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Thread: Advice please: weird electrical problem

  1. #1
    'LS' Member
    Join Date
    Aug 2013
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    Question Advice please: weird electrical problem

    (X-posted at The AllCorsa forum)

    Apologies in advance if too much detail, but rather that than too little.

    Head unit packed up in SWMBO's Corsa C. Took the opportunity to replace with one that has the bluetooth, etc, that she wanted: Sony MEX-BT3100U

    When I installed it, it wouldn't work; I tried the old trick of swapping the yellow and reds around and all seemed OK -- until the ignition key is removed, when it loses all memory.

    I also have a SONY (older, etc) in my car, so I tried to isolate the fault by swapping them round. Both work fine in my car; neither works properly in hers, so I resorted to a voltmeter. This is where things got weird!

    With the ignition off and radio disconnected, the permanent supply (yellow) and switched supply are both 12.xV
    When I connect the radio, the yellow pd drops to around 8V.
    When I connect the radio and switch on the ignition, the yellow pd drops to around 2.5V

    The red pd appears to behave as it should (drops to a tad under 12V until engine is running).

    I assume this behaviour on the permanent supply is not normal, so:
    • Is it a known "feature" and, if so, how do I rectify it?
    • If it's not easily fixable, is there somewhere else I can run a permanent supply from easily?
    • If I run a permanent supply from elsewhere, shuld I fuse it, and with what value fuse?
    • Is there another obvious-to-everyone-else question I should have asked (& if so what is it and its answer?)?

    I know next to nothing about auto-electrics, so imbecile-proof responses will be welcome!
    Last edited by Tetenterre; 08-08-2013 at 06:10 AM. Reason: tidy up

  2. #2
    'GLS' Member
    Join Date
    Jul 2013
    West Nottinghamshire
    1.0 Corsa 'C'
    Thanked 20 Times in 19 Posts
    Welcome to the Forum.
    I see that you still haven't got a definitive answer to this problem yet, so let's try and reason it out.
    It seems to me that what is happening with the yellow (are you sure that it is yellow?) feed when you measure the voltage there, is that there is some high resistance somewhere 'upstream' I would bet diamonds that if you were to use a test light in place of your voltmeter, the bulb would fail to light, due to it being relatively low resistance and is swamping the supply, whereas the high impedance of your VM registers the voltage, but alas there isn't enough current there (due to to the 'upstream' resistance) to allow your radio to operate.

    I would suggest that you try cleaning all of the connections, including the fuse holder (you MAY find that it is the fuse located behind the battery - not sure though)


  3. The Following 2 Users Say Thank You to walksall For This Useful Post:

    Aaron (13-08-2013),Tetenterre (13-08-2013)

  4. #3
    'LS' Member
    Join Date
    Aug 2013
    Thanked 0 Times in 0 Posts
    Thank you! It was indeed the fuse-holder behind the battery. All cleaned up, working like a dream.

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