The result:





So the car started exhibiting a random miss at cruise of about 2000 rpm. This random miss occurrence began late last year before I put the car away for hibernation. In the spring I checked my timing and carb for proper function. Did a compression test. All cylinders 190-200 psi. Test drive. Still the random miss but not on every drive. So wife and I head out to dinner last week. Got 5 miles from home and car starts missing again. Another 1/4 mile and car died at a stop sign trying to turn and go home. Flatbed it home instead.

Anyway, next morning, I pulled the dist. It's a Mallory Maxfire with programmable mechanical and vacuum advance. Also it's a CD ignition and no external box required just an external coil. Dist'r gear had very little wear but required some shimming. Removed coil and bench tested. Ohm's appeared to be within spec as far as coils go. Brass terminals on top of distributor like new except one. The coil terminal. Build up of 'crud' for lack of a better word. Upon closer inspection of coil terminal, the same thing. I looked at coil wire. Terminals inside the plug boots are corroded. Did ohm test. No continuity!!! Checked wiring for voltage from distributor to coil + and - terminals. Less than 0.5V. I should be expecting 12V. I didn't have a spare plug wire for a coil wire but voltage test seemed way low. Electronic ignitions are not my forte. Pull everything and put back my original HEI with MSD upgrade kit. Starts first click.

So, I assume the distributor electronics are fried. My question is this. Did the corroded coil wire terminals somehow fry the distributor electronics??? Again, I'm assuming it's fried.

 

Old tech to the rescue

new electronic stuff is amazing, but sometimes it seems like it has a ways to go to be as reliable as the time tested technology. At least you got a standby until new stuff shows up.

 

Seems Mallory no longer has Max Fire?

Hard to beat a good points or HEI distributor.

 

(I'm not a big fan of MSD or Mallory, so I shouldn't comment, but here it is anyway.) If this distributor is a CD system, the only time (typically) that there is voltage to or across the coil is at the moment that the Capacitor Discharges into the coil, causing the coil secondary voltage to try to make its way through the corroded coil wire.

It appears that the corroded coil wire could emulate an open/disconnected/resistive coil wire, It might be worthwhile to ask the Mallory techline guy what happens to the CD module/controller if the coil wire open-circuited or got highly resistive.

 

I may just stick with the upgraded HEI and limit the mechanical advance again. The stock vacuum canister is 10* advance max so that's ok. It was just soooo easy to program an advance curve with the Maxfire. I already knew that dist'r went out of production and Mallory sold off to Holley/MSD. Tech support and replacement parts may be limited or non-existent but I will give them a call regarding the coil wire situation.

69427 or anyone else, any ideas as to how/why the coil wire and terminals became corroded in order to prevent this from happening in future? As I mentioned before, the distributor terminals are like new.

 

dielectric grease

 

Moisture was not the cause of the corrosion. The spark plug terminals on the dist'r cap are like new. Car is only driven on warm sunny days and is garage kept.

 

I have a complete MSD ignition system on my 1968 C3 and have had no issues except for ignition coils burning out. I am on my 3rd MSD round red coil (In 25+ years) and now the MSD guys said that I should be using a firewall mounted coil that makes more power and is not as sensitive to vibration. Now they tell me....

The MSD 6AL works great on my car as the spark sounds like an arc welder. My distributor is also the MSD Billet pro with mechanical Tach drive (1968 C3) and it is wonderful. The setup instructions helped me understand how and what I was doing it for. I send people links to the MSD distributor setup instructions as it is explained well and written clearly. The kit came with a bag of springs and bushings which you can use to modify the vacuum curve and vacuum limits any way you want to. The trigger (Hall Effect sensor) can be replaced easily on these distributors as well. I am enclosing a link to the instructions for the mechanical tach drive like mine which you fortunately DON'T need.
Check out these instructions:
https://static.summitracing.com/glob...2_frm28723.pdf

I see by your compression numbers that you must be running a higher than stock compression ratio. I am as well and use a "Digital Retard Box" from MSD to retard my timing 20* to allow easier starting. Between that 20* and the gear drive starter my 12.25-1 Compression ratio engine cranks like a Chrysler. The higher compression requires a hotter spark then normal and this is what burns out the coils according to MSD. I also have MSD's 8 mm ignition "Super Conductor" spark plug wires and I fill the boot with dielectric grease before sliding the spark plug boot on the plug. I have had very good experience with the MSD wires and use them on both of my Corvettes.

My C3 came with the Transistorized Ignition and to preserve those parts I removed them cleaned them and bagged them all up. The MSD ignition system works great with my engine and the original parts all worked fine when removed from the car.

One thing to watch out for. Many years ago we had a GM pick up truck with a V-6 4.3 liter engine. I replaced the ignition coil with a aftermarket (MSD) and from then on I was replacing the distributor cap every 6-8 months. I looked at everything and it was set up properly, it turned out that the air inside the distributor was getting Ionized and making Carbon tracks shorting the cap out. I re-installed the original coil and the problem went away. The cap on a V-6 is smaller in diameter and used an external coil but it developed Carbon tracking everywhere inside the cap that could only be seen with a black light or infrared light. Too much of a good thing is not always good for the car.

Just out of curiosity, what kind of plugs do you run and what gap do you use on them?

I wish you the very best in solving the issues! That is a beautiful Corvette you have there!

Best regards,
Chris

 

That looks like you said "CRUD" Coil and distributor terminal. Did I not read this or did the forum gremlin write it?

 

I would disconnect the wires off the coil and check ohms on the primary and secondary winding. Check across + and _, and then From + to the center (where the plug wire goes). Replace the coil and coil wire as needed. I doubt the CD box went. They usually just burn up coils.

