Sometimes, no spark!

[m3a1]

Ok guys, my 1970 pre-emissions A2 continues to act up and I'm really in need of some advice.

Situation is this - the MUTT will fire right up when cold. For this reason, I'm convinced the timing is spot-on. If I take it for short drives, particularly where I'm just MUTTing around and not pushing the truck into the higher RPM range, the truck will restart with no difficulty. However, anything more than just loping around leaves me with a truck that will not restart after being shut down and the reason for it is there is no spark.

For clarification, the truck never just dies but it won't restart after it has been shut off. While running, the ammeter needle is pretty much at the extreme right edge of the green during these outings. If allowed to sit for quite some time (like 3-4 hours) it usually restarts as though nothing ever happened.

Any ideas?

Cheers,
TJ

[rickf]

Possible coil or ignition module. Do an ohms test on the coil and is it a Prestolite module or Swiss controls?

[m3a1]

Rick, thanks for the quick reply. I'm not sure. How do I tell the difference?

As for the coil, I'm not electronics savvy so I'm going to need some instruction on checking the coil. I will add that I've had two different coils in the truck and have the same problem no matter what coil is in there.

[Hambone]

As Rick said, it almost has to be coil or ignition, if you have power to distributor then all spark is created in there.
http://www.g838.org/viewtopic.php?f=3&t=6222

[rickf]

There is one other possibility and that is the input capacitor where the wire attaches to the distributor. It is rare that that capacitor goes bad but they do after 40 or 50 year of age. In order to check it you will have to be kind of quick so you know you are checking it at a time when there is no spark. You will have to remove the distributor cap and check for voltage on the positive side of the coil, should have 24 volts. Maybe a bit less if the module is in the closed position. As far as which module you have the Prestolite uses the same small rotor as the points setup and the Swiss controls uses a large rotor. IF you find you have no voltage at the coil quickly pull the wire off the distributor and check that and if you do have it there put it back and check the coil again. If still no at the coil then the cap is bad. All that is is a radio suppressor so it can be eliminated with some creative modification of the capacitor and soldering of the wire back to the terminal.

[Hambone]

If it turns out to be the capacitor, I have a sack full of NOS ones, as said, it can be eliminated but then you won't be EMP proof.

[m3a1]

OK, thanks fellas. I won't be able to get right on this but it's starting to make some sense. Frankly, I though coils either worked, or didn't work and that was that. I never had one that was intermittent. After reading the link you sent, I see that it is entirely possible so, perhaps the two coils I have both have issues.

When I get the time to pursue this, I will report back with the results.

Cheers,
TJ

[rickf]

Inside the coils you have winding's and the primary side is fine wire, If that wire breaks it will arc for a while as it heats up and expands. Once it expands to the point it cannot bridge the gap with the arc the vehicle shuts of. Also, while it is arcing the gap is growing so each time will be shorter. In your case if it is the coil it could be because the coil is heat soaking as it sits that is opening the gap. That is why it is important to narrow down where the problem lies.I would get one of those capacitors from Hambone and trow it in and see if that does anything, cheaper than a coil and that would eliminate one of the variables. Did you figure out which ignition you have? Prestolites had a serious problem with the potting material melting out of them and then the electronic components would short out. This could also be heat related.

[paracord]

Now that explains while even with the new coil no spark, when my coil went it trashed my electronic module also?

[rickf]

Not likely, when the coil goes 95% of the time it fails open circuit so the module is getting no power at all from the coil. NOW, If the coil failed because you left the ignition on then that is a different story.

[m3a1]

To all who responded, thank you very much. Ken, your kind efforts in providing that link really helped put me on the path and a clear up some misconceptions I had about coils and ignition systems in general. In all this has been a nice learning experience. The problem was a headache, though, once costing me a three mile walk home at mid-day.

Just a brief recap. My truck was starting like a champ and would run just fine, that is... until it was turned off..and then it would not restart due to lack of spark. The problem did not occur on extremely short drives. Fearing I had a coil issue, I would swap in another coil and still, the truck would not restart until cooling off completely (which takes a while in the Texas summer heat). That coil swap pretty much eliminated the coil as the culprit because a cool coil did not eliminate the issue.

I ordered a coil and a capacitor from RAPCO. I bought the coil simply because I wanted to make a clean sweep of this. My understanding is the capacitor works almost exactly like a condenser does on an ignition with points. After swapping both coil and capacitor in, the problem appears to have been solved though I am going to take a few more shakedown runs before claiming total victory. As of this time, I'm officially blaming the capacitor. I believe it was mentioned that Hambone has some and if you have an Oh-finless brown trout-Kit, I would strongly recommend having an extra one of these on hand. Perhaps Hambone would be willing to part with some.

Cheers,
TJ

[Fil Bonica]

Keep in mind the bypass capacitor on the input of the distributor s only there for radio interference purposes.
A simple jumper can suffice.The ignition system will work correctly without it.
A good thing to remember if you have an in the field failure.


Fil Bonica