No Spark. Why?

[Formerly Steve Wilkinson]

Car is a twin-plugged, carbureted '83 SC with twin MSD ignition boxes and twin Blaster coils, absolutely no other engine electronics. Meaning it's an old-fashioned air+ fuel + spark = guaranteed ignition car. Drove just fine yesterday--totally normal, no missing, no loss of power, nothing out of the ordinary. Shut it down in the driveway. And today it won't start.

Cranks just fine, fuel pressure is three psi on the gauge, there's still plenty of atmospheric air here in New York, so I pulled one of the two coil leads and with a bent paperclip inserted into its socket tried to coax a spark, while my daughter cranked, to jump a small gap to the fan housing. (Normally, these MSDs will send a monster spark an inch or two.)

No spark. (I didn't try the second coil, because assumedly if that one's working I'd still get a start on just one set of plugs.)

Is there a failure point anybody can come up with that would simultaneously disable both independent ignition circuits with absolutely no warning or prior symptoms? The only other anomaly the car has been showing is that, since coming out of winter storage (and after hundreds of miles of normal driving), the tach is inaccurate at the low end of the scale; it seems to be accurate from 2,000 rpm up, but it usually settles on about 2,000 at idle when I know the car is doing 850 or so rpm.

Any ideas?

[Beetle Bailey]

Do they both get their power (main)from the same source........Also keyed power?

[RoninLB]

after checking like Beetle says disconnect the plug to the dizzy and jump the 2 wires on the MSD side to look for spark... assuming your using stock magnetic pickup.

[fastfredracing]

I have had msd boxes themselves fail, but I seriously doubt that both of them went down simultaneously. What are you using to trigger them?? I am assuming that you are using the sc distributor , maybe the green wire has failed???

[rennworks]

Your tach could be killing your trigger signal. Disconnect the tach lead and ck for spark.

[Formerly Steve Wilkinson]

I will do all of the above, but understand that there are no magnetic pickups involved, just an old-fashioned RS distributor with a rotor, and yes, everything in the car gets its power from the ignition key on.

[911pcars]

Like some owners , don't the spark boxes normally produce a whining sound when switched ON? If so and they don't, you may want to look for a system fuse or other common (or independent) source wire, then diagnose with your multimeter to see where the voltage doesn't continue.

It's not common for devices to fail at the same time although it could happen. I'd look outside of the spark boxes if both exhibit the same symptoms; common ground wire, signal wire, harness connector, malfunctioning ignition switch?

Sherwood

[Steve@Rennsport]

Originally Posted by Formerly Steve Wilkinson

I will do all of the above, but understand that there are no magnetic pickups involved, just an old-fashioned RS distributor with a rotor, and yes, everything in the car gets its power from the ignition key on.

Refresh my memory (it sure ain't what it used to be), but you have the twin-plug dizzy with the Bosch cap, yes?

Looking at my records, I've answered my own question,... Yes, you do.

Thats the SC distributor that I converted to accept the RSR cap & rotor and it IS magnetically triggered. The internal trigger is quite reliable but the same cannot be said for the connector at the side of the dizzy and the green trigger cable. Those are coaxial and prone to failure over time.

I would be checking continuity between the connector that plugs into the distributor body and each MSD trigger pair of wires (the green and purple ones).

Lemme know what you find.

[Eagledriver]

I had a similar problem that turned out to be a bad ground. Are the two MSD boxes using the same ground connection? Double check that they are both grounded well and both have power. You can check these boxes by using the white points trigger wire (unused in your application) to fire the boxes.

-Andy

[Jeff Alton]

If you think it is 12V+ related getting to your MSD boxes, don't screw around, jumper 12V off the battery to them for a quick test. Then if it proves to be power related, rewire or hunt for the problem circuit.

Cheers

[Beetle Bailey]

Any luck yet?

[Formerly Steve Wilkinson]

All I've done so far is pull the distributor, unplug the "green wire" and check it for continuity. Checks fine, but I've already ordered a new one from Wayne because Steve Weiner says he routinely replaces them as a precaution against future semi-inevitable failures. Next I'll check the power wires to the MSD boxes to see if they're getting battery voltage when the key is on.

But tomorrow morning, I give a webcam lecture to a BOCES near Rochester that has been reading my Gold-Plated Porsche book--these are welding students who have never read a book in their lives--so that's my primary mission now. I do love helping young people learn...

[Beetle Bailey]

Good luck with that!!!! I'm surprised they still give kids books.
You know it's gonna be something simple, no ground or power from key or a tiny snail has slimed a connection!! On my 69 it's something different everytime it rains.

[fastfredracing]

Not to hijack this thread , but you wrote that book Stephan???. That is great, I cant remember who, but one of my porsche buddies gave it to me a couple of years ago, I read it, loved it, and passed it on to another Porsche friend. He loved it as well.