Technical: Condensor
Dave Hennessey

“Condensor” is an archaic term for a capacitor. The only people who use the term condensor are old guys who play with old cars, trucks, motorcycles and tractors. That would be us. In Electricity 101, we learned that the purpose of a condensor is to store electricity. Most of us have had a condensor discharge in our hand, and the sharp pain told us that they truly do store electricity. In normal use, condensors seldom totally charge or discharge. Instead, they act like a holding tank, constantly giving and taking. Condensors for general use come in sizes ranging from 1 picofarad (pf or 0.000000000001 farad) to 1 milli-farad (mf or .001 farad). There are some huge kilo-farad (kf or 1000 farad) condensors used for things like radar and laser weapons, but the ones we see on a motorcycle are in the micro-farad (μf or .000001 farad) range. Why would we want to use these things on a motorcycle?

Ignition System – The function of a condensor in the ignition system is to give the electricity somewhere to go. Without a condensor, when the points start to open, the electricity would continue to flow across the gap until the points were fully open. The yellow-blue flash of light you see is the points “arcing”. Arcing causes metal to transfer between the points, and without a condensor, the points would quickly become unusable. The condensor simply gives the electricity somewhere else to go when the points start to open, so the points break cleanly with minimal arcing. The condensor in our circuit breakers and magnetos is usually in the 20 - 30 μf (microfarad) range.

Battery Eliminators – Some custom bikes use a large condensor as a substitute for a battery. They aren't very good substitutes, but they will keep the ignition running at low rpm, and prevent the generator from burning up the light bulbs. These soup-can sized condensors are in the range of 20,000 to 50,000 μf.

XLCH “Horn Condensor” – From very late 1963 to 1969, the XLCH gained a large external condensor connected to the voltage regulator. Late is “above engine number 63 XLH 3015”, ~August 1963, about 85 bikes from end of 1963 production. This condensor, Harley #32727-63, appears in the 1965 and 1967 Parts Catalog in the Circuit Breaker section, near the ignition condensor. V-Twin sells a replica, which they describe as “500 MFD”, but I think they really mean 500 μf. V-Twin calls it a “Horn Capacitor”, but then they say its purpose is to “regulate light intensity”.

What does it really do? – Well, one thing it won't do is reduce electrical noise from the ignition system, since it's not connected to the ignition system. It will reduce electrical noise created by the generator, but I don't think that is its purpose, because it's not used on XLH or any other models which use the same generator. I think its purpose is three-fold (a) to reduce arcing of the contact points in the regulator; (b) to smooth the operation of the lights; and (c) overcome horn inertia.

On a battery bike, the cutout relay section of the voltage regulator usually activates when the bike is started, and deactivates when the bike is turned off. On the XLCH, at low rpm, the cutout relay is opening and closing as the generator output fluctuates around the 5.5 volt mark. (Note, on XLH, the cutout relay activates around 6.4 volts.) Like an ignition condensor, it gives the electricity somewhere else to go, and thus reduces arcing of the cutout relay points.

As the cutout relay operates, the lights (and horn) suddenly turn on and off. While a 500 μf condensor doesn't store a lot of electricity, it does store enough to “ramp” the flow of electricity up and down, instead of the abrupt on/off. This probably increases light bulb life.

DOT safety requirements started taking effect in the mid-1960s. We got seat belts, turn signals, and other safety requirements. The XLCH horn didn't beep very well at low rpm, and the condensor may have been an attempt to improve its performance to pass the tests. The condensor wouldn't store enough electricity to operate the horn for more than a split second, but it might hold enough to overcome horn inertia when conditions were right at the edge of beep / no-beep.

Where does it go?


1969 was the last year for the magneto, so 1970 and later XLCH do not have a condensor.

The micro-farad is designated μf, using the Greek letter “mu” You often see condensors mislabelled with mf or mfd, since typewriters don't have a mu. But “mf” is really milli-farad. Fortunately for us, milli-farads are not very common.

Capacitor testers can be purchased in the $15 to $20 range. Check with Amazon.

Ever wonder what the terminal letters on a Bosch regulator mean? And what's that 61 terminal? Ask Robert Bosch