Dexta charging light

Discussion in 'Classic Machinery' started by timff, Jan 12, 2019.

  1. timff

    timff Member

    Hi all. In the past, the old girl's red light has stayed on until she'd run a few minutes at some revs but now it's staying on. Before I buy a new/recon dynamo, is there anything else I should check? The two wires are connected to the dynamo okay. The present dynamo is at least 40 years old so it's probably worn out? I inherited the old girl from Dad who bought her new in 1958!
    If you reckon it's the dynamo, rather than buy a new/recon, is it worth getting it repaired by a specialist?
    I'll be off air till Monday so many thanks in advance for any replies!
  2. davidroberts30

    Might be the control box?
    Battery connected positive earth?
    tomlad likes this.
  3. timff

    timff Member

    Thanks David. I sparked the control box yesterday but no change. I reckon at 60 years old its either the control box or dynamo so I'm awaiting a call from a chap who knows....
  4. db9go

    db9go Member

    Change both
    Right-arm fast likes this.
  5. Oh the days of sorting out dynamos. I used to know how to set those lucas regulator boxes with my eyes shut and which terminal went where etc, but now I cant hardly even remember what those relay bobins and contacts under the aluminium cover even look like its been that long. :(
  6. davidroberts30

    Dynamo is 40 quid and reg box is 25
  7. davidroberts30

    U spark from battery to dynamo to polarize the dynamo

    Mursal likes this.
  8. timff

    timff Member

    Thanks David & db9, it's not an expensive fix. Will get on to Agriline
  9. Polarise your dynamo 1st check your earth's too
    If you remove the wires you can test the output of the dynamo or remove belt and short circuit it it should turn slowly like a motor this means the dynamo is functioning

    If you do swap try and fit in pairs

    But I would polarise the old one 1st

    And you have to polarise the new one also
    And make sure it's negative earth too
    davidroberts30 likes this.
  10. tomlad

    tomlad Member

    nr. preston
    My experience of iteams like this from agriline is poor quality im afraid
    I don't know if u can by lucas parts as such ???
  11. essexpete

    essexpete Member

    Why were the old dynamo systems set up with pos earth?
  12. davidroberts30

    We're they sent out positive earth?
  13. timff

    timff Member

    Thanks all, awaiting a call from a sparky. Thanks for all your guidance chaps
  14. db9go

    db9go Member

    That is how they came out of the factory set pos earth
    It only changed to neg earth in 1965 when the new range came out
  15. TheTallGuy

    TheTallGuy Member

    It was largely a hangover from the early days of telegraph where the standard was to have positive earth/negative signal as it was easier to manage the galvanic corrosion on the earth circuit that way around. It wasn't until silicon diodes & transistors became viable & cheap that negative chassis became the standard system & some early alternators were positive chassis!

    Strictly speaking, cars & tractors don't have "earth"/"ground" - it's "chassis" as there's no direct connection to the general mass of earth.
    essexpete likes this.
  16. The new dynamos are neg earth only,
    If the tractor is attached as pos earth then it won't charge and polarising won't have any effect

    Unsure if dextas are neg or pos earth from new as most people like to bigger about with things and I've not done much with dextas if honest either

    Had 1 dealing with one we restored and it kept putting the light on dim at tickover adjusting the reg box would sort this but it would cutout higher up the rev range,
    Adjusting the ticking a 100rpm cured the issue after altering the reg box back to standard
    davidroberts30 likes this.
  17. Mursal

    Mursal Member

    And the manufacturers moved from positive "earth" because you always have a chemical reaction on the positive terminal of the battery from charging. This reaction caused corrosion where the positive "earth" bolted to the chassis. The negative terminal doesn't suffer from this .......

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