450sl ignition wiring diagram?

Discussion in 'General Discussion' started by NOMONEYBUTAMERC, Mar 23, 2014.

  1. NOMONEYBUTAMERC

    NOMONEYBUTAMERC Hardcore MB Enthusiast

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    After "tidying up " my engine bay , cleaning up wiring , and spraying the inner wings , i put everything back in place , or so i thought. However , being my usual badly organised self , i lost the piece of paper that i had noted down all the wiring on!:doh: Unlike the later SL models the 73 450sl has two ballast resistors in the coil circuit , to reduce the throughput to around 9 volts. At present there is no spark at the coil , and i suspect i have replaced something incorrectly.I have tried running an additional earth from the coil to the chassis, but still getting no spark. The car is negative earth , and checking on an American forum it appears the coil should be earthed from the positive terminal? There is a break in the ceramic housing on one of the resistors , but the "element " appears intact. A local motor factor is trying to locate a new coil and resistors. It appears the coil is specific to a few 70s mercs. Any ideas/diagrams would be much appreciated. I had thought that any 12 volt coil could be substituted , but apparently not.
     
  2. Ted

    Ted MB Club Veteran

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    The coils are designed to work at 9v and The purpose of the ballast resistor is to reduce the voltage to the coils to 9v
    This is then overridden by a contact on the starter so the full battery voltage is applied during cranking which produces a stronger spark during that period.
    So
    You should have 12v on one side of the ballast with ign on
    This should drop to 9v on the other side of the resistor if the coil is in circuit AND current is flowing through the coil via the ignition amp.
    In my experience no part of the coil should be earthed. The body doesn't need it, one side goes to ignition the other to the amp (which switches the current to earth)
    The positive side should go to the ballast, the negative side to the amp. This is not crucial but you lose out on a few hundred volts for the spark due to the way the coil is wound internally i.e. the voltage created by the primary coil collapsing will not be added to the secondary coil.
     
  3. OP
    OP
    NOMONEYBUTAMERC

    NOMONEYBUTAMERC Hardcore MB Enthusiast

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    Thanks Ted. I am fairly certain I have got the wires mixed up , but the coil was definitely earthed . There are 2 ballast resistors , 1 rated at 0.6 and one at 0.4 ( which is apparently obsolete ,and of course this is the one with the cracked ceramic! ). Druk has very kindly offered to send me a diagram , which will probably confirm that i should not be trusted with anything that has more than 2 wires. Should have a new coil tomorrow , so fingers crossed.
     
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  4. grober

    grober MB Club Veteran

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    Last edited: Mar 24, 2014
  5. OP
    OP
    NOMONEYBUTAMERC

    NOMONEYBUTAMERC Hardcore MB Enthusiast

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    Thanks , and welcome back Graeme . Sorry , but i cannot reciprocate with advice on computers , now if you want to build an abacus ..................
     
  6. OP
    OP
    NOMONEYBUTAMERC

    NOMONEYBUTAMERC Hardcore MB Enthusiast

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    Many thanks guys , after studying the wiring diagrams , made a few "adjustments " , and started up first time. Much obliged for your help.Not totally convinced that the Intermotor coil is the correct replacement , but it
    appears to be running reasonably well. As soon as we get a dry day I will take it for a spin.
     
    2 people like this.

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