Agreed, that's my 78 SC ROW there and Warren's advice was especially helpful and the thread has been recommended for Wiki.

Tom

Bump - Steve, did this have any sort of resolution yet?

Dave, it really hasn't, other than the fact that the problem hasn't reappeared in the last several months. It's maybe one of those mysterious easy-come-easy-go 911 problems of which we're all so fond. I replaced the green signal wire to the distributor many months ago, and that didn't help. Also replaced the entire electric cassette part of the ignition switch. But there was no point at which the problem definitively went away and never reappeared. I won't be surprised if it shows up again next May, when I start driving it again.

Problem is, when people suggest that I "clean all the grounds, tighten everything, replace all the fuses and confirm the integrity of every single circuit," I just roll-eyes and wonder whether it isn't time to buy a nice, reliable Aston Martin. Some people say "the car's 25 years old, what do you expect?" Well, I'm surrounded by 25-year old Saabs, Volvos and even Subarus that just keep on keepin' on, and it's an excuse I no longer accept.

Damn car is in the shop right now--since I'm not about to lie on my back in the snowy driveway hugging the gearbox--getting its three-year-old starter solenoid massaged. Another ordinary 911 problem...

sounds like exact thing that happened to me.



1st time engine and tach just died for a second at around 75mph.

I was on the ET Highway and looked above and waited to be abducted.. no kidding

Didn't happen for the rest of the day, probably another 400mi.

Within a couple of days it was happening once or twice a day. That's like once or twice every 600mi.

Finally it killed the engine for 5-30 minutes. Good.

Now it won't start and time to trouble shoot. Wire feeds and coil all checked out. Good.

It wasn't enough headache that I had to spend 2 hours emptying and repacking the trunk to get at my spares. I jump on the phone and order a coil and CD from pelican to be delivered to pwd72s' home in Oregun. I didn't care if it was either one.

Bingo.... never happened again

But I have two coils, Ron, and two MSD units. They couldn't all fail simultaneously...

As you can see from people's experiences, there are several theories as to why this occurs. Instead of feeling the urge to begin replacing parts, I'd try to systematically troubleshoot this.

It'd be better if there was no juice at all since it would be apparent when checking for source voltage in the source wire. This is an open circuit that only occurs intermittently, often the most difficult to locate.

When the vehicle dies, do other systems continue working (e.g. headlights, fan motor, etc.)? If so, this is one data point to identify the affected circuit.

If it's just the ignition circuit, look at the source circuit which begins at the ignition switch (actually the fuse box). Connect a voltmeter to the MSD source wire and, while observing it, start wiggling wires and wire harnesses and see if you can isolate the trouble area. The fuse box is a good area to begin a visual inspection. In my car, fuse no. 1 feeds the ignition circuit. Once upon a time, the screw terminal attaching the wire to the fuse box became loose and introduced the same symptoms you have. A wiggle here and there revealed the loose connection. In my case, I was fortunate it was in an accessible area.

Like many aftermarket ignition boxes, I believe there are two voltage sources to the box. The connection to constant 12 VDC (from starter motor battery terminal, etc.) doesn't/shouldn't affect the circuit until a second 12 volt signal is applied to the box from the ignition switch. Again.... the ign. switch circuit.

Hope this helps some.
Sherwood
http://members.rennlist.org/911pcars

A buddy of mine had the same type of problem 5 years ago. It turned out to be a bad ignition coil. Something to consider ...

All that gold plating is screwing up the ignition ... :)

Car has carbs, so it can't be fuel. Manual throttle, so it can't be air. That leaves spark.

It can really only be power (or ground) to the MSD boxes or the trigger signal, and I'd think the trigger signal is more likely.

As far as assuming new parts are good, many many moons ago I tore my freshly rebuilt beetle engine down because of lack of power. Thought I screwed up cam timing. Split the case and all. Turned out to be a brand new Bosch spark plug. Wasn't shorted, wasn't cracked, just wouldn't spark - probably one in 10 million. Since they were new plugs, I never pulled them - just yanked the engine instead. Smart, huh? :)

ok.. and I guess you took MSD constant "on" power from the starter?

if you said the radio spikes then it would be the ignition switch, its source, a weak switch wire connector, the bulkhead plug on top of the pedals, etc.
My radio never shut down when this happened.

maybe like Sherwood said, try wiggling wires. I'd also wiggle the relatively small two red wires off the positive terminal of the battery that leads into the cabin.

in the meantime if it was me I'd carry around some wire and be ready to supply voltage to the MSD boxes on/off small diameter red wires... because I'd assume it'll be completely dead in the future.

that's my wild guess as I assume the battery main terminals, distributor and MSD wiring is ok


Happy New year everybody




.

The car will run on one plug, so it has to be something that kills both ignitions at the same time.

Could you tap in to the same hot and ground you use to power the MSD boxes and to drive a small volt meter in the cabin?

How did you split the trigger signal to the 2 MSDs?

Tom

possibly an intermittent open in the hall effect coil in the distributor. seen it many times before in all types of vehicles. the coil is very fine wire almost like hair. when the wire gets warm it will create an open circuit just for a moment. sometimes it will stay open sometimes it will just open for a second and wont do it again for days , hours, weeks.

good luck.

signing-up.