Finally found the cause of my "miss", now have questions!

[ashyers]

Last Friday on the ride home the truck developed a miss under load. I spend some time last weekend and today sorting it out. I wish there was a way to put a scope on one of these igniters!!!

Here's my questions:
What's the resistance of a good distributor cap measured from inner plug terminal to outer plug terminal? I have 5 caps and they are all over the map.
Brown NOS cap 1: 5Kohms (in truck now)
Brown NOS cap 2: 5Kohms on all terminals but one, it's in the Mohm range, in truck when miss developed
Black cap 1: 10Kohms, tried in truck, ran same as above, removed while shot gunning parts...
Black cap 2: 10ohms on 4 terminals, Mohms for others
Black cap 3: one of the "pads" on the plug side of the terminal fell out exposing a small resistor????

When chasing the miss I pulled the dizzy/plugs/wires multiple times as I was unable to narrow it down to a single cylinder. Fun!!!

Finally I replaced the cap/rotor and was able to narrow the miss down to a dead plug wire in 3. The wire showed no signs of distress, even after removed from its sleeve.

I'd like to know if there's a resistance spec on the caps. I'm tempted to swap them in and see if they were part of the problem or not. Hate to save 'em if they are junk, hate to toss 'em if they're not!

I'm also curious if there is wire and terminals available for the waterproof wires. It was easy to remove the core of the wire and it looks like rebuilding one would be simple if parts are out there. All I'd need would be some 5mm (?) wire, a pair of insulators, and a pair of crimp on screws for the springs on the tips.

Andy

[isaac_alaska]

Following...i'd like to know the answers to these questions too! I'm going to use a thermal camera to help me determine if my truck is running on all cylinders, at least at idle/high idle. how did you determine your plug wire was bad? continuity check?

also, which insulators are you talking about? i don't have a plug wire in my hands to check it out...i'll have to do this soon

[ashyers]

Isaac,
I found the bad miss by finding the sooty plug in 3 once I'd gone through the dizzy for the 2nd time and blasted around the block. I am not entirely sure if I have issues with some of the caps or not. Hopefully some of the crew will chime in. I don't really feel like pulling the dizzy to test all the caps...

The wire that failed only had issues under load, idle was fine. This made diagnosis a bit more annoying. The insulation had a pin hole and it was arcing to the outer shield. I'll try and post a photo of the failure, it's pretty tiny.


Edit: Added a photo of the area where the wire's insulation failed. It failed instantly, I opened the throttle to go around another driver and zap there's the miss... I suspect the wire was nicked when the students were pulling the plugs that day.



Andy

[isaac_alaska]

Wow, that's hard to see! Makes me feel better about having a set of new plug wires in the mail...

i didn't realize the wires just slide out of the shield like that, really makes me wonder if you could find the individual pieces for the wires. maybe i won't bother selling my short set, since it looks like they could be parted out and pieced back together into a full-length set

[ashyers]

Isaac,
It looks like 5mm wires are available, I suspect that the terminals may not be. I'm thinking about picking up some wire and making the terminals on a lathe. Not sure I'll be able to pull it off, but seems worth a try. I won't get better at this machining stuff it I don't try!

Andy

[isaac_alaska]

if i were making the terminals, i'd probably start with brass tube, you might be able to get it in the right size. and then just thread the outside of it and cut it off. if you can find a "rolling" die that would definitely be the way to go for putting threads on something that small. I wonder if that's the original wire....unless yours is maybe a canadian truck? i wouldn't expect to see anything "5mm" on an american truck this old, but i could be totally wrong.

actually...now that i think about it, i need to get one of my plug wires and figure out what size thread those terminals are. i might get lucky and have access to a rolling die...it's a die the shop teacher made for rolling threads onto titanium bicycle spokes

[isaac_alaska]

something like this! the real trick is if you can find one with the correct thread...

thread rolling die

but it might be easier/cheaper to drill out brass screws. usually they are fully threaded though and would be hard to crimp

[ashyers]

Isaac,
I don't think the thread is too critical, you just need something that would hold the spring on the end of the wire. My guess is you could just get the OD in the ballpark and the pitch would make little difference since all you need is some teeth for the spring to grab. The spoke threading die sounds like a good idea.

