Condenser functionality explained

Moto Guzzi V700, V7 Special, Ambassador, 850 GT, 850 GT California, Eldorado, and 850 California Police models

Updated: 2012 August 20

Thanks to Bill Dudley for providing the following information as to how a condenser works on the old Yahoo! Loopframe_Guzzi news group (which has now moved to Groups.io). In Bill's own words:

I would like to try to clarify what the condenser is, and what it does, and what it does not do, since I've seen all kinds of strange claims in this thread.

A condenser (capacitor, same thing) is like a miniature battery. It stores charge. Not much, and not for very long. It's a shitty battery, in other words.

The purpose of the condenser in a coil-points ignition system is to act as a snubber and cut down on the arcing that would occur across the points. This prolongs the life of the points. (Technical discussion of how deleted for brevity).

A condenser measures as an OPEN CIRCUIT when tested with an ohm-meter. An OPEN measurement (infinity ohms) is not a guarantee of a good condenser, but any condenser measuring ANY resistance is a bad condenser.

The ONLY way that a condenser can fail that will KILL THE BIKE is if the condenser SHORTS. Therefore, if your bike DIES and you have NO SPARK, DISCONNECT THE CONDENSOR. If the condenser was shorted, then the spark will return. You can ride the bike home without a condenser. Eventually, the points will burn out (erode away) if you run the motor long enough without a condenser. I don't know what long enough is: hours?

The condenser body is one of the two terminals of the condenser, and is connected to ground via the screw through the tab that is spot welded to the condenser body.

Condensers rarely fail, and many experienced mechanics won't replace a known working condenser with a new one, as the old one is likely to work forever, but the new one might be bad out of the box.

As others have stated, a good condenser test is to measure the resistance of the condenser with your ohm-meter, The meter should read infinite ohms. Then, reverse the test leads to the condenser. The meter (this is way better with an analog test meter, it's hard to see on a digital meter) will BRIEFLY show low ohms, and then the needle will return to infinite ohms. Setting the ohmmeter to a high resistance scale (R × 10K, for example) will enhance this effect. You can reverse the leads and see the needle twitch as many times as you like (fun if you're easily amused).

This statement: if your coil is below spec (say 2 ohms or less resistance) the coil will try to use your condenser as a resistor. doesn't make sense to me as an electrical engineer. If your coil doesn't have enough resistance (a 2 ohm coil where a 6 ohm coil is called for, for example) then the current through the points will be higher than spec and you may burn out the points prematurely. I'm not sure if this will have any effect one way or another on the life of the condenser.