crg53

I checked my distributor caps today and found that I have a fairly new one and what looks to be a pretty old one. Below are a couple pictures of the older one after I cleaned it up a bit; what do you think, should I get a new one or is this OK. Also ,what should the resistence be between the contacts?

Indycam

That caps not new , nor is it so old as to be needing to be replaced , imho .

"Also ,what should the resistence be between the contacts?"
Eh ?

darth

Cap looks pretty bad. The black carbon lines are where the electrons have been travelling and shouldn't be going anywhere except thru the contacts and down the spark plug wires. All the contacts should be insulated from one another or have infinite resitance between them. The micro scratches I assume you created by cleaning invite more carbon tracking. If you want to save the cost of a new cap and want to spend time rejuvinating the old one, you have to remove all the black carbon tracks with a sharp instrument like an exacto knife or dental pick then polish the scratches out with something like a Dremel tool using an abrasive like Brasso. Any deep scratches you can't polish out have to be filled with a non conductive spray coating like a clearcoat spray (i.e. I used an electronic spray coating manufacture by MS) in order to make a smooth surface. The carbon on the contacts can be left as is. It's a tough job with all those nooks and cranys inside the cap, depends how much time you want to spend on it or what your time is worth. Also, you have to consider the top of the cap as electricity can leak out there too. I had a carbon track on the top of one of my coils between the high tension contact and the +ve terminal which I polished out but that was an easy surface to work on.

Bill

Indycam

"The black carbon lines are where the electrons have been travelling"
The tracks are not touching metal so where are the electrons running from and to ?

darth

"The black carbon lines are where the electrons have been travelling"
"The tracks are not touching metal so where are the electrons running from and to ?"
Rules of the universe or laws of physics says, from a hi potential to low potential (electrical). An analogy would be gravity from hi potential to a low potential (height). Everything in the universe wants to be at ground potential or at a lower potential. The track is in its infancy if left long enough it will likely touch, unless it finds an easier path to another lower potential along the way, possibly the center conductor but probably not as it is at a higher potential 6X more often than the contact . Right now it is just a stepping stone from one metal contact to the one beside it.

BlueHeeler

IMHO what is the point of questionable parts? You can buy new caps, rotors, and plugs on ebay for $190 shipped and do not have to worry about it again. Plus you get to spend quality time with your car for installation.

I am a prevention over cure if you already have it apart kind of guy when it comes to the car.

amr89c4

Cap is arcing and contacts have high carbon buildup and therefore increased resistance. Its time for a new one. On top of the 'laws of the universe', electricity in ignition systems can do some downright funky things, that you wouldn't think possible.

Indycam

"high carbon buildup" ?
Where did this carbon come from ?
The spark contains no carbon
and the post is not carbon , so where does the carbon come from ?

"and therefore increased resistance" ?
That beautiful finish on the post lowers resistance .

tgage

One of the caps was clearly not buttoned down correctly as it has scrapes on it showing it was run off-centered. I'd try just cleaning first... then because an inferior part will just bug the crap out of you, you might as well order a replacement and enjoy your cleaned up part for a week..... Sort of best of both worlds.....

springer3

Plastic is a hydrocarbon. Lots of carbon. Dry-band arcing when moisture is present leaves a carbon deposit, that eventually shorts out the insulation. This is a very common problem in dielectric materials, and the reason you replace the distributor caps.

Indycam

So the "carbon" on the posts is the plastic from the cap ?
Is that what you are telling me ?

Indycam

"it has scrapes on it showing it was run off-centered."
I don't think so .
If the rotor was hitting the cap ,
the metal tip on the rotor would leave a mark .
The "scrapes" are from the machining of the cap posts .

http://media.photobucket.com/image/d.../Pict0613a.jpg

darth

The posts on a new cap will have a carbon build up on them after a couple thousand miles or less. The center conductor is carbon and conducting carbon dust from this center conductor and or bearings as they wear combine with moisture (or oil from a leaking oil seal) to form a wet conducting contaminate (carbon conducts electricity). This conducting film, along with the large electrical potential difference between distributor block electrodes, causes a current of a few milliamps to flow through the moist layer causing slight heating. This heating leads to the formation of an occasional very narrow "dry gap" in the conducting film. Concentrating most of the voltage across the tiny dry gap resulting in tiny arcs. Such dry gaps and arcs occurs randomly over the surface. Carbon arc tracking creates a high-voltage short circuit causing the wrong, or multiple spark plugs to fire. Each tiny arc deposits a small spot of carbon. Over time a complete "carbon track" path forms to enable flashover. By the time you can see the "carbon track" your engine has been firing the wrong plugs at the wrong time; a situation that could result in preignition and severe engine damage. The carbon traces between what I would call walls or dams between the electrodes is most concerning and if they don't clean up and possibly even if they do cleanup then replace the cap unless you're gonna coat the inside with the Miller Stevenson urethane spray I mentioned having excellent dielectric properties up to 15000volts but I believe a can of the stuff will cost you just as much or more than a new cap. Some things just arn't worth fixing. Consider replacing the wires and spark plugs as they all work together as a system and replacing the cap and nothing else will lead to premature wearing of the cap not to mention you will be running with a subpar part and a system is only as good as its' weakest link.

pete000

I replace my cap and rotor on my old VW Vanagon all the time, cheap and seems to run a lot better with the newer cap and rotor.

I did the same with the 964, while not as cheap its just worth it to replace them when they show signs of pitting and arching.

springer3

Not entirely. You asked for one example of where carbon could come from. I am sorry I was rushed and did not leave a better answer.

Most contaminants also contain carbon - oil (deposited when vapor enters from the engine), traces of fuel (also gets in as a vapor), exhaust fumes, pollen, mildew, bacteria are all carbon-based. Leave a finger print behind, and you left traces of carbon from skin oil and skin cells. Ionic contaminants (salts) make any water condensed on the surface conductive. The heat, either from the warming engine or from from the current flowing in the moisture, drives off the water. The surface becomes dielectric (insulating) again when it dries. Here is the tricky part: arcs start to jump from wet surfaces to adhavent wet surfaces across dry bands that form in the drying water film. I have seen this phenomenon many times where I work. We run a test to determine which insulations are more resistant to carbon tracking. It is the sparks in the dry bands that burn plastic insulation and any carbon-based contaminants present. The carbon tracks are conductive even when dry, and they eventually bridge between the voltage terminals.

Google "dry band arcing", and I am sure there are better explanations. I am the mechanical engineer at the lab, and they just let me watch the high voltage stuff.