I thought it was just to supress radio emissions but it is actually part of a tuned circuit to improve spark and to minimize current damage to coil, breaker points or transistors.

Stanley, I will draw a distinction between the earlier points driven cars and the later electronic ignition cars:

 

For the earlier points driven ignition cars, the condenser should be on the negative side of the coil (same side as the points) and the function is to prevent the points from arcing as described above. If you have a points distributor and your condenser is on the + side of the coil (same side as the ballast resistor), then you've got the condenser installed incorrectly.

 

For the later electronic pickup variable reluctor style cars (starting in 74), they started including a condenser on the + side of the coil, and I'm not totally sure of the function. Never thought about it much... Maybe the Datsun engineers felt like they always included a condenser on the coil and were uncomfortable without putting one on there.

 

Somewhere.
 

Yeah, I've also got the one on the negative side, attached to the distributor. Found out about that a few years back when my points kept wearing out. Read somewhere that the condenser kept the points from frying (more to it than that, as described above in detail, but that's the bottom line). Installed a new Nissan tune-up set at that time,that included a condenser, and the points are still like new. The other condenser on the positive terminal supposedly acts like an open to DC (otherwise it would drain off the current for the ignition system) but shunts RFI/EMI to ground, protecting the coil and electronic equipment and preventing static on the AM radio. Don't know if it was stock on my '73, but looks like it's been there forever. If it's on the schematic I can't find it. Possibly the dealer installed it.

 

BTW, I just edited post #13, changing "wired in series" to "wired in parallel" which seems right, or at least less wrong.

So you have two condensers? One on the coil positive and one on the coil negative? Well like I said, I'm not sure what they would consider the purpose of the one on the positive side, but it sure won't cause any problems. I'm not so sure about the noise reduction stuff though. Most, if not all of the noise picked up by the radio is caused by the points arc. That little arc is a noise radiation source that is broadcast through the air and then picked up by the antenna. Some noise might be conducted through the wiring, but not much. Anyway, the condenser on the + doesn't hurt. And if you've got two of them, then you're guaranteed to have one in the correct location, right? 

 

And for a little electrical theory. It's not so simple as to say that a condenser acts like an open to DC...

A FULL condenser acts like an open circuit to DC, but an empty  one acts like a dead short. So when you first connect voltage to an empty condenser, it will take all the current you can give it (like a short). But then once it's charged up (full) it won't take anymore current and it looks like an open circuit.

 

And since we're talking theory... I'm really sorry, but I just can't stop myself without including the seemingly converse description of a coil (AKA an inductor).

 

A charged coil (one that has had power applied to it for a long time) acts like a dead short to DC, but one that has just had power applied to it (uncharged) acts like an open circuit. So when you first connect voltage to a coil, it won't flow any current at all, but as time goes by, it takes more and more current until it looks (theoretically) like a dead short.

 

And if we continue to talk about this, I'm going to have to start wielding my calculus. Nobody wants that.

No! No they don't...  The first time I took calculus I withdrew with like a 25 out of 100. Got through with a "D" the next time. Done! I love math but calculus is not math. I don't know what it is...

Those condensers are hard to find. Can't find the Chevy one, seems to have disappeared from internet. There's "Standard" brand RFI condensers for various cars ranging from $3 to $35, but no specs for any in their catalog. Pep boys doesn't have any. The old guy at Autozone that knew everything seems to have retired, so if they have any, no one there can find them. Maybe try Napa. The independent parts store I go to had one RFI condenser, supposed to go on a voltage regulator, but it's 0.5 uF not .22 uF as Blue suggested (and backed up on some sites where I looked it up). The .5 uF one is way bigger than the old one on my car, might be wrong, expensive too. Per post#2 it might be different with the 36 to 40 K volt coils we seem to use.

I noticed yesterday the wire in mine is coming out. Stuck it back in. Great.

At autozone http://www.autozone.com/1/products/52465-radio-capacitor-duralast-urs1531.html

Edited April 29, 20159 yr by Stanley

Those condensers are hard to find.

Rockauto has 20 different choices. Many are on closeout, though.

I was guessing the one on the + side of coil was different spec than the one on the distributor, but maybe not; they don't list specs. Autozone online lists a bunch of RFI capacitors.

Just to muddy the waters furter, I read somewhere not long ago that capacitors have a shelf life and may lose effectiveness with age.  Anyone know more about this?  I suppose a bench test could verify 'good' vs 'bad'. 

You are correct. They can be tested with a meter that measures capacitance. 

 

http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=capacitance+meter&rh=i%3Aaps%2Ck%3Acapacitance+meter

Trying not to get into too much detail, but there are several different internal constructions used in capacitors. One of the major groups is called "electrolytic" capacitors which means "it contains an electrolyte". The addition of the electrolyte allows for a much higher capacitance per unit volume. In other words, you can have the same amount of capacitance in a physically smaller cap.

 

So the the up side is a physically smaller cap for the same value, but the down side is that the electrolyte is sometimes not a solid and that non-solid electrolyte can leak or evaporate over time reducing the value of the cap over time.

 

Better quality caps last longer. Caps at lower temp last longer. Caps that are operated at lower voltages last longer.

 

I've never dissected an automotive condenser, but I suspect it's what they call an "aluminum electrolytic", and yes, it's capacitance would probably drop with time, heat, and voltage. And as SteveJ suggested, bench testing can't really be done without a meter or circuit designed to measure capacitance.

 

All that said... I don't think that the value of the condenser on the coil POSITVE isn't that important. Certainly not as important as the value of the one used on the coil negative. I'd have no issue using a 0.5 uF instead of a 0.22uF on the positive side of the coil.

I've read (since this thread) about caps on coils and voltage regulators, and saw a Z this week with MSD that had a big cap on the alternator. Also read, on an aviation site, about the dangers of RFI/EMI interference damaging or affecting the performance of vital electronic equipment. Maybe it's a bigger deal for those using MSD etc.

 

Seems somewhat less mysterious  now. Autozone will have my RFI condenser (for GM cars from 60's & 70's, newer cars have different-looking RFI condensers) tomorrow, although the car seems OK without it.

Call me a skeptic, but I think a big cap on the alternator is snake oil. The battery is pretty much like a hugely gigantic capacitor already. I could see some benefit to having a small faster reacting cap located right at the alternator or regulator, but a big expensive chromed one is a gimmick.