The wire that failed is an Autolite wire and the truck is not Canadian. I think plug wires are just one of those things that's metric. Here's the wires I found after a short search:

http://www.jegs.com/p/Accel/Accel-5mm-3 ... 3/10002/-1

I think they may work, and they come in so many colors . There may be other solid core wire out there similar to what's in the truck, I just have not come across any yet.

Andy

[w30bob]

Hi Guys,

Andy, those wires must be relatively new........the older original Auto-Lite spark plug wires from back in the 50's were made of 7mm stainless stranded core covered by a glass braid and a black Neoprene outer cover. I've seen repair kits for the M37 and M38 waterproof wires, but I haven't seen one lately. Since the wires are still in production you could probably ask John at Midwest who the manufacturer is.........and see if you could contact them for parts. I'll hunt around and see what I can find.

regards,
bob

[ashyers]

Bob,
I suspect they are the 2nd generation of wires. I looked up the plug wires in the parts manual to find #'s last night and there was a running change. The one that failed was a bit different. It did not utilize a large spring for sealing, it used a taller section of silicone insulating material. Would have worked fine, I think it just died from being removed and installed frequently in class. If you come across one of those repair kits I'd be interested in seeing what they supply. It would be nice to fix up a spare set of wires for the next diagnostic session .

Andy

[w30bob]

Andy,

Here's the spark plug wire repair kit I was talking about............looks like the auction is still active.

http://www.ebay.com/itm/131439378229?it ... m=&vxp=mtr

regards,
bob

[isaac_alaska]

Very cool, I feel like I should order one because I have a bunch of cables that definitely need repair...

[ashyers]

Fix one miss, drive for a while, develop another!

The truck developed another miss under power that kept us busy, too busy! After doing a true shot gun approach we finally fixed it this evening, with some inspiration from Bill L. Thanks Bill!

We replaced wires w/ a NOS set I had picked up. NO CHANGE. We replaced plugs. NO CHANGE. We checked the dizzy on the Sun 504, it looked fine, we replaced. NO CHANGE. We installed a Pertronix. NO CHANGE. For grins and giggles we rebuilt the carb. NO CHANGE. We pulled the dizzy again and pulled the cap (<500mi.) from the housing and found the issue.

#2 Tower is fried!

bad cap1 small.JPG (23.91 KiB) Viewed 2093 times

Both times the miss took place somewhat randomly under load. I think the plugs were marginal and stressed the system, killing the wire first and cap second.

The water proof ignition is pretty cool, but I wish I could put a scope on it!

Andy

PS:
Anyone care to fill me in on 12 ohm vs 6 ohm coils? I have one of each. The 12 ohm is in the truck, the 6 ohm is a spare. If the time comes to use the 6 ohm, do I need to ad a ballast resistor? If so what's the preferred method??

[jim lee]

I'm pretty sure you could put a scope on it. I've not done it with my 24V M37 but in the past I've used a standard electronics test oscilloscope for looking at 12V ignition patterns. I can't see why the same technique wouldn't work on the M37.

-jim lee

[w30bob]

Hi Andy,

The 12 ohm vs 6 ohm coil issue has never really been explained to anyone's satisfaction, as far as I can tell. The earliest M37s used a 6 ohm coil with a wafer capacitor mounted directly underneath it in the distributor. Testing those early coil/capacitor units showed 12 ohms. That design went away pretty quickly and the coils were manufactured with the capacitor mounted internally, and those coils also test to 12 ohms. I remember when AB LINN started selling coils on Ebay his listing stated "the resistor was built in" these coils. I didn't know what he meant by that. Then I did a little digging and found out the early M37s used the external wafer under the coil.

When the supply of US made coils dried up it seems some of the overseas copies were showing 6 ohms when new. I don't know if they all were that way or just some. I also don't know if they've been corrected and now show 12 ohms. There's also no real difference between the aftermarket military 12v coils and the 24v coils other than the exterior marking.......so some folks confusingly mix up the two. Obviously using a 6 ohm coil when it should be 12 ohms is going to stress the coil (and probably burn up the points fairly quickly).....but I really don't know what current production coils are reading. The other thing to check is the voltage regulation. The mechanical regulators can be adjusted, but I don't think the solid state ones can be. If you're running a mechanical regulator and it's putting too much voltage into the system the coil will suffer. I know I didn't answer your question, but maybe this info is a start and others can add to it. It would be nice to get the coil issue sorted out once and for all.

regards,
bob