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alternator being fixed.. will my car be okay with the battery disconnected?

Discussion in 'Electrics, Vacuum, Ignition and ECU' started by james93, Oct 18, 2012.

  1. james93

    james93 Active Member

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    Hi everyone.

    Been a bit worried as I have my alternator in bits at the moment and while im waiting for the car I obviously cannot reconnect the battery as the main power to the alternator is disconnected so connecting it would cause sparks all over the shop.

    But I am really nervous as after reading about apparently having the battery disconnected can cause the rear sam to pack up!!

    I really cannot afford this as well to pack up.

    at the moment the battery is disconnected but am I really likely to break things by having the battery disconnected for a few days?

    James
     
  2. wireman

    wireman Senior Member

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    Insulate the fat cables lug at the alternator if you want the power on, a poly bag and insulation tape will do, tyrap the cable where its end wont get damaged/trapped/short.
     
  3. television

    television Always remembered RIP

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    Yes I know what you mean with an early 203 it really needs a power source less than 12v to power up with before connecting the main battery
     
  4. OP
    james93

    james93 Active Member

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    Thanks, I have reconnected it all while I wait for a new regulator to be delivered. What do you suggest to power the car with before connecting the battery?
     
  5. television

    television Always remembered RIP

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    Just do as you have done now, do not charge the battery and you should be OK
     
  6. Rappey69

    Rappey69 Senior Member

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    I know about how dodgy the w203 can be but before i knew this have jumped my car from another, used mine to jump another car, disconnected the battery a fair few times and just re-connected when ready and have had no problems.. guess i have been very lucky !
     
  7. Naraic

    Naraic Moderator

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    And me...
     
  8. cleverdicky

    cleverdicky Senior Member

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    So have I. Except I wasn't. :(

    If in doubt pull the fat red cable from the back of the sam before refitting the battery.
    Once battery refitted after a few min's reconnect it. Simple.
     
  9. OP
    james93

    james93 Active Member

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    Thanks for the help. This has to be the best forum I have been apart of in a long time, your all so quick to reply with your help!

    Just so you all know, as I have low funds at the moment and my car has a water cooled alternator I am repairing the voltage regulator with the alternator still in situ. I'm sure it been done before but if anyone is thinking of it, there is actually quite a bit of access around the back of the alternator from under the car.

    Fingers crossed it is the voltage regulator though! To do the diode pack is a tad more complicated!!

    James
     
  10. television

    television Always remembered RIP

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    Good one James and look forward on seeing how that one goes, it id seldom the diodes that fail, they are hefty things, if the slip rings are good you should be OK
     
  11. OP
    james93

    james93 Active Member

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    Thanks buddy!

    yeah that was what my thoughts were. the regulator seems to be quite a complex unit on these and chips can be quite sensitive at times.
     
  12. OP
    james93

    james93 Active Member

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    Hello again!

    quick update.

    I got my alternator all put pack together. plugged it in and all has been well no problems.
    until tonight.

    the undervoltage message came on again....

    I drove home and checked the battery voltage of which is at 11.6 volts, now im sure are night, cold and raining the drain on the battery after a windy 12 mile drive if the alternator was not charging anything would bring the battery voltage down to less than 11.6 as its normally that anyway. I checked the battery with the car running and it still read the same voltage.

    Now my latest thinking is that could it be the bumpy lanes where I live are causing something to be temperamental under the bonnet somewhere? its interesting how it was fine and then it kicks up again. . .

    if the alternator is not at fault where else should i check?

    James
     
  13. wireman

    wireman Senior Member

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    The alternator requires that a supply is delivered from V+ to the excitation/sensing (small) terminal, this is almost universally through the warning lamp (its connected from ign to alt sensing) and if you have a dodgy warning lamp or connection anywhere in its circuit it might be your problem.
    The warning lamp will not illuminate if this is the cause of the fault, the possibility of it being an intermittent fault could make it hard to pin down, such things as poor lamp contacts or in the instrument panel are potential candidates, perhaps even a dirty fuse/holder.

    Check that 12V appears at the small alternator terminal with the ignition on, grounding the wire should illuminate the warning. Check the current into the alternator excitation, expect about 100-200mA or thereabouts.
     
  14. television

    television Always remembered RIP

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    We have had a few cars of late where the exciter wire has been damaged, this being the case when you turn the key the lamp would not be on for the charging
     
  15. OP
    james93

    james93 Active Member

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    thanks for the info.

    now my findings today show me that i have 10/11v in both wires that plug into the voltage regulator..

    that does not seem right does it? in my haynes it has the blue wire shown going to the engine fuse box but it does not show the other wire. does this mean it is meant to ground somewhere?

    james
     
  16. wireman

    wireman Senior Member

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    Both wires should have the battery voltage on them, except when the engine is stopped when the thin excitation wire will have considerably less, perhaps as low as a volt or two.

    The essential ground connection is normally derived from the regulators mounting bolt or a sprung contact on the reg/brush pack against the alternator casing.

    With the engine not rotating, The excitation/sensing wire supplies power to the rotating field coils to provide the magnetic field which makes the alternator generate power when rotating, unlike the old dynamo there is no retained magnetic field and electricity must be supplied to "boot up" the alternator.
    Once rotating an extra set of output diodes from the stator windings supplies the excitation power and the current drawn into the excitation terminal via the warning lamp diminishes to almost zero. Under very light electrical load the current could in fact become slightly negative.

    The regulator is interposed between the excitation terminal and the field coil, it measures the voltage on the excite/sense terminal and switches the field off if the voltage is over >~14v and back on when it falls bellow <~14v, the cycle time is short, in the order of a few tens of milliseconds or less.

    Did you inspect the condition of the slip rings and were the old brushes worn down?
    The electrical resistance of a typical field winding (between the slip rings) is around 2-10 Ohms.
     
    Last edited: Nov 2, 2012
  17. OP
    james93

    james93 Active Member

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    Update!!

    took the brushes out had a look.. nothing there put it all back again and when I started it this time I heard a slight cruch noise after it started. and now its charging.

    So would this be pointing to that free wheeling pulley that seems fine when i had the alternator off be having a fault where its free-wheeling when it shouldn't be?

    I did also charge the battery while I had it unplugged....
     

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