Anyone ever see a Distributor Rotor Do This?

jluetjen · jluetjen's Garage

I was driving my 911 to a customer meeting today, and after coming off an exit ramp, the engine died after coming to a stop-light. I started cursing my MSD ignition box since this seemed to be the exact same symptoms that I saw when my last module burned out. Unfortunately I was about 40 miles from my house, but only about 1 mile from my customer. So I called them up and they picked me up for our meeting and I figured that I'd let the car cool since that had seemed to do the trick in the past. Given the distance from home, I ended up calling up a tow rather then risk having the car die on the highway.

We got back to the car as the tow truck was just starting to put it on the ramp. I tried starting the car, but it wouldn't start. After dinner I tried again with the same results. After swapping out the coil with no affect, I swapped out the ignition module with my spare (the burned-out unit after MSD repaired it). Still no start. I asked my wife to watch the distributor wire for a spark. Judging by her tone of voice I seemed to be getting lots of spark.

So I pulled the distributor cap (why didn't I think of that earlier???) and found this in side.

Has anyone ever seen a rotor self destruct like this? What could cause this? The engine was running great up to the point when it died and I've been getting a pretty consistent 23 MPG on each tankful of gas.

rwest · rwest's Garage

The MSD has such a "hot" spark, that it can melt the resistor in the rotor, search the forum for MSD rotor mods or similar, and there are several good threads that show how to prevent that, I had noticed mine melting before I had seen the threads and modified it and it is working fine.

Rutager

Rot 911 · 08-20-2009, 06:01 PM

Had a Fiat in Germany way back when did the exact same thing on the autobahn.

pwd72s · 08-20-2009, 06:01 PM

Hard to tell with the destruction, but I've heard that MSD units can burn out the rev limiting rotors in 911's...just too hot for the rev limiting piece.

rwest · rwest's Garage

John,

There was a real good thread with photos showing how to modify a rotor so it doesn't melt, I just did some searching and couldn't find it. what the problem is, is that there is a 5k ohm resistor under the black epoxy in the rotor, I believe it is to eliminate radio interference, but you dont need it, and that is what gets too hot and causes the mess you had. So what you do is take a dremel and carfully grind away the black epoxy, and on each end you will have brass contacts, and you solder a piece of 12 gauge soild copper between those brass ends, check to see that you have zero ohms, and then re-fill the cavity with epoxy. Hope this helps, you might try searching though, because someone fairly recently posted nice pics of the process!

Rutager

snbush67 · snbush67's Garage

Yes, I have had a total melted rotor and simply fried on the inside rotors, I went through several before I was given a meihoff rotor as a replacement which has held up fine.

jluetjen · jluetjen's Garage

Just an update...

Last night, I had put a 73.5 2.4TK rotor in as an interim measure to get the car started -- which it did. Based on everyone's input (and a search of the Pelican history on the subject) I found a rotor from a '72 Beetle (Bosch number 04038 per RoninLB) at a local NAPA parts store and bought a couple. I also ordered a couple of '78 SC rotors ( 930-602-902-00-M14 ) from our host. I put one of the 04038 rotors into the car and have kept one as a spare or for experimenting. Some observations...

1) The resistance on the 73.5 2.4TK rotor was 4.5K ohms while the resistance on the VW 04038 rotor was .4 ohms. This is similar to the resistance that I was able to measure on what's left of the destroyed rotor. So the 73.5 2.4TK rotor definitely has too much resistance.

2) The (remains) of the rotor that melted down looks suspiciously like the rotors that I just bought from NAPA. Unfortunately I can't measure the resistance of the complete original rotor for comparison. Neither have any part numbers on them, but the casting marks all appeared the same. Going from memory, I'm pretty sure that I had bought the destroyed rotor from NAPA a couple of years ago.

3) The NAPA rotors both have a potted area in the arm, which makes me suspect that there is still a resister in there. I haven't had a chance to play with the Dremel yet.

I'll post what I learn from the '78 SC rotors after they arrive next week. I suspect that I'll need to brush-up on my soldering.

dailyATX82sc · 08-21-2009, 08:00 AM

here is one that a buddy( he used to build a ton of RC stuff and can solder in his sleep) and I did....worked out great and only takes a few min

sorry about the pic's (iphone

)

and what u want to see.....

jluetjen · jluetjen's Garage

Oh -- you just dispensed with the wire and laid a bead of solder down?

jluetjen · jluetjen's Garage

Hello. Just a brief follow-up, the '78SC rotor (930-602-902-00-M14) is just the wrong part for an early application -- it doesn't even fit under the cap. For those owners of early cars who want a rotor without a rev limiter, you can use the VW number that I listed above, or the 912 rotor (616-602-221-05-M14) from our host. Once I get the 616-602-221-05-M14 rotors in I'll post the resistance that I find across them. I've milled out the potting on one of the VW rotors and didn't find a resister, just a long copper contact. I filled the cavity with solder and got essentially 0 ohms from the solder to the outer lead, but still about 4 ohms from the inner lead to the solder. I'm pretty sure that I don't have a good solder connection to the contact at the center of the rotor, so I'll try reflowing it again when I get five minutes.

jpnovak · 08-26-2009, 01:39 PM

John,

You have it right. I have done this to a few rotors after my Crane ignition overpowered the resistor.

