Distributor condensers (capacitors)

We call them condensers but they are capacitors. The old MoPar pieces are very good. Sometimes you can find them on line. It does not matter if they are 6 or 12 volt. They are about .27 uF. The old MoPar capacitors are easy to spot – they have a copper strap. Some “Standard “ or Blue Streak did too. Those with the copper strap are almost always perfect – they are hermetically sealed. You can also use a radio capacitor Panasonic .27 / 630 v. It won’t be stock looking and you need a radio 2 or 3 terminal strip to mount but it is bulletproof quality, sealed in epoxy.

The bad ones from Asia have a black rubber cone at the wire and a dent or X embossed into blank end of can. That dent or X presses the can to the foil where it is supposed to make ground. That connection corrodes in a year or two and then the capacitor gets intermittent. Further, these capacitors are not sealed well at all and you get aluminum oxide corrosion. It is a terrible design, good for a year then becomes intermittent (how lovely that part is), and you get low spark energy. The car runs awful and is hard to start.

How to test a capacitor

Set your analog ohmmeter on the highest scale.
On a good capacitor, the needle should swing up then settle down to infinity.
If you do a second test, discharge the capacitor first.
If the capacitor has a short, the meter needle stays up.
If the capacitor is open, the meter needle does not move.
If there is leakage from old damage or bad insulation,
the meter needle will not settle down to infinite ohms.
Click here for a video.

Why you need a capacitor

If you didn’t have a capacitor, the volts on the primary side of the coil will jump sky high when the points open. The coil is a transformer with something like a 100:1 ratio. So “60 kv “ on the secondary side would mean 600 v on the primary side. That 600 v will immediately draw an arc as the points open. That arc burns the points but even worse, the arcing wastes the energy in the coil. You end up with a weak, low voltage spark at the plug.

Coil voltage is described by L (size of coil) and di/dt (how fast the amps change as points opens. (E= L di/dt). If you have a quick change in current of about 7 amps to zero your coil will make big volts. But if there is an arc at points, this means a very slow change in current so E goes way down. So to fight the arc, a capacitor is connected across the points. It has a large surface area inside of two sheets of wound aluminum foil facing each other through a paper or plastic insulation film. You need to charge up all that surface before point volts can change at all; that charging up takes current flow in. So the instant points open, the volts can’t change until the current flows long enough to charge the capacitor. Charging the capacitor holds volts low and lets points open nicely with no arc. Then the current rushes into the capacitor very fast and we get a fat spark with all the coil energy going to the plug.
Next, the capacitor charges to 200-600v then dies down on its own to 12v again after spark at the plug. When the points close it shorts out the 12v to zero and coil starts filling up again. The capacitor is empty, at zero volts, ready for next spark. That spark event is about 400 microseconds long. The next spark event is in about 2,000 microseconds at high rpm.

It is important to know that the spark plug fires at 15-20 kv. If you pull a plug wire to check spark, the following current / power has no where to go. Coil volts go way up to 60,000 instead of 20,000 and that can arc inside the coil windings. You have just permanently injured your coil. The capacitor also sees 600 v instead of normal 200 v and you can burn out or damage the capacitor. It is now “half bad” and you never know that until some dark night on a back road and your ignition fails, or begins to break down at high rpm. NEVER pull a wire to check spark unless you put a spark plug on it and solidly ground the shell or use a neon spark tester to ground.