distributor cap test

While watching the snow melt, I thought I'd play around with the new cap & rotor for my 3.2. According to the Bentley book, I can test the cap by "checking resistance of cap between tower and its matching contact inside cap. Resistance should be nearly zero ohms."
I set my multimeter to 20k ohms, then tested it for calibration by placing the probes together and received a 0 reading (apart, the scale is at one)
When I tried to test the cap, I found that there was complete resistance (reading 1) on each contact point. It wasn't until I scratched the surface of the inside contact points with the tip of the probe that I could get any reading at all.
1) It appears that the inside contacts must be coated with some clear lacquer like coating that's maybe left over from the manufacturing process. SHOULD I GIVE THE INSIDE CONTACT POINTS A LIGHT SANDING BEFORE INSTALLING?
2) When I do scratch below the surface of the inner contact point, my reading is about .96, not the recommended 0 resistance. What's the deal?
This is a brand new Bosch distributor cap. The rotor checks out fine. Any thoughts?
regards,
jlex.

No takers? IMHO, a new distributor cap's internal brass contact points should be bare metal for maximum efficiency. On the new Bosch cap, they're coated with lacquer, just like a brass bathroom fixture would be. Tip of the rotor is bare metal, why not the cap's pickup points?
Also, I think Bentley's spec of 0 resistance is wrong; should probably be .96-.99.
regards,
jlex.

jlex,

Is the reading you obtained 0.96 Ohms, or 0.960 K Ohms, which equals 960 Ohms? If it is slightly less than 1 Ohm, then don't worry about it ... on most digital multimeters you will NEVER see a reading of 0.000 Ohms!

Yes, use a wire brush or some method of removing the coating ... how about a bit of spray carb. cleaner such as Berryman B-12 Chemtool on a Q-tip or paper towel to remove the sealer? After cleaning, if a measurement less than one Ohm is obtained, consider the cleaning sufficient! Be sure to use plenty of silicone grease such as Dow Corning 4, 4X, 111, or 112, or Syl-Glyd, etc. on the plug wire ends to keep out oxygen and moisture! Good luck!

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Warren Hall
1973 911S Targa

Thanks for the reply, Warren. I guess Bently book is wrong if you say 0 resistance can't be obtained. .96 is all she'll go down to.
I guess I'm most surprised at the lacquer-like coating on what I would have thought should be absolutely bare metal. It sure looks real shiny inside the cap, but who cares? I want maximum conductivity for the spark coming off the rotor!
Will see if the carb cleaner fazes it. Looks like pretty tough clear coat, though, so I may have to get some 440 grit out to break it up before installation. Who knows, they may be thinking at Bosch that the spark will be so hot, it burns it off in short order.
You can be sure I'll be using a bit of silicon grease on the wire ends... getting the wires off without it is murder! Thsnks again.
regards,
jlex.

I don't know this for a fact but I do not believe the rotor actually makes "solid" contact with the contacts on the distributor cap itself rather its comes close and that is why old caps and rotors have burnt marks from arcing.

That's right, George... must be a millimeter or two of space between the internal contact and the tip of the rotor. The spark arcs across this gap. My point here is why impede the arc by having any kind of a coating on the contact? I think it's just a manufacturing glitch that Bosch didn't address before the cap was shipped out. I'm kind of wondering if they just skipped a final dressing process which would have made sure the contacts were bare metal... oh well, I've opted to sand off the lacquer. If anyone else buys a new distributor cap, you may want to do the same.
regards,
jlex.

No, the spring-loaded carbon rubbing block in the center of the cap DOES MAKE CINTACT with the rotor, and if you look, you will see a shiny spot on the rotor! There is a tiny gap out at the tip of the rotor ... perhaps 0.5 mm to 1.0 mm, though.

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Warren Hall
1973 911S Targa

[rob911]

I have just done this exact test, looking for the cause of "hard start" and rough running on 1984 3.2. Checking for a nice blue spark I see only yellow / orange - so tested ignition components.
I too had to clean up the internal connection (due to oxidation?? I suspect) before getting any reading. Still do not get anywhere close to Bentley manual zero ohms.

Measured the resistance of the distruibutor cap
Centre coil connection (exterior) to carbon brush (interior) = 1114 ohms
Each plug lead to the internal connector = 900 ohms approx

The plug leads are exactly as expected
end to end value = 3k ohms
So is the rotor
Rotor centre connctor to tip = 984 ohms
The meter does read zero when I touch both leads together.

Do I have a faulty cap?

[ed martin]

If the manufacturer laquered the contacts, they probably did it for a good reason, like to prevent corrosion for example. Corrosion over time would give far more resistance than a thin film of laquer. Certainly the voltages used in the system are great enough to overcome a lot a minor barriars like small gaps etc. I would think. A yellow spark is probably an indication of a bad coil.

[BURN-BROS]

This post is from 2001...