Rotor Fried

DSPTurtle · 05-05-2002, 11:38 AM

I was driving down the road, on my way home, after a nice Sunday morning drive when all of the sudden my 1.8L Carbed '74 loses power. I pull to the side of the road and start checking electrics. Once I get the cap off I find my surprise... the rotor has melted (see picture). I go and get a new rotor and install it and the car starts right up...for 45 seconds and then dies. Now it will not start. I thought it could be my MSD box so I bypassed that and nothing. I am not getting a spark to come out of the coil. Is it possible that a bad Pertronix could cause this? I pulled the wire off the distributor that comes from the coil and cannot get it to spark at all. Please help...I have fuel and air but no spark. I remember what a pain it was to get the Pertronix in there so I don't want to jump right to that conclusion unless someone else has had this experience. This would be pretty fitting huh...the guy who always says great things about the Pertronix is now the guy that can't get his car to start??? On a side note, the flat bed tow truck that towed me home (AAA rules) has the same shift pattern as our 914's. Interesting huh??? NOT Please help!!!

Rouser · Rouser's Garage

"I" haven't heard of an Ignitor unit shorting out and frying a rotor in that manner, since it's just a fancy (Hall Effect) switch for the coil. When they have failed, they either "opened" or "shorted", and refused to switch anymore.

I've heard of coils shorting out internally and getting hot, but I thought the MSD unit had protection against this type of external failure(s). Isn't it designed to shutdown by blowing a (replaceable) fuse? Am I wrong in this line of thinking?

Anyway, you need to determine if power is getting to the ignition circuit/system; see if maybe you blew a fuse or are losing power further up the Chain-Of-Command. Frustrating, I know, but it'll be something you'll be able to isolate with a wiring diagram in hand.

DSPTurtle · 05-05-2002, 06:52 PM

I went back and checked that I had 12 Volts going to the coil and I did. Then I checked the coil and it seemed ok. So I replaced the new rotor with an old rotor from a while back. I pushed down a little harder on this one and felt a good click. Popped the cap back on and she fired right up!!! Hmm... unverifiable failures are pretty nerve racking. I am still holding out hope that someone has some experience with fried rotors and why they happpen!!!

J P Stein · 05-05-2002, 07:36 PM

I assume you have a hot coil to go with your MSD.

My WAG would be you found the weakest link in the spark chain.
Is that a Bosch rotor? Thought the Bosch were tan colored.
It may have defective or ...........
Either way, I'm putting my Bosch rotor back in my dizzy.
I installed a NAPA jobbie while trouble shooting.

DSPTurtle · 05-06-2002, 07:25 AM

You are right about the coil...MSD Blaster 2. Does anyone even sell Bosch stuff anymore? All I can find is aftermarket stuff. Beck Arnley and Standard are the two big suppliers in my area.

John Rogers · 05-06-2002, 08:48 PM

I had to check, but I have burned up several rotors, one on our 930 and two on the race car. Both use a very high secondary voltage at high RPM and both get really hot which cause the middle to burn out just like your picture. The fix I guess is to go to a weaker system, not very good or do what I do, check them frequently and have spare(s). Check the inside of the distributor for moisture/oil and keep it very clean and look for any arching and replace the cap frequently. Good luck.

DDS · 05-07-2002, 03:55 AM

Here's a WAG based on my recent ignition system research and absolutely no personal experience.

Is there a resistor under that potting compound you see in the top of the rotor? If so, I've read that high current ignitions can cause that resistor to fail and the cure is to bypass that resistor with a length of appropriate sized wire soldered in place.

Is that it?

DSPTurtle · 05-07-2002, 04:02 AM

I think you need a resistor somewhere in the system. I do not have an external resistor. I am thinking of going with John Rogers advice...too bad MSD does not make a rotor that can handle all this power. I guess they want you to buy their distributor!!!

DDS · 05-07-2002, 04:28 AM

The par. below is from the tech area at www.aircooled.net

What is the purpose of that resistor?

