How to test a condensor from a small engine

Discussion in 'General Electronics Chat' started by Gdrumm, Apr 13, 2010.

  1. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
    684
    36
    How do I test a condensor from a small engine?
    I'm getting no spark, and the points look fantastic.

    I have a multimeter.

    Thanks,
    Gary
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    How solid is the stator plate and capacitor connection to the engine (quality of the ground)? Do the points really conduct? Is the insulation on the plug lead still sound? If it's a Briggs engine, be prepared to go round and round.

    Never saw one go bad, but apply the meter leads to the cap while set on ohms. You may see a brief bit of conduction, but the cap should charge and show open pretty quickly.
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    What size engine is it?
     
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,006
    1,528
    Some thing that I have found for a no spark condition-- Rust on the coil laminations and on the magnets themselves. Clean the rust off and reset the coil to magnet gap at around 0.010- 0.015 inches, (two sheets of printer paper) and your good to go.
     
    SgtWookie likes this.
  5. Bychon

    Member

    Mar 12, 2010
    469
    41
    The ohm meter will indicate if the capacitor capacitates, but the voltage of the meter is nothing compared to the voltage it has to survive in action. Don't bet the ohm meter will tell you the whole story.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hey Gary,
    Do you have a DC power supply available that will put out somewhere in the range of 15-50 volts?

    If so, you can use a 10k Ohm resistor in series with the cap and power supply with your meter to test it.

    Beenthere's method works with the old-style multimeters that have D'Arsonval movements (a needle that moves across the scale) but modern digital multimeters have very high impedances that make that sort of test difficult to interpret.

    However, the resistor in series with the cap connected to a supply can be used with either type of meter set to measure voltage with success.

    With the power to the supply off, connect the cap and 10k Ohm resistor in series with the supply's + and - leads.
    Connect the meter's test leads across the 10k Ohm resistor, with the meter set to a DC range that is equal to or higher than the output of the supply. Observe correct polarity.

    Turn on the power to the supply. Initially (for a brief moment), you should read full supply voltage across the 10k Ohm resistor. However, that should at first quickly decay, and then fairly slowly decay to nearly 0v.

    If the voltage remains higher than a few mV, the cap has too high of a leakage rate.

    If the voltage didn't jump to nearly the supply voltage, the cap is open.

    [eta]
    If I remember correctly, automobiles up to the early 70's had "condensers" that measured somewhere around 0.22uF (220nF.) I don't have a B&S engine, so I don't have a cap handy to measure.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I'd forgotten about rust being an issue. Odd thing is that we have one dead mower right now (ignition) and the coil laminations are rusty as the dickens - so is the flywheel magnet.

    One sheet of copy paper is about 0.003", so more like 3 to 5 sheets should be about right.

    Gary, another issue can be the ignition kill wire. With modern safety requirements, mowers have to shut off unless the operator is holding on to the controls. If the controls are released, the throttle cable (or another cable) may close a switch that grounds the ignition coil via a wire.

    The routing for this ignition coil ground wire is not always optimal. It can get rubbed on things, or melted by hot parts, and the insulation can get worn/melted through, leading to it being always shorted against the motor housing or associated parts. If you find something like that, you can repair it using several layers of heat-shrink tubing.
     
  8. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    check the diode on the line from the coil. I have been foiled by these little guys many times in dealing with small engine repair.

    If I am correct, the diode is used in reverse to help build the charge, then when the breakdown is reached, the cap dumps into the plug.
     
  9. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
    684
    36
    I'll give these tests a try tomorrow.
    It's a small chainsaw engine. 2 cycle.

    During disassembly, I may have discovered the problem, it may have been a kill switch issue, ie; broken, maladjusted, or dislodged, I'm not certain.

    I'll still check the components tomorrow.

    The coil has two small wires, one going to the points (and condensor), and one going somewhere else? I can't recall to what right now..., then of course the plug wire. I tested those two wires with a conductivity probe, and found contuctivity between the two wires, and conductivity between each wire, and ground. Does that sound correct?

    I'll post again tomorrow.

    There was no rust that I recall, but I'll check again.

    Thanks,
    Gary
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    that second wire should have a diode in line and go to ground.
     
  11. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
    684
    36
    Using Sgt. Wookie's method, it showed 29V right away (24VDC Charger), then dropped down to about 12V, then slowly build back up and topped out at 18.77 volts.
    If I'm reading his post correctly, that would indicate a leakage rate that is too high.

