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Shift knob electrical connector only giving 5v, constantly.

Discussion in 'Z3 Roadster & Coupé' started by DivideBYZero, Feb 3, 2014.

  1. DivideBYZero

    DivideBYZero Zorg Legend

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    As title. I fitted a lit shift knob, official BMW M sport 5 speed unit, but when testing the unit prior to putting it all back together, the knob wouldn't light. Tested with lights on, off, with engine on, all sorts. Nothing. So got the multimeter out, and the connector was only supplying 5v, but all the time, regardless of ignition, lights, anything.

    Anyone got any tips on why this might only be pushing 7v too few and all the time?
     
  2. Brian H

    Brian H Zorg Guru (V)
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    The illuminated gear knob works of the same circuit as the instrument lighting, turning the rheostat on the light switch from side to side when the light switch is either being used for side lights or low beam controls the brightness of the instrument lights. Are your instrument lights working OK?

    Can you confirm the colour of the wires going to the gear knob? They should be a brown (gnd) and a switched wire (slate/red).

    A word of warning, if you short out the circuit it will blow the rheostat in the light switch and ALL instrument lighting will be lost.
     
  3. DivideBYZero

    DivideBYZero Zorg Legend

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    The wires were brown and slate with a red stripe. The black connector only fits one way.

    All the dash lights work perfectly. Odd.
     
  4. FRANKIE

    FRANKIE Zorg Guru (V)
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    About 10 years ago, I took a class on micro electronics. It was at the local vo-tec. college and I was supposed to use this in conjunction with slot machine repair. The micro circuits, including leds all used a 5v system. Whether this has anything to do with your application or not, I can't say. Frankie.
     
  5. DivideBYZero

    DivideBYZero Zorg Legend

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    Anything is possible, Frankie. Most, if not all feeds should be 12v negative earth. However, a work colleague only yesterday told me a story about a 1977 Ford Fiesta that had a 7v feed direct from the alternator...!
     
  6. FRANKIE

    FRANKIE Zorg Guru (V)
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    I agree that all feeds are 12v, however, the feed here is the US in the house is 120v, but the computer components don't run on 120v, and the tv doesn't run on 120v. All voltages are reduced once they enter the component. The electric that runs through the circuit board in the radio in your car isn't 12v. So what I'm saying is that the led in your shift knob may not necessarily use anything but 5v. I just really don't know. What I'm saying is that don't automatically dismiss the 5v reading you're getting for the shift knob as an error until you're sure that's not what it is supposed to be for the knob to operate correctly.(I must admit that I'm rather surprised, at this point, knowing the mindset some of our fellow forum members can get into, that no one has broached the topic of knobs with lights. They must be sleeping?) Frankie
     
  7. DivideBYZero

    DivideBYZero Zorg Legend

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    The knob is a standard bmw part, so I wouldn't expect too much uproar. I had an illuminated shift knob in my e46 M3 as standard equipment . :)

    I know what your saying regarding voltage control within components, but this should be a simple 12v bulb at the end of two wires.
     
  8. FRANKIE

    FRANKIE Zorg Guru (V)
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    Then you're in trouble.
     
  9. Brian H

    Brian H Zorg Guru (V)
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    Can you try and measure the voltage on the Slate/Red wire using another ground? if the instument lighting including the back lights for the heater control etc. all work correctly it sound like faulty wiring to the black connector. I have not traced the loom as it disappears under the centre console so I don't know if it splices in to the heater control backlights, Ill have a look at the ETM later on.

    Edit: Had a look at the diagram and it is a simple ciruit that is common to all dash and switch illumination. It has a feed wire from the light switch and a ground.

    [​IMG]
     
    #9 Brian H, Feb 5, 2014
    Last edited: Feb 5, 2014
  10. DivideBYZero

    DivideBYZero Zorg Legend

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    Thanks, Brian. I'll try testing with another ground. If it's no good I'll probably end up using the ashtray light feed. I don't use the ashtray and its illumination is pointless. I've only recently had the entire center console out, so I'm ok with working under there.

    There is also a very small white connection under there too, but it doesn't expose enough metal for me to get my multimeter pins into it to see what it has.
     
    #10 DivideBYZero, Feb 5, 2014
    Last edited: Feb 5, 2014
  11. jonco

    jonco Zorg Guru (III)
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    @Brian H - you may recall the saga with my alarm push button circuit you helped me with last year. The permanent supply to that was 5v - is this not a similar low power circuit?
     
  12. Brian H

    Brian H Zorg Guru (V)
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    @jonco I initially though of this but then though it should have 3 wires if it was for the alarm switch.

    Interesting you have a small white connector as I replaced my illuminated gear knob last year and had to cut off the original white connector as the new gear knob had a black connector.

    Original connector I had to cut off.

    [​IMG]
     
  13. DivideBYZero

    DivideBYZero Zorg Legend

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    Yep, that's the white connector... Your previous illuminated knob used it, did it?
     
  14. Brian H

    Brian H Zorg Guru (V)
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    Yes, my new gear knob came with a different black connector to the original one so I cut both connectors off and used one of my own.

    [​IMG]
     
  15. DivideBYZero

    DivideBYZero Zorg Legend

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    Looks like that's my route. Thanks guys.