Last weekend, I lost in 3rd round due to an ignition misfire. This post tells the story, some background to the problem, and my initial attempts to solve it.

Another $30 win

To make this post more complete, here is the previous trouble shooting I did:

This is what I checked on Sunday:

- Spark plugs all looked good
- Spark plug wire resistance matched previous measurements in April 2005, including coil wire
I checked the plug wires as they were attached to the distributor cap
- Distributor cap looked good, inside and out, terminals good, rotor good
- All wiring connections were checked for tightness
- I did the MSD check from the instructions, removing distributor pickup wires and shorting/breaking connection.
It sparked a plug I put on the coil output during that test, so the coil apprears OK too.
- I measured the distributor (stock) magnetic pickup and it was 345 ohms, about right.
- Distributor reluctor gap was 0.006", good. There is a little play in the distributor bearings, but not much 0.002"
- The fuseblock has a separate circuit for the MSD low power (turn on) wire, and the fuse didn't seem tight when I pulled it out. I reinstalled it fully, but I don't know if it was really making a bad connection or not. The main power wires from the MSD go directly to the battery terminals.

Unless I found a loose wire, I didn't expect this to find the problem. It runs fine at low rpm, but over 4000-5000 it acts up badly. Next time I work on it, I will check more wires under the dash for chafing. I will put it back together (plugs etc) and try running the engine again. Then if it still has a problem, I will put the old alternator back in. This was a new Power Master 100 amp, 1-wire alternator I put in just before going to the track. Remember, it was running fine for 5 passes, then it started. Something must have happened, a part went bad, etc.
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On Tuesday after work, I took the MSD box (Digital 6) to Speed Unlimited (over an hour drive each way) and they tested it on their MSD display. He varied the RPM, tested the rev limiter, etc. and it worked fine. I got home and put it back in the car, installed the spark plugs etc. But it was too late in the evening to run the engine in the neighborhood with open headers, especially since I need to take it up to 5000 rpm to check things. I even took a spare distributor and pluged it in, then spun the distributor by hand, it fired the plug just fine. I am suspecting the coil is going bad, I checked the resistance (for what that is worth) and it showed 0.2 ohms and the catalog says 0.02 ohms (Blaster HVC #8252). I will try it with the old coil and the Blaster 2 (round, red) coil from the Cuda for comparison.

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Today is now Wednesday. I tried the Blaster 2 coil and that didn't help. I replaced the magnetic pickup and that didn't help. I put the old Delco alternator back in (63 amp) and it now would run smoothly past 6500 rpm. This was with the fan off (see background topic). With the fan on it still misfired. At least I was back to where I was a few weeks ago, turning the fan off to run, turning it on while on the return road.

While the motor was all warmed up and running smoothly, I checked the rev limiter (MSD Digital 6) against the tach. It was real close at 5000, and 6000 rpm, but when set at 7500 it limited about 7100, and setting it at 7600 it limited at 7200. I left it there as I cross the finish line at 6900-7000.

I was feeling better, now I can race again and the Pittsburgh Mopar race is coming later this month. So, I took a break and went in the house (air conditioning) and called MSD to discuss my problems.

Vic said the Delco 1-wire alternators are the worst for interfering with the MSD box. And the Chrysler factory magnetic pickup acts like an antenna. With my big block, that alternator is right next to the distributor! He recommended the shielded pickup cable and to test by wrapping the wire with aluminum foil. I told him I already have the shielded cable, but there is about 8" of exposed wire coming out of the distributor because I have an adapter cable to go from the Chrysler connector to the MSD connector. So I will try wrapping that part.

I went out to the garage and wrapped the unshielded part of the wires and ran the engine. It didn't change, OK with fan off, not with fan on. By now I am sure it is alternator interference, and I figure the more amps the alternator is putting out (fan on), the more interference. So I tried wrapping the alternator in foil, and that was futile...

I decided to modify my distributor to get rid of the unshielded part of the cable. The MSD cable now plugs right into the distributor. The phillips screw locks the vacuum advance plate since I removed the vacuum can.

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And now it fires perfectly even with the fan on!
Man this electrical interference thing is a pain, but it is now fixed. Here is a pic of the installation in the car:

have you tried to unplug a field wire on the alternator and then run the engine with the fan on?. could the amp dray from the fan be casuing some interferance and mabe its not the alternator at all?.

ooops i see its fixed, good deal!