    I don't see a Diode inline, only a black wire going to ground.

    Thanks,
    Gary
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Well then, replace the cap/condenser.
     
  13. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,006
    1,528
    Never seen that with a points style ignition. The new style electronic though would have one.
     
  14. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,006
    1,528
    Sgt., I must have the good stuff, my paper is .005-.007 ! I pulled my guts out on a 5hp. wood chipper before finding the rust on the laminations and magnets. The older engines magnets didn't rust, the newer ones are ferrite instead of Alnico, I think. After using the wire brush I put a thin layer of engine oil on them, kind of like the old style "rust bluing" process on muzzle loaders.
     
    Last edited: Apr 14, 2010
  15. Gdrumm

    Thread Starter Distinguished Member

    Aug 29, 2008
    684
    36
    I went to a local parts house today, and the condensor isn't available.
    I'll have to travel several miles to another parts place later this week.

    I asked about the coil, and they said that to have it checked, I would have to go to yet a third shop, where they have a device called a buzz box. Any insights on that?
    Any insights on checking a coil without a buzz box?

    I'm starting to think I should convert this to an electronic ignition, that costs about $20.00....

    Thanks,
    Gary
     
  16. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    That seems to be a reasonable option. $20 isnt a bad investment if the rest of the motor is sound.. You would spend more at a movie.
     
  17. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,006
    1,528
    The small engine electronic ignition modules that I've seen still use the original coil from the engine. The original coil makes both the low and high voltages for the ignition.

    Did you check the coil with your meter?

    The thin wires that go to the points should have continuity, to each other but not to the laminations. If you get continuity to laminations the primary coil is bad - need new coil.

    The secondary wire SHOULD have continuity to the laminations of the coil. If no continuity to laminations secondary is bad--need new coil. BUT, make sure spark plug wire is pushed all of the way into the coil and that spark plug connector is connected to the wire core. Pulling connector off of plug sometimes strains the high tension wire connections.

    There should be two small wires coming from the coil- one goes to the points, the other to ground. If there is a ignition switch make sure it has continuity. When switch is on the side going to the coil should conduct to the ground of the engine.

    Like I said in my first post the magnets and coil laminations have to be rust free AND gapped correct to the magnets.

    If the cap is the old style (not part of the point set) but the kind with a wire lead, go to a auto parts store and get one thats physically the same size/dimensions. The points won't know or care that it's not for that motor.

    If the cap is the new style, did you file the points? NOT sandpaper but a file, even a fingernail file will work. After setting the point gap, .020 inch with the rub block on the high part of the point cam. Then clean the points by running a slip of clean white paper between the contacts with them closed. Then make sure where's no bits of paper between the contacts. The number one reason the old single cylinder engines won't start is oil getting on the points. From the oil seal on the crank wearing out.

    If you buy a new capacitor, always get a new set of points. Then you won't have to pull the flywheel again soon.

    Hope this helps.
     
  18. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I've found that fine wet/dry sandpaper works pretty well for points. The grit doesn't come off like flint, garnet, oxide papers and emery cloth. If grit gets embedded in your points, they will burn up quickly.

    I though the points were in good shape? If they are bright & shiny with no big "peaks", don't file them.

    The idea is to make the points touch all across their faces. If they aren't lined up correctly, they'll burn in one spot.

    Isopropyl alcohol works pretty well as a de-greaser. Dip a strip of clean typewriter/copy paper in some alcohol, and drag it through the points. Do that several times.

    Sometimes people put too much grease on the rubbing block that rides on the crank lobe. Just a little bit does it. Put the grease on the side of the block that will cause the rotation of the crank to pull the grease in.

    Side note...
    Mic'd a sheet of copy paper; came in right at 4 mils. Used a wire brush on both the coil (magneto) faces for the laminated E-core, and the magnet in the flywheel; and used a small amount of white grease to keep the moisture off. Used the paper folded up 3 ways to get 12 mil spacing between flywheel magnet and the E-core. Replaced the plug. Good spark, still wouldn't start. Turned out lacquer in the float bowl had clogged the fuel inlet to the venturi pickup. Some lacquer thinner and work with fine wire got things cleaned out and going - fired up 1st pull.
     
Loading...