Can't tell you the value but I have been told by old timers that all points condensers are the same value and can be interchanged, provided they match 6 or 12 volt and positive or negative earth . Never tried it myself though.

According to my Haynes Automobile Electrical Manual "A capacitor used in an ignition circuit has a capacitance of about 0.2uF." (That's microfarads)

It doesn't provide answers for dielectric or working voltage, but at a guess (but I'm not an expert) any voltage greater than about 20v and probably paper. Probably not critical.

Rob

Yes, about 0.1uF, ±150%. There's probably an optimum value that suits a particular induction coil best, but they're all similar. They increase the power of the spark, stop the contacts arcing, and reduce radio interference. Not needed if an electronic ignition is fitted.

The originals were paper capacitors, rather unreliable, with ±20% tolerance. The electrical duty isn't demanding but they work inside a hot dirty vibrating engine bay. I think any modern non-polarised 0.1uF capacitor of more than 100V working would do. Car condensers are special in that they come in a metal can with a lug and lead sized to fit securely inside the distributor; you don't want an ordinary capacitor coming loose when the car is in motion!

Dave

Yes some are rated at a few hundred volts so there must be some spiking due to inductance or some such electrickery. Definitely not for the 25,000 volts or whatever on the high tension side of the coil.

I think if you get a condenser from one 1950s car it should work OK on another. Plenty of that old Lucas stuff is still available, and a lot of it is still being made. Ditto the American points ignition stuff. But trying to find a match from an electronics supplier relying on specs could be tricky and may not be up to the heat and vibration as SOD says above.

Didn't the cap sit on the moveable contact plate and the lug on the dizzy body? Otherwise you have a lot more dismantling before you can get at the bob weights

strangely I was reading this last week link

http://www.da7c.co.uk/technical_torque_articles/capacitors.htm

H

Pedant alert, if the value was – 150% it would be negative, never seen a negative capacitor

I have! An electrolytic went bang, filled the room with acrid smoke, and ruined a brand-new carpet. When house-proud young wifey arrived to start a frank and open discussion about the catastrophe I left them to it…

Dave

300V Would be my guess. These are to suppress sperks – and you don’g get sparks like these at low voltage, as it is the induced back-emf from the coil windings.

Must exist, after all negative resistance exists..

Hi,Voltage measured across points and capacitor on Fiesta Mk1 1100cc engine.

Maurice

It's the sort of question that hurts my brain. If the capacitor is left out, the back-emf rises rapidly and the coil generates a voltage spike – certainly hundreds of volts. The energy is wasted because it sparks across the distributor contacts rather than where it's needed at the spark plug, and it soon erodes the contacts. Not having a capacitor is bad!

However, when a capacitor is fitted, it absorbs the back-emf by charging up and prevents the voltage rising anywhere near spark voltage level. To a voltage spike, the capacitor looks like a dead short. How high the voltage rises as the capacitor charges when the breaker opens is beyond my understanding, but I guess it's closer to 12V than 120V. *** Edit: Posted before Maurice's reply above, where his oscilloscope shows a 120V peak on the capacitor.  The capacitor has to be rated to take that, so I now say 200V.***  If the capacitor is too small, the voltage will rise higher, causing sparks and risking popping the capacitor. If the capacitor is too big, it will smooth out contact breaker action so much that the coil won't fire.

In a common sense way the action of a capacitor is simple: one of roughly the right size, about 0.1uF, makes the engine go better, the contacts last longer, and you can hear the radio! Seems simple to us but it took about 30 years of research early in the last century to get right.

How a capacitor really works is more difficult in this application because the designer has to size it to compensate for several rate of change effects: how fast the engine is turning, how quickly the contact breaker opens, how long it takes the ignition coil to charge and discharge, and how stiff the power supply is.

The chosen value is a compromise, one that allows the engine to perform reasonably well between, say, 800 and 3000rpm. Unfortunately, no matter how well made, an ignition system combining condenser, distributor, contact breaker, and coil is never optimum. For that reason modern cars now use electronic systems that can vary timing on the fly and have no contact breaker, distributor or capacitor. The deliver better starting, fuel-economy, performance, and less pollution. An engine management unit replaces the vacuum adjuster and provides timing accurately tuned to rpm and load as they change on the road. Much more reliable too. On the downside, they're far harder to understand and fix. No skilful mechanics adjusting the breaker with a bit of fag paper, the garage has to connect the car to a diagnostic unit…

Dave

Edited By SillyOldDuffer on 17/02/2023 10:15:18

Edited By SillyOldDuffer on 17/02/2023 10:22:47