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W126 420SEL M116.965 no spark

Discussion in 'Electrics, Vacuum, Ignition and ECU' started by mej, Dec 21, 2012.

  1. mej

    mej Senior Member

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    Your Mercedes:
    S124 1994 OM606, ditto OM605, ditto M111, W126, S211, W169
    I wonder if anyone has ever come across a break out box/cable for the EZL module - EZL/AKR adaptor cable (117 589 01 21 00). The WIS (15-541 Testing electronic ignition system with variable ignition characteristics and anti-knock control (EZL/AKR) - page 2) suggests the use of this, and I am looking for short cuts. Have pretty much excluded the coil (by replacing it with a cheap second hand one from a srappy I trust, so prob a functioning unit) and am left with the CKP or (please NO) the EZL. Laid it up last autumn, and when I went back to start it after a few weeks idle (battery off, and recharged before reconnecting) the spark-le had gone out of it.



    Malcolm
     
  2. television

    television Always remembered RIP

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    I have never seen a breakout box for sale,what few there are around the garages hang on to them.

    Is it possible to do any checking with a blink code reader, I can check later in the day
     
  3. Alex Crow

    Alex Crow Senior Member

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    Your Mercedes:
    W169, W124, w202, W203, KTM 250 EXC, VW T25 camper and a Polo in a pear tree
    Where are you testing for spark? Hopefully on the king lead from the coil, to rule out the cap/arm.
    If not, do.

    Beyond that, you just need to check the inputs - these modules do fail, and I would think hard to repair (as they are probably potted).
    You can improvise without the test leads, although they will be many hanging up gathering dust somewhere!

    Also, beware usuing the wrong impedence coil, that alone could kill the module.
     
    Last edited: Dec 21, 2012
  4. OP
    mej

    mej Senior Member

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    Your Mercedes:
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    I have the blink code reader for the W124s, but there is no facility to use this on the W126. You're always on the WIS Malcolm, so you'll see the device I'm referring to on the reference I gave.

    If not, I may have to remove the CKP to check the flywheel end, but it appears to be a bugger of a job on this model.

    Regards

    Another Malcolm
     
  5. OP
    mej

    mej Senior Member

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    Good point Alex. Am about to re-attack this car as I'm around for a few days. I believe I only checked with a spare plug in lead 1. I'll go for the HT lead later today.

    Will start from scratch on resistances etc, as given on the WIS 'engine not running' instructions.

    The coil is the same p/n, so should be correct.

    Regards

    Malcolm
     
  6. television

    television Always remembered RIP

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    I did try repairing one but impossible, the potting and crude construction stopped me from doing anything.

    Bosch paid £4 each for a batch some 6 years ago
     
  7. OP
    mej

    mej Senior Member

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    I finally had chance yesterday to spend some time working through the diagnostics in the WIS(15-541 Testing electronic ignition system with variable ignition characteristics and anti-knock control (EZL/AKR) - page 2), or at least as far as one can with a Maplin's DMM! Both original and replacement coil give .8 ohms between the two nut topped terminals and 8.1 ohms between the nut terminals and the HT lead socket. The .8 is a bit high (s/be .2 - .4) and I'm looking for someone who actually knows what they're doing to tell me if this is significant.
     
  8. Alex Crow

    Alex Crow Senior Member

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    The biggest mistake commonly made here is not checking the resistance of the leads and in the meter.
    Reading very low resistances with a cheap meter is never going to be all that accurate, but for best results always test the leads together, and subtract that value from the 0.8 - you will probably find this gives you the correct figure.
    I am sure both coils are OK.

    There are a lot of pins on the EZL/AKR modules, as they have knock sensors added, but only certain inputs are needed to run - ie crank signal, power and earth, you need to check these first.
    Then check for power at that coil (term 15+).
     
  9. television

    television Always remembered RIP

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    It is near impossible to measure a coil for anything other than open circuit or not, the battery in the test meter sets up a field in the coil that is out of phase and apposes the reading. the secondary reading looks far to low as there are some 15k turns of wire, and you cannot measure 1 short turn.


    You can test a coil quite easily, but it really needs a cap to do so, but to test a capacitor of between 2mfd and 4mfd 400vdc working will do-

    The coil has 2 terminal a + & - the center is output
    connect the cap across the + & -

    You need a 12 volt DC supply. ( the car battery will do)

    Connect a wire to the - terminal and this will be the neg - part of the circuit

    Have a wire with the plus + 12volt to hand

    You need to set up a spark gap, say about ½", from the coil center output to battery supply neg terminal. You could use a sparking plug with a wire around the body going to the neg - terminal and just pop the terminal on the sparking plug in the hole in the top of the coil.

    Now take that plus + wire and touch it on the plus + terminal on the coil and take it away again, as you take the wire away there should be a good spark over the sparking plug tip or spark gap.

    How a coil works
    If you put a voltage on a coil with an iron core a magnetic field is produced, if the voltage is removed the magnetic field collapses back into the coil and this produces a voltage that is out of phase back into the coil.