Glad ya worked it out. Thats a good idea there makin it so it plugs into the dist. Now get hold of MIKESDUSTER and help him out.

Since I am using Delco 1-wire alternators, there is no field wire connectors to unplug...

I don't think it is the fan itself, since the 100 amp alternator was causing a misfire with the fan on or off. The smaller alternator must just make less interference at a given amp draw. FWIW, my fan runs off a relay. The power wire comes right off the alternator, through an in-line fuse, to the relay and to the fan. Those wires are not near anything else, they run to the passenger side of the car (where the alternator is).

That's my thinking anyway.

Is MIKESDUSTER having MSD misfire problems? I will look for his post.

now, that was worth reading. I am going to remember this one.

did you solder the wires in the dist?

Outstanding...I've been following your ignition misfire thread and I'm glad that you did a follow-up report on finding the trouble, and then correcting the issue

I'm wondering if a three wire alternator would cause the same problem ?...... and if either a 1 or 3 wire alternator was mounted lower ( not in the stock location) that the interference would not be an issue ?

I will say this, you did one heck of a job trouble shooting the issue, and I really like how you tucked the wires in the distributor.....

Nice Job!

Mike

Wow, that's some serious detective work! I'll remember that one for sure. Glad you found it.

Understood Jim,

Consider this a bump...really good info here


MODS....can we have this thread saved, so it makes it into the tech archieves

Thanks!

After all the mods I did to my factory 400 distributor, it now has some of the features of the MSD distributor! LOL
Connector plugs right into the body
No vacuum advance to bounce around
And my mechanical advance is locked out.

And you know what? I turned the start retard feature off (built-in to the Digital 6 box) for testing. And the motor starts just fine with 34º locked out ignition timing, even hot. I guess with all the duration and overlap of the MP 0.620" cam (283º at 0.050"), 106 LSA, and only 12.2 CR... it doesn't need it.

Nice solution, and great d etective work!!

440Jim, did you change anything before this problem started? I'm having the same problem with a Digital 7 box I just installed, it starts misfiring around 5000-5500. I put the 7AL-2 box back in the car and the problem goes away. I'm getting ready to send the Digital 7 box to MSD to have them take a look at it.

Bill,
IMO, the Digital MSD boxes are more sensitive to electrical interference than the analog boxes. This started when I put a 15 amp electrical fan on the car, and I couldn't make a pass unless I turned the fan OFF. The 63 amp Delco 1-wire alternator made more interference as it put out more current. Then I put a 100 amp Delco 1-wire alternator in thinking the old alternator might be overloaded. I think the 100 amp alternator made more electrical interference at lower amp draw, so the misfire occured even with the fan OFF.

So, if you have a Delco style alternator, especially with high electrical load (fans, water pumps, etc.) and an unshielded magnetic pickup distributor, you might have the same solution.

Thanks for the info 440Jim. I do have the 100 amp powermaster alt, electric waterpump, and electric fan. I don't have the shielded mag pickup wires.Maybe I'll try shielding the wires before I send the box back. I was thinking the same thing about these boxes being alot more sensative then the analog boxes. I've already replaced the cap,rotor,plugs and plug wires trying to fix this problem........

Bill,
What distributor are you running? I think a shielded cable is a good idea for a magnetic pickup. And don't run the distributor wire next to the alternator power wire or the coil primary wire (output from the MSD).

I like the charging characteristics of the Delco (GM) alternator, good low rpm output. But MSD even says in their FAQ that the Delco 1-wire alternators are big offenders.

I'm running the MSD Pro Billet.I ran the mag wires away from all the other power wires like MSD tech suggested.They didn't mention anything about the alternator or the shielded wires.

Jim, not that it would've made the difference but do you have any capacitors in the car? alternators, electric motors, primary (race ignitions run high voltage), secondary ignition circuits and solenoids all make noise than can cause problems. capacitors can help soak up some of the noise in the electrical system. even if the car seems OK i'd install one...can't hurt.

Question:

I thought that it was a no no to run solid core spark plug wires with an MSD ignition?

Or does this change when you run a digital MSD?

I have a brand new set of solid core wires like yours, but the old MSD 6al-2 and have never put them on, because of that.

Is this right or wrong, or wrong and right?

BP