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Technical Problems with SW Tach - needle shakes

Discussion in 'The Hokey Ass Message Board' started by edwardlloyd, Oct 2, 2014.

  1. edwardlloyd
    Joined: Aug 2, 2003
    Posts: 2,072

    edwardlloyd
    Member
    from Germany

    Got a bit of a problem with a new SW electronic tach. I'm running a 1940 flathead V8 with stock coil and distributor. The car is 12V so the coil has a resistor before it.
    When ticking over the tach needle jumps now and again. At higher revs the needle jumps around a bit.
    I disconnected the generator - problem didn't go away.
    I ran tach power and earth to a separate starter battery to supply clean power - problem didn't go away.
    So the problem must be coil / distributor. Coil is NOS. Points and condenser are new from Macs.
    Anyone know what's causing this? Does this indicate an ignition problem?
    Ed
     
  2. edwardlloyd
    Joined: Aug 2, 2003
    Posts: 2,072

    edwardlloyd
    Member
    from Germany

  3. ROADSTER1927
    Joined: Feb 14, 2009
    Posts: 3,127

    ROADSTER1927
    Member

    Can you try it on another car? That may give some clues! Gary
     
  4. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    I had that problem on an old Volvo that my son installed an electronic tach into. I ended up making a tach filter, using some resistors and capacitors, which solved the problem.

    3648TachFilterDiagram.jpg
     

  5. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    If you understand a bit of electronics, this might be helpful....the first picture is the signal at the coil to points connection, the second picture is the signal after the filter. The high voltage ringing gets filtered out, leaving a simple pulse that the tach can deal with more easily.

    filterscope1.jpg filterscope2.jpg
     
  6. seb fontana
    Joined: Sep 1, 2005
    Posts: 8,444

    seb fontana
    Member
    from ct

    Thanks Jim..Radio Shack should have the resistors and capacitor? Think would work with a Mallory that has built in [adjustable] rpm limiter? Tach is a little twitchy below 2k rpm..
     
  7. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    give it a try.

    There are different kinds of filters for different ignition systems, and different tachs. This one seems to work well with points and newer tachs.

    radio shack has the parts. The values are not critical, you can be off by maybe as much as 50% on the values and it should work fine.
     
  8. edwardlloyd
    Joined: Aug 2, 2003
    Posts: 2,072

    edwardlloyd
    Member
    from Germany

    Thanks Jim. Makes you wonder why SW don't build this into their tach in the first place.
     
  9. RICK R 44
    Joined: Dec 13, 2009
    Posts: 474

    RICK R 44
    Member

    Had the same problem. Have chryco electronic ign on my flathead. the distributor that I bought it from supplied a filter that did not help. Stewart Warner are assembled in Mexico by Maxima Technologies of PA. After about 2 years and about $100 in long distance calls to both PA and Mexico, they finally replaced the tac with one that they claimed was custom built. Worked ok until the needle fell off after about a year. After several more long distance calls, I finally talked a girl in Mexico into sending me a couple of needles. I pryed the face off and fixed it myself. As far as I am concerned , no one at Maxima knows the meaning of customer service. Good luck. My suggestion is to scrap it and take your loss, buy another brand. Off shore would be an improvement in my opinion.
     

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