High Output Engines

Another issue that you must be aware of is that high output ignitions MAY have a problem with the small diameter distributor caps on the Bosch units. There is a reason the MSD and Mallory distributors have a large cap, and that is to minimize the possibility of the spark jumping to a contact that it is not intended for! Another problem is that the Bosch rotor has a resistor in it, which WILL burn out in a short period of time if used with a high output ignition. You can dig it out, and solder a solid piece of brass in its place (it's between the tip and the cap contact area). Our new distributor has a large cap and bulletproofrotor, so that's easy!

DSPTurtle · 05-07-2002, 04:48 AM

Cool post Dave!!!! My plan is to eventually end up with the Mallory Distributor (it is supposed to make ten extra horses according to the TIV experts). I have an old distributor I will open up and post pics of... just got done ordering two new rotors and a new cap from our friendly BBS host. Hopefully it will be real Bosch stuff!!! Just another thing to pack in my "914 Trip" bag.
Maybe tonight I will dissect that rotor and maybe a few of us can collaborate on a Tech Article.
My only concern would be as to why the resistor is there in the first place...ie is there a safety thing that would be bypassed?

JB

DDS · 05-07-2002, 05:03 AM

I may go Mallory too, though I am curious about ignition sytems generally (I don't know enough) and specifically about the function of the various componenets, including that resistor.

JohnConnolly · 07:05 AM

The resistor simply reduces ignition noise to the radio. Increased ignition energy overwhelms the rotors wattage rating and burns it out. Replace it anytime you add ANY CDI to the ignition system before it burns out when it feels like it.

It's interesting to note we have offerred modified rotors since we opened over 3 years ago for $18, if you don't want to do it yourself. It's not difficult, but some people may not want to bother.

AFAIK, we are the only company that offers the rotor, or that even acknowledges the stock rotor as a problem with CDI ignitions, everyone else either doesn't know, or pretends they do not.

John
Aircooled.Net Inc.

DSPTurtle · 05-07-2002, 07:32 AM

So does the new and improved rotor screw up the radio reception? Do you need to add the resistor from MSD that is available as an option. Or were some German's just over engineering their electrical system?

Dave at Pelican Parts · Dave at Pelican Parts's Garage

Well, I did say something about the resistor inside the rotor on one of the e-mail lists... (Not sure which one any more; this post has shown up in a number of places.)

I believe that the Bosch rotor does have the resistor in it. There are aftermarket ones you can get (possibly from NAPA?) that do not.

I ccould never remember which ignitions had problems with which rotors... From John C's note above, it sounds like high-output CDIs have problems with resistor-equipped rotors.

--DD

JohnConnolly · 05-07-2002, 02:11 PM

if you look at the rotor, you can identify the resistance. I haven't looked in a while, but "1k" or "5k" identify 1000 and 5000 ohm resistance respectively.

If you use good wires the removal of the resistor only increases reliability. Plug wires is another mysterious autopart. ;-)

John
Aircooled.Net Inc.

fty · 05-09-2002, 07:52 PM

I fried my rotor just like that one on my 944...dont know how or why...

jpnovak · 05-10-2002, 06:15 AM

I have to admit I don't own a 914, but am warming to the idea. I had a Crane HI6 in my 911 for about a week. It fried my rotor and was arcing to the cap. Its been in the box ever since. I like the idea of soldering a jumper in place of the resistor. I might try that on one that is burned out.

Jamie

JohnConnolly · 05-10-2002, 09:03 AM

Here's a link for those that want to know how to do it. We sell them already done if you don't feel up to it, or if your time isn't worth the bother!

http://www.angelfire.com/nm/GoPed/RotorTech.html

it solves this weekpoint in the ignition system.

John
Aircooled.Net Inc.

zuffenhausen · 05-10-2002, 09:17 PM

After installing a Crane box and the hot PS92 coil on my 911 a few years ago, I went to a CarGuys DE at Road Atlanta and I didn't finish the first lap! Got towed in and found the Bosch rotor was cracked in two. The overheated wire outgassed and split the black epoxy. I went to the local NAPA store, bought the three rotors they had in stock (amazed they had them any all), and have used them without a glitch since. They have a solid copper strip all the way from the center contact to the outer edge. Hard to imagine a NAPA part being superior to a Bosch part, but the NAPA rotors are the ticket if you're running a hot coil.

-zuff