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  1. #1
    Join Date
    May 2005
    Location
    Near Guildford, Surrey, England.
    Posts
    1,470

    Cobra free of charge!

    Hi,

    its been a long time since I posted...hope you're all well. I still have the car and its still going ok (Pilgrim sumo Mk3 , RV8 completed about 10 years ago) I took it for its MOT today and found the battery isn't charging. Before I start unwiring things and doing some tests, is there anything obvious to check?

    The alternator is spinning, belt has good tension, but I'm not getting any additional voltage back to the battery. I've confirmed this with a meter.

    I recall reading something about there being some delicate electronics in alternators that can fail?
    I don't recall there being a fuse or anything in that circuit?

    What's also slightly odd is that the volt meter on the dash is reading about 10V, yet a handheld meter shows the battery is fully charged and holding just under 13 (car starts perfectly)

    Anyone got any advice of what to check next?

    cheers

    Graham
    Last edited by chedz; 06-08-20 at 09:59 AM. Reason: SQL Error:100 - row not found
    Share your Cobra knowledge here on the Sumo Wiki
    ...and don't forget, building cars is meant to be fun!!

  2. #2
    Join Date
    Feb 2007
    Location
    West Sussex
    Posts
    9,445
    Hi Graham!
    Could be a dead diode on the rectification pack on the back of the alternator.
    If you have access to an oscilloscope, this would tell you.
    Or you could try switching the mutlimeter to AC Volts as well as DC volts and comparing.
    More here:
    https://www.brighthubengineering.com...-oscilloscope/

    If not, I'd suggest new alternator.

    Kevin
    Crendon Chassis No.49
    Huddart FE428 + toploader

    Not listed in the Shelby Register.

  3. #3
    Join Date
    Jan 2009
    Location
    Dronten, the Netherlands
    Posts
    357
    Hi Graham, check if your charge light on the dash is still ok, so with engine off and contact on it's lit, it provides the alternator with initial voltage to start charging proces. About the lower voltage on your dash meter, can it be it takes measurement after the instrument voltage regulator? Some require one of these to provide them with 10 Volt, at least some of my Smith's meters do...
    BR, Theo

  4. #4
    Join Date
    Jan 2006
    Location
    Chelmsford Essex
    Posts
    1,585
    Hi Graham,

    If you have a bad connection or fuse blown for your S cable switched live ( brown or violet colour wire )to the alternator the alternator will not put any output voltage. Dash light will be on and not going out showing no output is confirmed.

    Check the terminal voltage at the alternator. 13.8v charging output.

    I think you might have a different problem with your gauge or a bad earth. Check the voltage at the gauge to prove this.

    If the other instruments are fine ( as they usually use a common earth. Check the earth is good or just run a temporary earth.

    It could be the alternator in the end but few things to check before buying a new one.
    Dax 351 Cleveland T5 2.88 PL

  5. #5
    Join Date
    Jun 2007
    Location
    Lanarkshire
    Age
    44
    Posts
    214
    Graham

    One to possibly keep in mind once you have been through the basic checks could be a starter problem. A number of years ago a friend had and MG RV8 and had an issue with charging that finally turned out to be an internal issue in the starter motor. From memory it was shorting once everything got warmed up and dragging the battery down. He would recharge battery and everything would be fine for another 20 miles.

    Good luck with investigations

    Andy
    Andy
    Dax- 350 Chevy, Holley Carb - Loads of smiles to the gallon SOLD
    AK- 383 Chevy - Even more smiles to even more gallons

  6. #6
    Join Date
    May 2005
    Location
    Near Guildford, Surrey, England.
    Posts
    1,470
    Thanks chaps,

    - the charge warning light on the dash is permanently on.
    - the other gauges seem to be ok.

    I wonder if it could be fuse 5 - this seems to be related and also powers the low brake fluid light.....and perhaps other things...i'll check that out too. (I remember now how horrible the wiring diagram is in the build manual )

    I'll have a poke round with a meter tonight and see what else I can find out.

    thanks

    Graham
    Share your Cobra knowledge here on the Sumo Wiki
    ...and don't forget, building cars is meant to be fun!!

  7. #7
    Join Date
    Nov 2012
    Location
    Lanarks
    Age
    64
    Posts
    670
    Quote Originally Posted by chedz View Post
    Thanks chaps,

    - the charge warning light on the dash is permanently on.
    (I remember now how horrible the wiring diagram is in the build manual )



    Graham
    Be brave - give the Alt good hit with a hammer.
    ( If too scared to try it - ask a good old fashioned mechanic about this trick. )
    or could be just a break in the small wire to the warning light

    Check the Ign light wire to the Alt 1st - then get oot the hammer !
    Attached Images Attached Images

  8. #8
    Join Date
    Sep 2007
    Location
    wiltshire
    Posts
    2,638
    2 wire alternator, big and small? If the red battery charge light is on the small wire must be connected and not broken, the alternator acts as the earth to the charge light bulb when engine is not running and earth when alternator has failed so putting the light on.
    You need to check at the rear of the alternator for charging as the large wire could have broken, poor connection somewhere.
    Last edited by corn plaster; 06-08-20 at 03:42 PM.
    Z4 E89 sDrive30i M Sport Highline

  9. #9
    Join Date
    Oct 2005
    Location
    Costa Del Aldershot, Hants
    Posts
    1,209
    Hi Graham
    Be good to catch up sometime
    Tony
    The Yellow Crendon 427SC, 448 FE Ford & Tremec TKO600. 2008 till RIP.

  10. #10
    Join Date
    May 2016
    Location
    Hockley ,Essex
    Age
    58
    Posts
    382
    Quote Originally Posted by chedz View Post
    Hi,

    its been a long time since I posted...hope you're all well. I still have the car and its still going ok (Pilgrim sumo Mk3 , RV8 completed about 10 years ago) I took it for its MOT today and found the battery isn't charging. Before I start unwiring things and doing some tests, is there anything obvious to check?

    The alternator is spinning, belt has good tension, but I'm not getting any additional voltage back to the battery. I've confirmed this with a meter.

    I recall reading something about there being some delicate electronics in alternators that can fail?
    I don't recall there being a fuse or anything in that circuit?

    What's also slightly odd is that the volt meter on the dash is reading about 10V, yet a handheld meter shows the battery is fully charged and holding just under 13 (car starts perfectly)

    Anyone got any advice of what to check next?

    cheers

    Graham
    Can you give me a clue on the make of Alternator ,
    Bosch or Lucas etc and have you any idea if is a nine or six diode set up .
    This makes a difference on how the warning light works.
    Most important how did you get on with your mot.
    I also have just had my cob mot'ed and was a bit apprehensive due to having to use an unfamiliar inspector.
    As it goes I was winding myself up unessarily and all was good with no advisories.
    Let me know about your alternator if you can ,
    Chris.

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