Distributor RF shield

[DanHoug]

i've never liked 'em on any car. they gather dirt and being made of conductive plastic-- that's how they shield the RF, always suspected them of conducting spark away too. well, here's proof as seen on coil tower. i removed the shield from a spare distributor and despite being a very clean shield and exposed towers, found corona damage carbon tracks under the shield. take 'em off!

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-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[Merian]

how do you know those aren't just rubber deposits from sitting there for a long time?
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....

[DanHoug]

the fan shape of mark on the coil tower indicates to me that there is high voltage corona action present. i see this in transmitters running 10kV plate voltage. BUT! there is not a carbon track down the coil tower nor individual cylinder towers. so it could be rightly argued that there is not actual flashover occuring.

here's where the rubber meets the road for me. these are marginal ignition systems at best compared to modern Coil On Plug or CD ignition. it's a standard Kettering affair with a solid state coil field collapse. any marginality-- whether it be from flashover, corona dissipation, or non-ionizing bleed is a loss to the joules of spark energy. clean, dry, isolated coil and plug towers deliver the most energy to the plug. adding a conductive shield immediately at the termination of the tower boot defeats the entire isolation of the dist cap itself. there just isn't a margin to afford that loss.

the shield was put in place to reduce static interference to the neighboring car at a stop light listening to AM radio. given the content of AM radio, i could care less if someone gets interference idling next to me.
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-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[dhaavers]

D@mn... and I just spent $2 extra for a cap with the shield...

- Dave
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86 White Wolfsburg Westy Weekender
"The WonderVan"
<EDITED TO PROTECT INNOCENT PIXELS>

[Wildthings]

If there was a carbon track running down the tower where the spark plug boot fits, I would be more in agreement with your argument.

.

[DanHoug]

[djkeev]

I have a used cap like this.

The plastic cover replaced the older metal cover that covered the cap and was grounded to the distributor body.

So you are saying the black plastic is conductive?

Mercedes has used these plastic cover caps for like...... oh ..... forever.

Interesting.

It makes sense they are somehow electrically active, but I would think in a negative way. Like electronic kryptonite to RF waves.

Dave
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Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[Ronzo_volvo_guy]

[Wildthings]

[DanHoug]

more forensics...
first, inside of RF shield. the 4 cross supports on the coil tower correspond to the 4 'corona' marks

tower prior to any cleaning

here's where it gets interesting, next photo is AFTER attempting cleaning with ammonia, carb cleaner, brake cleaner, and paint gun cleaner. no solvent really touched the marks even with vigorous rubbing. it's either carbon or Kryponite. or Carbonite.

lastly, i took my fiberglass bristle pen to it for some abrasive action. VERY resistant to removal but you can see a) the gloss finish was removed, and b) even with material removal it appears these marks are deep.

so. if it was just dirt it would have washed away. i'm going to contend that there was high voltage breakdown THROUGH the coil tower to the conductive RF shield. there is no indication it went down the side of the tower, those are clean and shiny.

for those that are doubtful allow me to give an example... i have a 'plasma ball' that has a high voltage source in the center, a noble gas filled interior, and a glass ball about 6" in diameter. the glass is totally non-conductive but the high voltage coronal arcing does indeed 'sense' the presence of my hand nearby and the electrons whoosh over to where my hand is looking for a return path to ground. it even tingles a bit through the glass as they do the seemingly impossibly thing of leaping thru non-conductive glass. the electrons want to get home in the worst possible way. it's not enough that a material is non-conductive... its dielectric strength is also a function of thickness. placing a conductive mask over the dist cap reduces the dielectric strength to merely the cap thickness instead of the cap thickness PLUS free air to the nearest conductor.
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-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[DanHoug]

[Wildthings]

I am wondering if whoever had that distributor previously had the correct rotor installed. Run a rotor with a narrow tip verses the correct one for the Digifant with the wider tip and you will get much higher voltages building up in the coil at times.

