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Thread: Low spark voltage

  1. #1
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    Low spark voltage

    Well I am back trying to find my intermittent problem that causes the engine to stumble and be unable to accelerate. It has now degenerated to where I canít get the engine to even start. I have previously been focusing on fuel issues, but I am becoming convinced that ignition may be the problem.
    The coil is new and the resistance in the primary and secondary coils is within specification.. The voltage at the coil is 12.1 volts. Grounding the high tension wire while cranking produces a good spark, but it is a little yellow in color.
    I have a 123 electronic distributer that previously produced a spark at the plugs, but now there is nothing. My theory is that the internal electronics have continued to degenerate until it will now not operate at all.
    Today I re-installed the original distributer which has a Pertronix electronic ignition module. It produces a spark at the plugs but the engine will still not fire. The spark also looks to be a little yellow in color.

    Using a Snap-On MT2700 meter borrowed from Michael Balk, I measured the actual spark voltage at 5KV. This seems to be well below the 10 to 20KV I would have expected and may be the reason for non-starting.
    Both the 123 and the Pertronix set ups have run the car successfully in the past, but I have heard of electrical components breaking down.

    Before I convert back to good old points and condenser, is there anything else that might cause the spark voltage to be low?

    Still searching,

    Greg
    Gregory H. Peirce
    Atlanta, Georgia

  2. #2
    Administrator JimVillers's Avatar
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    Re: Low spark voltage

    Greg .... Back to basics. Spark is spark; you just need a spark, any spark at the plug to make fire. The high energy ignitions are for very high compression or very high RPM (neither of which describe a 190 SL).

    Try a little starting fluid, if it pops or starts then it is a fuel delivery problem. If you get nothing, it can be your ignition. The other item is to replace your plugs. Some water ingestion (or in the fuel could) be shorting our the plugs.
    Jim Villers
    1961 190SL, 230SL 5-speed, MGB 5-speed, Boxster 'S'; Porsche 356C; 1967 Porsche 911; 1950 Jeep CJ3A; 1958 190SL "Frosty"

  3. #3
    23K Original Miles slover's Avatar
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    Re: Low spark voltage

    It couldn't be a defective coil?
    John Lewenauer - Newsletter Editor - Regional Director
    1961 190 SL
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  4. #4
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    Re: Low spark voltage

    I've tried 2 different coils and they both test OK for resistance.
    Gregory H. Peirce
    Atlanta, Georgia

  5. #5
    Administrator wpuryear's Avatar
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    Spark plug wires (I use solid copper), rotor, and rotor cap are all possibilities. I'd suggest reinstalling mechanical points and go from there.

    Walt

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    Re: Low spark voltage

    Wires, cap and rotor are all new. I have another distributer with points but it's from a "56
    Does anyone know if a VJR 4 BR11 can be substituted for a VJR 4 BR24?
    If not I guess I could steal the points from it.
    Gregory H. Peirce
    Atlanta, Georgia

  7. #7
    Administrator JimVillers's Avatar
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    Re: Low spark voltage

    Greg .... Yes, you can swap the BR11 for a BR24. You will never know the difference.
    Jim Villers
    1961 190SL, 230SL 5-speed, MGB 5-speed, Boxster 'S'; Porsche 356C; 1967 Porsche 911; 1950 Jeep CJ3A; 1958 190SL "Frosty"

  8. #8
    Registered User tommd's Avatar
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    Re: Low spark voltage

    Greg, you said you do have spark. And 5kv sounds good. I would double check timing and then fuel as Jim said. Any chance the timing chain has jumped?
    Tom D.
    1961 190SL

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    Re: Low spark voltage

    Greg,

    It may not make a difference just for a start-up test but I think the VJR4BR11 distributor had a different spec for static timing- 9 degrees BTDC. I think it's covered in the workshop manual.

    Good luck- we're counting on seeing your car in New Hampshire!

    Steve
    Steve Maiocco

    1956 190SL #6500717 ("Lucy")- needs refreshing
    1961 190SL #20074- in pieces
    1984 380SL ("Faith")
    1958 Chevy Apache

  10. #10
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    Re: Low spark voltage

    Pertonix and 123 don't like solid copper wires due to interference issues from what I recall....
    Bob West
    '52 300s Coupe
    '60 220se Cab
    '55 300sl

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    Re: Low spark voltage

    I think timing may be the issue. Can you static time a Pertronix ignition? That's interesting about solid copper wires and electronic ignition, but the car ran very well for over a year on the same wires with the 123. That's why I think it is some kind of electrical breakdown in the 123.
    It's time to go back to basics...points...and start from there.
    Gregory H. Peirce
    Atlanta, Georgia

  12. #12
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    Re: Low spark voltage

    Curious thing on another of my cars, not a 190SL: new rotor wouldn't fire up the engine. Double checked with the old rotor in the distributor and found the new, Napa Parts rotor fits exactly 180* off! Back to the old rotor and it lit up immediately. Not likely your problem, of course.
    Jack English
    1960 #15229 Since 1972
    1971 MBZ 300SEL 6.3 Since 1983
    1999 MBZ 430E Factory Delivery

  13. #13
    Registered User tommd's Avatar
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    Re: Low spark voltage

    Quote Originally Posted by BWest View Post
    Pertonix and 123 don't like solid copper wires due to interference issues from what I recall....
    Bob, I know many electronic systems do not like very low secondary wiring resistance. Later MB models used copper wires with resistance in the spark plug connector. I somewhat assumed the original 190SL plugs ends do, but I do not have mine here to check. Anyone know if they do?
    Tom D.
    1961 190SL

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