 

ctmc..., I'm running NGK v-groove FR4's gapped to 0.045". Brand new a few hundred miles on them. Taylor 8mm spiro pro plug wires and coil wire. Ohm's check on plug wires ok. Coil wire toast. As I mentioned earlier, the beauty of the Mallory Maxfire is that it does not require an external box. It's all built into the distributor except for the ignition coil. Anyway, my old upgraded HEI is working for now. Over the winter I'll re-install the Maxfire and ignition coil and try a new coil wire and see if it fires. Still curious as to why the corrosion/crud was isolated to the coil wire and not the plug wires and terminals. Galvanic possibly? Thanks for the comments guys!

 

Let us know if you figure out the corrosion issue. I'm not a metals expert, but I am curious if the issue is current/galvanic action related (the coil wire does have current going through it 8x as often as each plug wire tower). Was also trying to figure out if it was a "plating" type action, as the polarity of the distributor cap center wire/tower interface is opposite that of the plug wire/cap interface. However, that doesn't explain why the coil/wire interface is also corroded.

I can think of one or two items that may be caused by a CD system, but I'll confess I'm a bit unclear how the system is a CD type, but small emough to fit inside the distributor.

Just talking out loud here. If you figure it out, let us know.

 

I know this wasn't caused by moisture. Being the ignition guru, I was hoping you could tell me the cause. Thanks for your input. The Maxfire was advertised with a built-in CD ignition and multi-spark up to 3000 rpm. But something caused the coil wire terminals to corrode and the coil wire core to "burn" (looked a bit burnt but not melted) at the ends and then no continuity of course. I did cut the coil wire boots off the wire. The cut ends of the remaining length of coil wire looked ok but ohms measured 1, no continuity. Coil bench test had following results: C+ was 8200 ohms and C- was 0.7 ohms or vice versa. Not going to lose any sleep over it. Just wondered if someone could provide a definitive answer. One of those ignition malfunction mysteries, I guess. Thanks again all.

 

I had problems with a distributor cap that developed corrosion between the aluminum posts and the steel piece in the wires. It is possible that the connection at the coil wire was not a perfect connection and the resistance built up and toasted the wire? I keep forgetting you are using a one piece unit with the CD built in. What kind of terminals does the cap have? I use Die-electric grease on the plug end and on the cap end and on the coil wire as well to keep air from getting to the connection as much as possible.

Those plugs should not have stressed your system, decent gap should have worked forever. When we open the gap on the plugs for high energy ignition systems we stress the coils according to what I was told by the folks at MSD. I used to run mine at 0.055" and dropped back to 0.045-0.050" to see if it helps my coil. I need to upgrade to a better coil to get the most out of my ignition system. Getting a good strong spark when Compressed can be a challenge at higher cylinder pressures. I am using NGK Iridium Plugs myself and am happy with the wear and tear. I tried a set of Champions and they got worn down fast in the engine. 500 miles looked like they had been used for years.

I am glad to hear that you have a spare distributor as that makes life a whole lot better. Spares are great as long as you can find them when you need them, that seems to be my issue.

Go out and enjoy that beautiful Corvette!

Best regards,
Chris

 

Is it possible to post any pictures of the corroded areas?

 

If you decide to go back to the CD unit make sure you use a solid copper or copper nickle core coil wire. Also use dielectric grease on the boots. This is to seal out O2 and moisture. You want the same connector material so copper to copper, steel to steel etc.. Look at it like this the coil and coil wire take every discharge while the plug wires and plugs only take 1 in 8 discharges. That is why you only had crud/corrosion on the coil terminals. In your case the coil wire was the weak link. In your first post you thought the CD unit was bad because you got 0.5 Volts. If you had cranked the engine over you would have seen the volts jump to 12 volts and back down as the engine turned over. (you can test this on your work bench before re installing it on you car.) If you don't get a good test result on the bench then you will know the CD box in the distributor is bad.

 

Where are you expecting to see 12v jumping up and down on a CD system?

 

Ok, I have some more info and pics. I needed to tweak my senior's memory and remembered that I did put a dab of dielectric grease on the coil wire plug ends and terminals. This is the crud I spoke of. Results from ignition coil bench test: C+ to CT = 8700 ohms, C- to CT = 8700 ohms and C+ to C- = 0.7 ohms. I cleaned up the terminals on the coil and dist. The coil wire ends are corroded even with the dielectric grease applied.







Did a bench test of the distributor hooked up to a small 12V, 3A voltage regulator. Turned shaft but no voltage on the coil terminal leads. I'm now wondering if it's just a bad coil coil wire that started breaking down causing high resistance with the corrosion and then finally went "open". The coil wire was probably causing the stumble when the engine was thoroughly warmed and then finally crapped out.

 

So I decided to take apart the distributor and have a look at the underside of the circuit board. Aha! Right away I saw what the issue is or a contributing factor.


One heat sink/chip is not like the others! The screw had backed out several threads making it loose. Anyone know what these heat sink/chip sets do? They are curved to match the distributor housing curvature and not much clearance between them.



The next two pics are the fourth heat sink/chip. This one was on the circuit board with the other three but the solder was no longer making a good connection. It came out with a very slight pull. I'm guessing it was ready to completely fall off with a bit more running time on the engine.




The plan now is to desolder the heat sink with the loose screw and tighten it. There's another component in the way and I'm not confident in bending the heat sink over to gain access. I'll just solder the other heat sink back into place should my limited soldering skills allow.

 

Digital to analog controller chips.