[tjet]

[Abscate]

[DanHoug]

to respond to some questions... the cap came off a dist i bought that is very clean and shiny with no corrosion, looks very low mile. i have no history of the cap and plug wires but i can say that the correct rotor was present when i disassembled.

the Vanagon RF shield was adopted from '89-'91.

i liken high voltage to a woodpecker on a tree... put a new tree in front of the woodpecker periodically and little damage is done. let the woodpecker whack away on the same tree over time and they'll drill a hole thru it. obviously, the RF shield is not a problem when everything is new and the cap is changed on a regular basis. but let things age a bit and the dielectric strength becomes compromised.

what is that time frame, how often should the cap be changed, what is the quality of new caps? dunno. but a cap without a shield is easy to inspect, keep clean with a rag, and presents the highest dielectric isolation to misfires. reason enough for me to ditch them at the possible expense of the AM radio listener in the car next to me.
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-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[djkeev]

And to think, here I was thinking that I should install the newer "better" cap with the RF shield!

Then again....... maybe not.

Dave
_________________
Stop Dead Photo Links how to post photos

Ghia
http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

Vanagon
http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

Beetle
https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[Ahwahnee]

Were there any issues with the metal shield used on earlier Vanagons?

Certainly seems effective, perhaps cost or parts rationalization was the reason for the change?

[DanHoug]

i grew up with a Dad that was an automotive tech school instructor, he was born in 1924 and after WWII entered the automotive world. so my bias is tainted with some rilly rilly old school do's and don'ts. he always pushed keeping the secondary ignition system clean including the cap, coil top, and wires.

so when i see RF shields right around the cap that a) are seductive to electrons zinging around inside the cap and b) prevent frequent inspection and cleaning my old-skool bias frowns. again, when everything is shiny and chrome i think it is okay. for a while. until it isn't.

remember, these things were put on to stem complaints from the vehicle owner that their AM radio had static. i remember hearing this all the time... a very fast ticking that would rise and fall with RPM but for me it was always the canary sentinel that SOMETHING was starting to deteriorate in the secondary ignition system. new plugs, plug wires, cap/rotor would usually stop the AM interference. my personal opinion is that RF shields gave a greater 'service interval' for AM complaints. i've got some really well-produced Delco-Remy Radio instruction books on radio servicing and AM noise complaints is a large section. for most purposes, there was not a large installed base of FM car radio in the US in the '60's and early 70's.

as we frequently say, we're driving antiques. these respond to old school basic automotive principles. the cleaner you keep the secondary system INCLUDING spacing from adjacent wires and conductors, the better. look at an old V8 from the 1960s... the entire plug wires were suspended on standoffs on the valve covers, with insulated wire guides separating the wires from each other and the valve covers. the distributor caps were out in the open using air as part of the dielectric insulation.

if AM reception is important to you, keep the RF shields on and really keep up on your annual tune-ups.
_________________
-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[Ronzo_volvo_guy]

I (*) agree with Dan's findings and explanations...it certainly looks like through the Cap to the Shield was the easier path...I just have to wonder why the wire and Spark Plug weren't...too high an HV from an "upgraded" ignition system maybe? Plus, every time one of those unplanned/unwanted EDM (Electronic Discharge Machining) zaps took place, it must have resulted in a noticeable misfire (even bucking) of the engine...which apparently was summarily ignored for a looooong time. Lesson to be learned: Don't ignore misfires!

Ahwanee; Your all-metal shield would be less effective as an RF Shield, as where the sparks occur is not really encased and covered that well, but it is also further away with a much bigger airgap for HV to jump, so actually better from a HV-withstand-ability standpoint, so as long as you keep the Ign Sys maintained and clean (as it sure looks), and you experience no misfires, I would say you are OK. How much ticking or "Ign singing" are you picking up on you AM radio...in between stations?...do you even have an AM radio?

Cheere

* Edit: Dan's and my posts crossed paths in the ether....please insert "strongly" here!...a clean Ignition Sys is a happy Ign Sys!