    Now if you add some more windings, the same induced voltage will appear on all of them, and we have just the 1 secondary winding on a car coil.

    One problem with this is that a lot of energy can run out of the primary winding, and we do not want that so a capacitor is wired over the primary terminals, the cap is charged and this too will be out of phase with the induced voltage, so it will appose this voltage long enough for all the energy to be pushed out of the secondary (where we need it) In a way it acts like a gate we could say.


    I am happy to talk you through it all if not clear :D
     
  10. Alex Crow

    Alex Crow Senior Member

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    Surely Malcolm, we test resistance with DC on a multimeter?
    So there will only be back EMF for a split second?

    I am sure the reading from a DMM will be fine (indeed, it is suggested to test coils this way in MB literature), as long as the end correction is applied, as I suggest above.
     
  11. television

    television Always remembered RIP

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    Things are better these days with digital meters as the voltage used to measure resistance is much lower than it used to be the old analogue meters but it is still present so you can get an indication mainly on the primary windings and maybe accurate enough for a go or no go situation, An inductance meter is the real way, but if you do not know what it should be then not much help.

    It is more often than not that if there is a problem on a coil it will be the secondary winding, and just 1 short turn in 15k will soon render the coil u/s, and one cannot measure this without a very special meter that test under load.
    When the secondary has a short, the coil output will be somewhere near normal for a short time, the short circuit winding apposes the output and the coil will heat up quite rapidly, as it heats the output will get lower and lower till a spark under compression cannot be produced.

    In practice the car would start and run fine, slowly the engine struggled to keep going and stop after about 5 min, let it cool down and away you went again.

    There is a massive difference between a spark in free air as to a spark in a compressed chamber. I used to have a spark box as I called it going around the country giving lectures, I had a good and a bad coil and would blow up the box with compressed air and you could see that the bad coil could produce no real spark under compression where as the good coil could.

    They were the good old days, good fun though :D
     
  12. OP
    mej

    mej Senior Member

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    Thanks Alex and Malcolm. Will move forwards re: the cap test next week. Submerged in g/kids today.

    Malcolm
     
  13. OP
    mej

    mej Senior Member

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    Checked the terminal to terminal resistance on the DMM itself as Alex suggested; .3 ohms. That puts the primary terminals resistance bang on after adjustment.

    Tried malcolm's coil test (I have Velosolexes, so have some grasp of coils, except they're magneto ignition). Without a cap between the LT terminals, simply emulating the action of the contact breaker as described above there's a spark at the plug. I will try to confirm the CKP (L5) in daylight, and if that checks out I reckon there's nowt for it but an EZL. Bugger.
     
  14. television

    television Always remembered RIP

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    The spark doubles with a cap or even more, if it goes about 5mm without then OK
     
  15. OP
    mej

    mej Senior Member

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    In a dim light indoors a spark could clearly be seen across the plug gap most times I flicked the + cable onto the coil LT terminal.
     
  16. television

    television Always remembered RIP

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    With a cap the spark would be blue I can slip one in the post to you free:D

    If you would like one is it a 12 volt coil
     
  17. OP
    mej

    mej Senior Member

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    That's most kind. Please do. Will PM details now.
     
  18. television

    television Always remembered RIP

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    I have sent you an Email
     
    Last edited: Dec 23, 2012
  19. OP
    mej

    mej Senior Member

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    Your Mercedes:
    S124 1994 OM606, ditto OM605, ditto M111, W126, S211, W169
    Following many helpful tips I attacked the car again today. This morning's results were as follows.

    Coil
    .2 Ohms between the 2 LT terminals (1 & 15 I believe)

    8.48kOhms between 1 and 4 (HT) and the same between 15 and 4

    Ignition on gives 11.85 vDC (it was a bit tired) at both LT terminals

    When I swapped batteries (I have a 66 Ah on a E280T S124) the rev counter needle clearly twitched upwards more than could be accounted for by vibration.

    L5 CKP
    768 Ohms between centre and outer terminal - within 15-541 limits

    .3 vAC between centre and outer terminal when cranking

    EZL
    As on p4/23 of 15-541, 8 pin plug A terminals 7 and 5 connected shows 1007 Ohms, 5 to ground and 7 to ground each show 1 (infinity?) respectively, so insulation good I imagine
    Plug A 6 connected to 2 gives 12.09 vDC, 1 connecetd to 2 gives 10.63 vDC (just before battery change)

    Also required on p4/23 the test diagnostic socket (round-ish, 9 pin) terminal 5 to ground gives 11.98 vDC, and 5 & 4 connected gives 0 volts.

    As there's no spark at the HT lead when you put a plug directly in there (instead of connecting it to the distributor) the problem seems to be upstream of the distributor. All the above seems to suggest the coil and the CKP (L5) are operational. This leads us to the fiscal cliff of a replacement EZL, unless Santa brings one tonight. Anyone any better ideas??
     
  20. television

    television Always remembered RIP

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    I do not have any yet, I boxed the caps for you, but after a disastrous night with Lilian I could not get to the post today
     

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