Rotor resistance

[Quickstep192]

Maybe a dumb question-

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

[John W.]

That should be zero.

[Quickstep192]

Thanks. That’s what I had thought. The one I’m testing reads open. I wonder how that could happen. Seems like there’s not a lot in there to go wrong!

[rwest]

There’s a resistor under all that black potted material- can’t remember the resistance off hand, but there will be some.

The resistor is a known “problem” area when running an MSD box as it will fry the resistor.

[Trackrash]

Yes there is resistance. I have read 3K ohms. Mine reads around 5K ohms.

[timmy2]

Originally Posted by Quickstep192

Thanks. That’s what I had thought. The one I’m testing reads open. I wonder how that could happen. Seems like there’s not a lot in there to go wrong!

What scale were you using on the meter?

[Trackrash]

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.

[Quickstep192]

Originally Posted by timmy2

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.

[Quickstep192]

Originally Posted by Trackrash

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?

[911pcars]

Originally Posted by Quickstep192

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

[dicklague]

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.

[mar2mar]

Just measured my 70S spare rotor with my cheap meter, 5.18K Ohms.

[Quickstep192]

Where can I get a rotor without the rev limiter?

[911pcars]

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.

[Jonny H]

Originally Posted by Quickstep192

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.

[Quickstep192]

Originally Posted by Jonny H

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.

[911pcars]

Looks like this:

[Jonny H]

Originally Posted by Quickstep192

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.