Rotor and Cap Continuity?

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mydart270

mydart270

myDart270
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Ok..real dumb question. Very embarrassing to me!

Why don't I measure continuity from cap hi-tension port to spark plug ports using my FLUKE (OHMS) scale when rotating shaft/gear? Distributor is on bench, clamped in vice using adj plate, rotor and cap installed/assembled.

Two styles of rotors, one "angled" contact and the other "stepped w/ horizontal contact" installed. Both rotors show NO CONTINUITY w/ cap light-brown (original) cap installed. Different caps also tried, same results w/ ea rotor. Positive contact assured w/ FLUKE and meter is GOOD. Also noticed that shaft can be "pulled" up/down very slightly, even w/ rebuild distributor and in OEM distributor (p/n 2875199) which was removed because very worn (varying dwell). Up/Down movement must be causing this no-continuity but not when installed....right???

Pls clarify and/or explain ...basics are not making sense to me.

A-Body forum is the best...dist being installed today before snow flies!!!!
 
My guess would be that the spark actually jumps across rotor and cap and that there is no actual contact between the rotor and cap.
 
Hello,
If I am understanding you correctly the reason is there is an air gap between the rotor and the spark plug towers in the cap when fully
assembled. The center tower (Coil) has a carbon contact that rubs on the center of the rotor But the end of the rotor never directly connects (air gap) to the spark plug towers.
Hope that helps
Happy Mopar:)
Arron
 
Like Mom said it jumps, you will never get continuity that way unless something is bent. Have a picture of both rotors, distrib and cap so I can help you out? The up and down play is normal up to .040 I believe.
 
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Leave cap out of it for now, hook meter leads up to dist pickup leads, spin dist, it should give you intermittent beeps.
 
Need an air gap or rotor/cap would destroy themselves.
Last week my truck croaked.
When i pulled cap, it was evident replacement was needed. Thought i took a pic, guess not. The carbon button was gone,centre of rotor was burned up, could see it had not touched for a while.

Would be like testing a spark plug for continuity.
 
halifaxhops said:
Like Mom said it jumps, you will never get continuity that way unless something is bent. Have a picture of both rotors, distrib and cap so I can help you out? The up and down play is normal up to .040 I believe.
Click to expand...

Well...that answers it totally...many thx.
I never knew we had an "air gap" and continuity was made by "a spark" between rotor tip and each s/p tab (port) inside the cap. Up/down shaft movement is natural and up to allowed tolerance. Thx for clarifying.
Doesn't air-gap "spark" destroy the s/p tabs inside the cap? Longevity of cap and rotor ultimately suffers. I guess that's why we replace both at ea tune-up. Ultimately I will install electronic ign and all this goes away.
A-Body forum is the best!!!!
 
Electronic ignition won't fix the rotor gap, unless you are talking about converting to coil-on-plug.

There are many problems with rotor gap, although that system has worked for decades, "just fine."

I guess you know, on a side note, that the model T Ford was "in effect" "coil on plug" having a spark coil for each cylinder
 
Yes there is a little wear/deposits on the cap posts over time. If they aren't too bad they can be cleaned or just replace the cap and rotor. Halifaxhops can help you with quality parts.
 
mydart270 said:
Well...that answers it totally...many thx.
I never knew we had an "air gap" and continuity was made by "a spark" between rotor tip and each s/p tab (port) inside the cap. Up/down shaft movement is natural and up to allowed tolerance. Thx for clarifying.
Doesn't air-gap "spark" destroy the s/p tabs inside the cap? Longevity of cap and rotor ultimately suffers. I guess that's why we replace both at ea tune-up. Ultimately I will install electronic ign and all this goes away.
A-Body forum is the best!!!!
Click to expand...
Points work fine on a slant unless your racing. Biggest thing don't leave the key on and use quality points and condensers. I only use old NOS/OSS
 
Had one come in like that kept destroying the cap due to shaft play. Actually destroyed the advance part of the shaft, the small part that the cam plate rides on. Good idea though.
 
Murray said:
Gap can be minimized by soldering a copper wire across the face of the rotor contact.
Click to expand...

Napa part number MO3000 rotor for the electronic distributor is .030 longer at the tip.
It cuts down on the damage caused by the spark jump, so I use this rotor with my HEI ignition.
 
My understanding is that the rotor/cap gap, combined with the spark plug gap, allow for more coil saturation and thus a hotter spark. This may not be true on newer electronic ignitions.
 
Murray said:
My understanding is that the rotor/cap gap, combined with the spark plug gap, allow for more coil saturation and thus a hotter spark. This may not be true on newer electronic ignitions.
Click to expand...

Actually coil saturation time (dwell) is determined by the ignition module or by points closed time on a points ignition.

That said, there is also something to the idea that a spark that jumps a gap has more inertia.
This is why a fouled plug can sometimes be made to fire by pulling the plug wire back and making the spark jump a bigger gap on it's way to the plug.

