Any theories on this?

[911-m5]

1970 911T

When I start the car within 30 seconds the ignition rotor gets so hot that the shiny black stuff behind the contact starts melting and smoke comes pouring out for under the cap. It has burnt 3 rotors. Kinda seems like it's not grounded to me - but when it comes to the electrical stuff, I know very little.

Any theories are greatly appreciated.

Thanks
Allan

[Steve87-911]

Is the rotor seated on the shaft properly? i.e., not so high that it's crushing the coil contact on the cap? Is the coil contact on the cap getting burned too? I mean can we rule out too much friction?

[911-m5]

The rotors seat just fine. It is definitly an electrical problem.

Thanks

[evill]

What type of coil are you using? I used a Crane PS-91 coil with a Crane HI-6 ingition box once and I kept melting the center of the rotors.
I put the regular Bosch coil with the HI-6 ignition box and all was fine, rotors stopped melting.

Ed

[randywebb]

what does the contact surface look like?

show a pic of the rotor and the inside of the cap...

[911-m5]

'im going to take some pics tomorrow. the car has a Bosch distrib. and I thin its a Bosch coil - I'll check tomorrow. The ignition boc is a relic aftermarket CDI box - goes by the name of...SPEED-O-TRON!!!

[911-m5]

Well, I took some pics... but now that I'm home, 25 miles away, they turned out to be the worst pics I have ever taken. I'll post them anyways.
Just to reiterate - It IS NOT a fit problem. It is an electrical problem.

Thanks

You can see where the black area on this rotor is melted - I have two more that have done the same thing.

[911-m5]

I dont think these will help either - I don't know how I took such bad pics!

[randywebb]

that's why the SLR was invented - so you can see if things are in focus on a macro shot...

anyway it looks bright and shiny so that rules out my first thought

[Gary Peters]

The same thing happened/ is happening to me. If I put in a cheap rotor they will do that in less then 50 miles. I have changed everything in my system (plugs, wires, harness, dizzy, coil and CDI box) and nothing has helped. As long as I use the ones like you have it doesn't do it often though. I'm so fed up that I'm putting in a complete SDS engine management system soon. I always have an extra one in my glovebox as well.

I wish that I could help you more. This is a problem that seems rare as no one I talked to could help. I'll wish you good luck and hopefully someone can shed some light on this problem.

[silverc4s]

Well speaking electronically, it takes current to cause that amount of heat along with the voltage that we KNOW is there.

CD Ignitions typically develop 2 to 3 times the High Voltage that a normal Kettering ignition does 30,000v to 40,000v in the secondary vs 12,000 to 15,000 normally. That higher voltage is only there for a MUCH shorter time, however- a few microseconds at most, vs milliseconds for the normal point based triggering pulse.

So, what am I saying? I find it hard to believe that the 400volt 100microsecond or so pulse output from a CD system, put through a ignition coil could generate enough average power to cause this melting of the rotor, or the epoxy within the rotor as may be.

i would say that there is a wiring fault that is applying battery / alternator voltage, unfused or fused at very high current to the secondary ignition through the rotor to ground. 13.5 volts dc through 0.5 ohms , by e*2/r would be about 365 watts, more than enough to melt the rotor epoxy as the metallic element turns into a 365w heater element.

You should be able to check with a voltmeter and find NO dc volts on the rotor element or the center connection of the cap under any conditions referenced to engine ground..

Happy hunting..

[SpeedracerIndy]

Can't help with your electrical problem, but with your digital pictures you should have a mode on your camera for close up shots. I had the same trouble when I started taking pics of my car parts, then I read the manual for my camera and figured out how to take them. On my camera (Cannon) there is a button with a picture of a flower. When I push the button a flower come up on the screen and a single box appears to aim with. Most cameras have something similar for closeups.

[911-m5]

About all I know of the electrical system I think you could fit inside a distributor cap.

If I can decipher what C4S is saying, I'll give it a shot. I dont even own a voltmeter - guess it's time to get one.

Speed - You are right. I forgot about the flower setting.

Gary - let me know what you do/ how it goes. I 'll do the same if I get it figured out.

[Early_S_Man]

Allan,

Well, that rotor is toast ... but, what happened to the rev-limiting parts that would normally reside out near the grounded, upside-down U-tab at the 7 o'clock position???

Also, I suggested a fix to your earlier (and current) burnt resistors in the rev-limiting rotor back on 27 March ... bridge the resistor element with solid wire!

Why didn't you try John Walker's suggestion to use an early, air-cooled VW rotor without resistor or rev-limiter?

[911-m5]

Warren,

I ordered two non- rev limiting rotors from Pelican. I got them on thursday, but they will not seat properly on my distributor - they will only go about half way down. It is almost as if the hole on those rotors is tapered. I even took the metal tension piece out of the inside on one of them thinking that maybe it was happering the fit, but it made no difference. Hosestly, I didn't try what you suggested because I'm not sure what exactly to do. Like I said I know very little about the electrics. I guess I need a picture or an illustration of what you are suggesting.

Thanks again,

Allan

[911-m5]

One last bump in the hope that somebody has had this problem and knows what it is.

[silverc4s]

I think you have a bad coil, OR a wire is shorted inside your distributor somewhere. Or the ground on your CD box is at +Battery voltage somehow??
That's the best I can do from here, with what you've told me.
I really like your SS Mufflers though...;-)

[Early_S_Man]

Allan,

What are the Porsche & Bosch part numbers for the plain rotors you got? Are they 356 or 912 parts?

Did you try to find an Echlin EP141 rotor at a NAPA store? That is the air-cooled VW non-resistor rotor.

[911-m5]

I haven't tried that Warren. Now that you have given me a part number I cetainly will, though. The car is about 30 minutes away from my house, but when i get up there again I'll get the part # off of them. Whatever they are, I ordered them from our host.