zero, as the motor represents a coil of wire. When turning, there is impedance developed as a result of "back EMF". Fuel pumps are not typical motors, and may more closely mimic a solenoid, depending on design. In either case, near zero resistance is to be expected when out of the car. (Caveat: make sure that the brushes are not situated over a split in the armature, which could appear as an open circuit.)

    That DJK is blowing a fuse suggests that the pump is not turning, and consequently no back EMF is being produced. This will result in extremely high surge currents, as the coil now appears as a short circuit to ground. An alternative is that the pump is laboring, which will also reduce the back EMF and result in high currents sufficient to blow the fuse. Finally, a short in the wire somewhere between the fuse connector and the pump is possible. All in all, most likely that the pump has gone south for the winter.

    A simple test of the last scenario would be to remove the positive lead from the pump, and turn on the ignition. If there is a short in the wire to the pump, the fuse will again blow. If the fuse does not blow, then the pump is the culprit.

    Off to read about Matt.

    Jim S.
     
  8. henryk

    henryk Formula Junior

    Dec 9, 2003
    479
    Door County, WI
    Jim S: Once again, thanks for a VERY complete, and easy to understand, explanation. I like the simple test you describe in finding a short in the wiring circuit.

    I am currently assemblying my TR motor, after doing the major service plus. In the process, I am recording the Ohm readings on the various sensors, and electrical components. This will be used as a reference. So that when I eventually get a problem (and we know I will), I can possibly use these readings to help eliminate, or find a faulty component.
     
  9. jselevan

    jselevan Formula 3

    Nov 2, 2003
    1,849
    Henry - you ARE organized. I never thought of measuring all these resistances when the engine was out for future reference. Tabulate these and publish on this site, and do mankind a great favor. Thanks.

    Jim S.
     

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