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'73 911 T Targa
 
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Rotor resistance

Maybe a dumb question-

Should I be able to get a resistance reading across the contacts on the rotor?



Old 09-16-2020, 04:49 PM
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John W
 
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That should be zero.
Old 09-16-2020, 04:54 PM
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'73 911 T Targa
 
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Thanks. Thatís what I had thought. The one Iím testing reads open. I wonder how that could happen. Seems like thereís not a lot in there to go wrong!
Old 09-16-2020, 06:10 PM
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Thereís a resistor under all that black potted material- canít remember the resistance off hand, but there will be some.

The resistor is a known ďproblemĒ area when running an MSD box as it will fry the resistor.
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Old 09-16-2020, 06:14 PM
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Yes there is resistance. I have read 3K ohms. Mine reads around 5K ohms.
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Old 09-16-2020, 06:24 PM
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Quote:
Originally Posted by Quickstep192 View Post
Thanks. Thatís what I had thought. The one Iím testing reads open. I wonder how that could happen. Seems like thereís not a lot in there to go wrong!
What scale were you using on the meter?
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Old 09-16-2020, 06:54 PM
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I have an old one that reads open as well. I believe the resistor just has burned out, as in open. It still works though.
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Old 09-16-2020, 06:58 PM
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'73 911 T Targa
 
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Quote:
Originally Posted by timmy2 View Post
What scale were you using on the meter?
I started out just checking for continuity. After that, I tried every scale. Nada...

It's an old one that I was going to remove the rev-limiter from to test something, but I think this one will seriously limit revs.
Old 09-16-2020, 07:04 PM
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Quote:
Originally Posted by Trackrash View Post
I have an old one that reads open as well. I believe the resistor just has burned out, as in open. It still works though.
Well now the plot thickens. How can it work if it reads open?
Old 09-16-2020, 07:10 PM
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Quote:
Originally Posted by Quickstep192 View Post
Well now the plot thickens. How can it work if it reads open?
About 10K voltage will bridge a gap wherever it exists in the secondary system, a gap in the rotor, a conductor open circuit in a spark plug wire, or the spark plug gap. For the long term, during low load driving, it isn't healthy for the ignition coil to produce excessive firing voltage. It can, over time, break down the insulation properties of the conductors along the secondary voltage circuit.

Does the ignition coil get hot to the touch after a drive?

In addition, a high voltage spark gap will tend to increase over time. Could eventually cause a misfire under load, equivalent to an excessively large spark plug gap.

Sherwood
Old 09-16-2020, 07:32 PM
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In my 1973 911 2.76 MFI I removed the rev limiting distributor with resistor when I up graded the CDI from OEM Bosch,

I installed a Daytona-Sensores CD1 system and love it. I use the internal built in electronic rev limiter and set it to 6600 RPM. I used a VW non resistor rotor.

The resistor rotor will burn out if you use a higher power CDI.

I have had this system in my 911 for a dozen years and over 100,000 miles....and I love it. I also have a Pertronix point replacement system. I never have to worry about point adjustment or wear. The system just works.
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Old 09-17-2020, 08:39 AM
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Just measured my 70S spare rotor with my cheap meter, 5.18K Ohms.
Old 09-17-2020, 09:57 AM
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Where can I get a rotor without the rev limiter?
Old 09-17-2020, 11:29 AM
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I remember sourcing a VW rotor for my S distributor many years ago. Don't remember what MY VW.

They should be cheap enough to have in stock at your local FLAPS. Measure and compare dist. shaft and rotor radius just to be sure.
Old 09-17-2020, 11:33 PM
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Quote:
Originally Posted by Quickstep192 View Post
Where can I get a rotor without the rev limiter?
No need to. If you have a limiting rotor like in the first photo, just remove the spring and take the plunger out. That will defeat the limiter.
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Old 09-18-2020, 12:36 AM
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Quote:
Originally Posted by Jonny H View Post
No need to. If you have a limiting rotor like in the first photo, just remove the spring and take the plunger out. That will defeat the limiter.
I mostly don't want to spend the money on a rev-limiting rotor just to rip out the plunger for an experiment. That's kinda what started this post. I wanted to rip the plunger out of an old rotor, but it appears that rotor is toast.
Old 09-18-2020, 07:17 AM
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Looks like this:

Old 09-18-2020, 08:05 AM
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Quote:
Originally Posted by Quickstep192 View Post
I mostly don't want to spend the money on a rev-limiting rotor just to rip out the plunger for an experiment. That's kinda what started this post. I wanted to rip the plunger out of an old rotor, but it appears that rotor is toast.
No need to ‘rip out’. Just take it out carefully. You can put it back if you want!!!!

The 3.2 non-limited rotor fits an SC but the resistance is 1k ohms. SC is 5k ohms.

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Last edited by Jonny H; 09-19-2020 at 01:04 AM..
Old 09-19-2020, 12:59 AM
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