A Tale of Two Condensers

[debe]

The Mercotronic tester on leakage test has 300v DC with 60Hz ripple in it. This is applied to the condencer, & the meter reads the current through it. Basicly the current on the meter goes high at the start of the test, then should drop back to the ok part of the scale. If it goes hard over & stays there its shorted. If theres no reading its open circuit. This basic description is from the US patent notes for a Mercotronic tester.

[legendre]

Out of town for a week – see you all when I can check-in!

[RICHARD A. WHITE]

Yes, them Merc O tronics are VERY handy…… Do not know what I would do without mine…

And for what it is worth, I still can’t wrap my mind how the condenser does what it does… I have always thought that it stores the charge for a split second before releasing it to the plug wire, but I am at a point that I have a Merc O tronic and I test ALL condensers on every motor I tinker with. I don’t question it, if it does not pass every test, it is junk….

http://www.richardsoutboardtools.com
classicomctools@gmail.com

[legendre]

It’s actually quite simple, Richard. The condenser does a couple of jobs, but the most important thing it does is prevent arcing at the points, when they open – and this provides two main benefits:

1) Prevents very rapid erosion (burning) of the points
2) Ensures that the largest possible share of energy in the charged-up ignition coil goes to fire the spark plug gap, rather than waste it arcing in the points gap

– If a condenser is open, it’s like not having one at all (see above).
– If a condenser is shorted, it’s like having a set of points that never open (condensers are wired in parallel with the points)
– If a condenser is leaky, it disrupts the reliable, efficient operation of the ignition – the spark becomes weaker and/or intermittent

[jim-moffatt]

I use a low voltage digital multimeter that tests capacitance value and leakage resistance but not ESR. I also have observed real time condenser failures in running motors. I will try to ad my thoughts.

First as an outboarder and not a tester Fan/collector I have found that the low voltage tests for value and leakage usually are all thats needed. Most all of my failed condensers have both high capacitance value and leakage below several megohms. The ESR is an important parameter however and values exceeding the coil primary resistance (usually around 0.7 ohms) may degrade performance. So take your pick of how much you want to get into advanced testing equipment. I have gotten along well with just the multimeter.

Now my capacitor failure experiences are all similar. The "about to fail" condenser tests a little poor on leakage but intiallly works wel and the motor runs fine. But then with as little as an hours use it begins to cause high speed missing after running at high speed. if you slow the motor down there are no misfires. But in another 30 min or so the misfire begins at lower RPMs and the motor is totally unusable. When you take the condenser out it tests very high for capacitance value and extremely poor on leakage ( below 2 megohms). This has happened on two motors before I became a believer in always replacing capacitors with a new (not NOS) one.

I agree with most all of legendres statements except that since the spark occurs when the points are open, the primary circuit is an LRC circuit not a RC one. The R comes from the primary coil resistance the capacitor ESR and the leakage resistance of the capacitor. The L is the primary winding inductance and the C is the capacitance of the capacitor. Since any resistance produces a loss all of the 3 resistance values diminish the spark intensity, hopefully not enough to prevent the motor from running

[legendre]

The condensers from eBay arrived while I was out of town. Oddly, they sent two distinctly different parts, and I’m not sure what to make of it – one is marked USA and has mounting holes for #10 screw. The other, which is identical to the pictured item, isn’t marked at all, and has mounting holes for #6 or #8 screw.

Seems like I got one new production, and one "NOS" (marked USA).. the NOS also has some rust starting on the case. I may raise these issues with the seller, haven’t decided.. to make it work, I removed the bracket from one of the old condensers and soldered it onto the new one.

Both devices measure much the same as each other, around 0.197uF and 0.65 ohm ESR. These specs meet those of "white", in the earlier discussion. They are installed in the mag plate of the TD20, which is making uniform, blue sparks at low cranking speed.

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