Hi all

I have a question around the noise suppression capacitors fitted to the coil and alternator on the older (maybe pre mid-80's?) Holdens. I went to purchase them recently for my HJ and found them nigh impossible to buy - generally people only sell the distributor condensor.

My general understanding is the suppression capacitors, fitted to the "line" side of the coil and the output of the alternator, were intended primarily (solely?) as a means for reducing RF (Radio Frequency) noise in the radio, specifically the AM band. (Other advancements in the very early days included type of spark-plug leads used, for instance, but put that aside). In comparison, the role of the condensor (though also a capacitor) in the distributor was to absorb energy caused by the back-EMF from the collapsing field in the ignition coil so as not to burn out the distributor contact points.

So, questions:

1. What are the relative values (uF and Voltage) of the distributor condensor capacitor vs. the noise suppression capacitors - in my case, a HJ Caprice with 308 V8?

2. Why were they phased out? I can only assume there are multiple reasons - advancement in RF noise suppression (both source reduction and better suppression techniques), change in ignition systems, few people using original A/M radios, etc.?

3. If one was hell-bent on re-fitting the original noise suppression capacitors to the coil and alternator (e.g., for originality), where can they be sourced?


Thanks in advance.

On further investigation, it seems that these suppression capacitors do vary from brand to brand & I guess from country to country.

I checked some old ones I had in the workshop & they vary from .47uF to as high as 2.2uF. Most seem to be .47uF or 1uF.

I would imagine the larger the capacitor, then more the it kills the RF noise. It probably also depends on the noise source & its frequency.

The frequencies involved are all fairly low. e.g. ignition noise @ 2000 rpm in a 6-cyl only 100 Hz & an alternator would be similar. An old DC generator (dynamo) would be higher but only by a factor of 4 or 5 times, depending on how many segments there are on the commutator & the pulley ratio.

If you are having noise issues with your car's audio system, another method commonly seen these days is to fit an inductive choke into the 12V supply line to the radio/CD/amp or whatever.

Dr Terry

Edited by user Thursday, 28 April 2016 7:59:25 AM(UTC)  | Reason: Not specified

Tuned in to Radio 2 picoCoulomb?

I did some work with the State Electricity Commission and a University here years ago, with a very directional microphone mounted on a crossbow with a telescopic sight as a means of identifying the source of the discharge. On several occasions the police took some convincing that no one was going to be shot.

The points switch at 100Hz, but the arc is at the resonant frequency of the coil and the capacitor, at least until the points are fully open. For coil primary inductances of around 4mH and capacitors of around 0.2uF this would be around 5kHz, with plenty of harmonics.

For HKTG vintage stuff, if the alternator is the Bosch type, there are points in the voltage regulator that switch the field supply at a high frequency, and are quite noisy. The Lucas voltage regulators are solid state and do not as generate as much RFI.

I saw a comment about plug leads in the initial post. Although they are often called "Suppression" leads, and contain a carbon string inner with significant resistance, one of the main functions of the resistive plug leads is to limit current fed to the plug and increase the duration of the spark when the plug fires. A solid wire plug lead causes the spark to quench too quickly.
There is some benefit for RFI reduction, but they are needed for the current limiting effect.

http://www.ebay.com.au/i...cf03:g:-QIAAOSwQJhUgp~V

http://www.ebay.com.au/i...0321:g:RTcAAOxyOM5RZqw1

Some on Ebay....Cheers

Edited by user Monday, 9 May 2016 6:33:44 AM(UTC)  | Reason: Not specified