Weird thing happened last night, I’ve never seen before like this. This is on an Off Topic modern Sprintcar but I figured the Magneto part is relevant. Anyway it melted the post on the cap on number 5 cylinder. Only that one post. This is a new style MSD mag that uses a control box. So it seems to me that there must be something causing high resistance on that cylinder and in my tiny brain it would seem to be down line after the rotor ??? Does this seem reasonable? Have any of you old timey ignition guys seen this? I’ve seen rotor buttons burn up before on divorced coil HEI caps but never a single post? Thanks in Advance
I'd have a really hard time...blaming...anything as in, something in the cylinder being at fault ! I'd say the culprit is / was, 1 - A bad (loose, dirty) connection of the conduit (wire) to the steel connector (end of wire). 2 - A dirty connection between the cap and wire connector. 3 - A loose (sparking) connection between the cap and the wire connector. 4 - A combination of #3 and moisture (water) in the cap tower. 5 - An "old" cap, and or rotor, causing a discharge buildup between the cap and rotor, causing excessive resistance (heat) . AND...what's with the (similar color) discoloration on the rest of the cap towers visible in the picture ? The beginning of the same problem as the obvious tower ? Mike
If they were open they possibly would. If we can get some opinions today it would be helpful, We would like to go racing…
What is the metal clip like thing sitting on the hold down screw for the cap? That wasn't somehow part of the problem that got cooked and fell down there?
Put a cap rotor an wires on it and go racing. One thing I do know is you want the rotor button to only make a very small contact patch with the button in the cap. The button should have a fairly sharp point on it. You achieve this by bending the tab on the rotor so that it is exactly 90 degrees to the button. I assume that mag has a rotor with a tab you can manipulate. Aside from that obviously the contacts need to be clean.
It’s all new this season, the generator/ cap/rotor wires ETC … we used the same drive. I think we have 11 nights on it. I’m not sure what discoloration you see or is from my phone pics are as bad as my eyes are, maybe the degreaser we use during the washing process or maybe sun fading?. The car gets incredibly dirty/dusty besides all the mud stuck all over it. This pic is taken right off the track last night, so it’s pretty dirty For the wash process the tach and box are on a removable plate and are removed before washing and we have wash caps that close off the injector and water tight quick disconnects on everything. This is basically how most world of outlaw type cars are and are washed down after racing. There is a good possibility there could be moisture in a connection that is certainly possible. Because the entire car inside and out (less box and tach) gets pressure washed after each night. I’m hoping we find an actual failed part or visible fault. We don’t want to load up and tow a few hours thinking it was probably a wet connection. Then have it happen again. I guess what I’m asking is that because it’s just the single post would that make it faulted on only that cylinder? And wouldn’t it likely have to be from the post out to the plug? I’m heading out there now to swap out mags I just like to know what caused the failure.
In my experience, that is usually a product of the wire or the plug. That mag is throwing out some serious power and if it runs into resistance, it starts making localized heat. Put new stuff on it and see if that cylinder does it again. -Abone.
See, this is my theory as wellit seems like it has to be “Downline” from the post and that’s all that’s “downline”. I’m gonna go ahead and swap out the mag, Cap, wires plugs etc… but I don’t like just throwing shit at it without knowing WHY it failed.
I've never worked with a magneto. But would it be possible, or even necessary, to vent the housing or cap? It would be interesting to see a pic of the inside of that cap and rotor if possible. Wonder if you might see anything helpful running that on an ignition scope. . .
I’m glad you chimed in Jim, because the only place that appears to be affected is that 1 post wouldn’t that indicate the resistance is in that post/wire connection or wire to plug connection or the wire or plug itself? Forgive my ignorance I’m just trying to think this through but I’m no electrical wizard
That's caused by arcing in the tower or wire boot. Because it's a "high performance, high voltage" ignition system the spark is probably still firing the plug, but it's jumping two gaps, the one on the spark plug and the one on burnt cap tower. There was a poor connection or gap (ie. resistance) somewhere in or near the tower.
That was a pretty common issue back when gm debuted the hei. Our fix was to use dielectric grease and make sure the top plastic wire hold down was installed.
Yeah. You get heat where you have a poor connection. Sometimes more obvious than other times. Note how the nut shines brightest in one spot where it touches the washer, probably the only spot it touches the washer as the nut isn't tightened.
The inside of the cap actually looked normal as well as the rotor and rotor button in the cap. I changed out the generator part of the mag, the points box and new wires and re used the same plugs after I checked the resistance in all the plugs and they were all the same. It fired off and seems back to normal again. When I checked resistance in the wires, a few of them showed between 20% and 30% more resistance. And one had a visible bad spot that looked like it was definitely arcing. In that area of where the damage was. We’ll send it all off to be checked over and rebuilt and call it good I guess. I didn’t find anything other than the bad wire that would really cause it to run bad. I’d feel much better about it if I found something really messed up or if the inside of the cap was all burnt up but it wasn’t? Maybe it was just a fluke deal? I wouldn’t have thought 1 bad wire would have made it lay down like it did but apparently it did??? Thanks for the replies fellas, I appreciate it.
Stop looking everywhere “downstream” of the melted parts, as @squirrel has tried to tell you numerous times the resistance was where the wire was on the cap, heat is generated AT the point of the high resistance…
You're having trouble seeing the forest because all the trees are blocking your view. Replace the cap and the wire for that cylinder and call it good. As has been pointed out more than a few times, the damage was caused by resistance (ie, a bad connection) which caused arcing, and fried that tower and the wire. It could have been as simple a thing as the wire was not firmly seated in the tower.
Thanks I appreciate it. It all seemed to be good there was no sign of high temps up at the top of the post at the connection or inside the cap at all just the melted part on the outside of the cap midway between the rotor and the plug boot connector. Not wanting to risk missing something I changed out the mag,cap, wires Etc. And used dielectric grease on the connections. We did also find some heat damage on the mag side plug running to the box but the damage seemed to also be more focused on the outside of the plug than in at the pins. It just all seemed weird several pretty knowledgeable guys at the track had also said they never seen one burn on the cap like that. Thanks again. I don’t get on here much anymore but I remember you from the early days. I appreciate your time Thanks again.
Have you ever run the car at night, in the dark, and observed the high tension side of the ignition...cap, wires and plugs? It can reveal arcing that can't be seen in daylight. I have been able to find problems/potential problems by observation this way. Once the high tension side of an ignition system gets a carbon trace to ground, that is the path the voltage will take.... good luck
The Jalopy Journal