Just lay a heavy section of wire or a piece of brass across the contacts and solder it in. I cover and fill the cavity with epoxy after. Its probably not needed. You will find that both the center and end contact have a small step in them. This is the perfect place to solder.

jluetjen · jluetjen's Garage

Just another update. I finally got the 616-602-221-05-M14 (912) rotors in and they measure 4.9 K-ohms of resistance, which I don't think is going to live very long with MSD's 1 amp/1000 RPM going through it. The VW rotor with .4 ohms should be better. For identification purposes, here's a picture of 3 of the rotors.

From left to right they are the melted rotor, a VW rotor and the 912 rotor from Pelican.

Curiously, even with only .4 ohms of resistance it sure looks like it was a VW rotor which melted down in my distributor. When I ground out the potting material on the VW rotor, I didn't find any descrete resistors, or even a thick-film resister -- just a copper lead. But measuring the resistance across the lead (from the center to the edge) it did have a measurable resistance, so I suspect that it is the copper itself which is providing the resistance.

Paulporsche · 08-30-2009, 06:50 PM

I've had an MSD installed for 4 yrs w/o this problem occuring. Have I just been lucky? Does anyone know why this sometimes happens and sometimes doesn't?

snbush67 · snbush67's Garage

Quote:

Originally Posted by Paulporsche

I've had an MSD installed for 4 yrs w/o this problem occuring. Have I just been lucky? Does anyone know why this sometimes happens and sometimes doesn't?

Sometimes the burnt rotor is not even visible, the resistor under the epoxy may simply just burn and then the spark will jump through the mess that is left behind. Your car may not run as strong and you will have a weaker spark and maybe a rich condition will result leading to you chasing fuel air mixes and CIS ghost (sound familiar?).

Another thing is that your MSD may not be completely grounded and maybe your spark doesn't have enough juice to burn out the rotor.

Having followed all your post and issues I say again, you most likely have electrical issues that are probably gound related and robbing you of HP.

sc_rufctr · 08-31-2009, 12:11 AM

I've posted this pic before in another thread but I thought it would be worth posting here.

This was my rotor after running an MSD ignition for about 6 weeks...
Note the running epoxy... I swapped it out for a rotor with the mod and the car runs great. No problems at all.

69911e · 08-31-2009, 04:22 AM

John: The .4 Ohms may be measuring the leads on your meter. For accurate measurements of low resistance values, always base line meter by shorting the leads and subtracting that from your measured value.

Ed Bogue

jluetjen · jluetjen's Garage

Peter, did you ever do a resistance check across that rotor? Judging from the red color, I suspect that it's a 912 rotor, which would have had 4.9K ohms of resistance. This would definitely be too much when running an MSD ignition.

jluetjen · jluetjen's Garage

Quote:

Originally Posted by 69911e

John: The .4 Ohms may be measuring the leads on your meter. For accurate measurements of low resistance values, always base line meter by shorting the leads and subtracting that from your measured value.

Ed Bogue

OK -- then it's down to less then .2. Why would that burn out with an MSD ignition if it's nothing more then a strip of copper in the rotor?

Paulporsche · 06:11 AM

snbush,

Thanks for that. I'll check it out.

jluetjen,

Sorry for the hijack.

Paulporsche · 06:29 PM

I just checked 2 rotors I have. The one in the diz is# 123433 followed by 2300R1 in a circle. It has a black potting and measured 1.2 ohms R. The other looks identical, except is # 123433, followed by 2215R5 in the circle, w/ a red potting, and measures 5.02 ohms.

I couldn't find the numbers on the Bosch site, but I'll check other sources and report.

EDIT:

The pic posted by RoninLB for Bosch 04038, which he uses, and is Pelican 930 602 902 00 M14 shows numbers on the u/s 12343x2300R1. The x is not entirely visible but the first digit looks like it could be a 3. It does however appear to be the same item. I have PMed RoninLB for a clarification.

BTW I use this rotor in a points diz for 74-77 911.