On Thursday, October 27, 2016 at 8:08:11 AM UTC-5, Ivan Vegvary wrote:
Thank you all. Scope will not be used on ignition. Car runs fine. Lesson(s) learned. Great group!
Ivan Vegvary

This is one of those times when an old CRO would come in handy. A 20 MHz job from the 1970s would be fine.

You would connect the vertical input to where the wires goes from the points/condenser to the coil. The other channel you use to trigger by loose coupling to cylinder #1.

Set the time base so you see eight pulses if it has a eight cylinder. They should all be the same amplitude. If some are higher the gap of the plug might be wider or the wire starting to fail. If some are lower that would indicate a spark leak, like loss of insulation or a fouled/semi fouled plug.

Once you eliminate all ignition faults having the plug gaps all the same and the wires all good, then if there are variations in the pulses that indicates the relative compression of that cylinder. A cylinder with low compression will yield a lower amplitude pulse if all other things are equal.

I don't remember what kind of voltage you get to the coil on one of those but I think it was around 300 volts. Many newer scopes only go up to 5V/div which with a 10X probe (always use a 10X probe to protect the front end, unless you REALLY need all that gain) gets you to 50V/div. You can go into "UNCAL" mode if it goes off the screen but you are at the upper limit of the front end of the scope now.

You can experiment with it by slowly pulling off one of the wires from the distributor. Don't pull it from the plug itself - I learned that the hard way. ZZZAPP !

You should see the pulse for that cylinder increase in amplitude. Take a wire and short the output of the distributor for that cylinder to ground and the amplitude will decrease. Don't let the thing run alot with o plug wire or anything on the distributor because those coils are current operated. When there is no arc the voltage tries to climb until there is a load and it might break down the insulative properties of the distributor cap or rotor. (this is even more important on newer cars)

Actually the easiest way to find a spark leak is a AM radio. Tue to the lower end of the dial and listen to the ZZZZZZZ sound which will increase in pitch as you rev the engine. But if you get a POP POP POP sound that means you probably need wires or a distributor cap. The does not tell you which cylinder but it doesn't matter, the wires should be changed as a set and the distributor cap is common to all so that is that.

The one time I ever used a scope on a car was a 1990 Olds with a VIN C 3800.. It stalled sometimes when hot and would not restart. The way it acted smelled like a semiconductor fault and sure enough the crank sensor was failing. We read up on how the system works and I stuck the scope probe where it needed to be. A garage would have taken a month to fix this but we were flipping cars at the time. I used to have a couple thousand worth of books on it but later sold them because I don't do that kind of work anymore for a couple of reasons.

Anyway, does that old Pontiac have a radio in it ? Now that can be a blast. You're looking at tubes ad a vibrator based B+ supply. I assume it is 6 volts, right ?

What can blow some people's mind is that those cars and even the radio will work if you put the battery in backward. It has been known to happen actually, that is why they used to polarize the generator. But they didn't really have to, nothing cared much about polarity. Even the radio, because the vibrator changed it to AC for a transformer the rectifier(s) after that took care of polarity, the filaments of the tubes don't care.

I think it did make a little bit of difference in the spark, but minor. The problem comes when you need or give a jumpstart. Sometimes you get that big spark and then you know that one or the other car is mis-polarized.

Later came the hybrid radios with the solid state output, then polarity became important, also then the alternator came out and it was not an issue anymore. It simply wouldn't run on reverse polarity because the diodes would short out the battery. In cars with a generator the charging current went through the ignition switch, or else the coils would discharge the batter forthwith. Therefore it was not a good idea to leave it on, to play the radio for example. then they came out with the "ACC" position on the ignition switch. It also wouldn't heat up the points if they happened to be closed.

While I greatly prefer 1960s and 1970s cars it is still a good thing to keep that old beast running.

I drove a 1950 Chevy once, it was almost like work. People don't know how good they got it now. They have no idea what it means to double clutch, and they have no idea how undeveloped the steering and suspension was back then.. Driving those old cars is almost like driving a semi truck, but the new ones aren't even that hard.

Well OK, it isn't really all that difficult but I bet you don't take that thing to work every day...