I use this method to help determine if a missing cylinder is a bad or foulded plug or possibly a valve or compression problem.
If you make the spark jump farther to the plug and that cylinder doesn't change any, there is probably a bigger or different problem.
It's not a tell all, but just a possible clue.
 
TrailBeast said:
Actually coil saturation time (dwell) is determined by the ignition module or by points closed time on a points ignition.

That said, there is also something to the idea that a spark that jumps a gap has more inertia.
This is why a fouled plug can sometimes be made to fire by pulling the plug wire back and making the spark jump a bigger gap on it's way to the plug.

I use this method to help determine if a missing cylinder is a bad or fouled plug or possibly a valve or compression problem.
If you make the spark jump farther to the plug and that cylinder doesn't change any, there is probably a bigger or different problem.
It's not a tell all, but just a possible clue.
Click to expand...
I like what TrailBeast said in this post. Not sure I will go the route of reducing gap between rotor and s/p tab under cap. Car died last Sunday, pulled distro, (no vacuum control, mechanical vacuum OK), installed re-manufactured distro, replaced points w/ better quality, set points 0.20 in, installed same distro and rotor, fired up and idles nicely. Dwell = 43 deg, timing=8 deg BTDC. Go figure.

Happy camper, love my Dodge Dart 270, SL-6.

Thx guys...A-Body Forum the best
 
mydart270 said:
I like what TrailBeast said in this post. Not sure I will go the route of reducing gap between rotor and s/p tab under cap. Car died last Sunday, pulled distro, (no vacuum control, mechanical vacuum OK), installed re-manufactured distro, replaced points w/ better quality, set points 0.20 in, installed same distro and rotor, fired up and idles nicely. Dwell = 43 deg, timing=8 deg BTDC. Go figure.

Happy camper, love my Dodge Dart 270, SL-6.

Thx guys...A-Body Forum the best
Click to expand...
sorry guys...installed same cap. Old distro returnedand received replacement based on warranty.
 
TrailBeast said:
Napa part number MO3000 rotor for the electronic distributor is .030 longer at the tip.
It cuts down on the damage caused by the spark jump, so I use this rotor with my HEI ignition.
Click to expand...
nice to know
 
halifaxhops said:
Run Better?
Click to expand...
Yes runs better, even idles at 680 rpm, new dist installed (re-manufactured) but don't know why it sputtered, died, and would not start. 2nd time this year, both dist at fault. 1st dist cam lobes no consistent 0.020 in setting, 2nd dist no vacuum advance. I always pull dist and set points on bench (extra work I know), ck vacuum advance, and hand feel mechanical advance. Only car at show w/ no electronic ign....healthy discussions for sure but originality prevails.
thx guys ...long live MOPARS
 
Appreciate Trailbeast input. Have actually done rotor trick to minimize gap with no noticeable change in performance. This all started in the "old days" when clear caps were available and you could see the gap and it looked huge to me.
 
mydart270 said:
Ok..real dumb question. Very embarrassing to me!

Why don't I measure continuity from cap hi-tension port to spark plug ports using my FLUKE (OHMS) scale when rotating shaft/gear? Distributor is on bench, clamped in vice using adj plate, rotor and cap installed/assembled.

Two styles of rotors, one "angled" contact and the other "stepped w/ horizontal contact" installed. Both rotors show NO CONTINUITY w/ cap light-brown (original) cap installed. Different caps also tried, same results w/ ea rotor. Positive contact assured w/ FLUKE and meter is GOOD. Also noticed that shaft can be "pulled" up/down very slightly, even w/ rebuild distributor and in OEM distributor (p/n 2875199) which was removed because very worn (varying dwell). Up/Down movement must be causing this no-continuity but not when installed....right???

Pls clarify and/or explain ...basics are not making sense to me.

A-Body forum is the best...dist being installed today before snow flies!!!!
Click to expand...
Contact and rotor do not actually touch, there for you wont read continuity with a Fluke. When engine is running, voltage is high enough to arc and bridge the gap
 
I have noticed that on a rotor spring with sharp top edges, it can scrape off the center contact in the cap, when wiggling the cap to get it on. That of course destroys the center contact and leaves carbon dust on the rotor and inside the distributor. The carbon dust conducts electricity and could cause arching.
When I could not find a replacement rotor without the sharp top edges, I filed them to remove the burr and make them smooth. As another precaution, I turned the rotor position to point at a side clip so that the cap would slide parallel to and on top of the spring, instead of across it, if the cap needed to be wiggled to get it on and the clips snapped in.
Some rebuilt distributors have tight fitting caps and it is difficult to get them on while in the vehicle. Good parts are hard to find. Buy made in USA, if you can find them.
Richard
 
Gap is not the "full story." In the 70's GM was screwing around with longer and shorter gaps in their V8 setups. Meanwhile Champion had come out with their "booster gap" spark plugs which actually had a gap in the ceramic insulator, and the GM shorter rotor nullified the Champion "booster gap."

This gets complicated, and is related to the load impedance the coil sees. This is not at all like your house power or a battery connection, which is a simpler amps/ vs volts/ vs circuit resistance deal
 
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