Only two questions today

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Bill Crowell

Bill Crowell

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1. Will you guys who have been around forever please take pity on me and tell me how you used to use an old-fashioned, free-standing oscilloscope to examine the spark on an engine? What kind of attenuator would you use, and what sweep rate?

2. What do you think of those "paint-can" smoke machines that are sold on eBay for around $65 or $70, and which one is your favorite?

Thanks very much.
 
Bill Crowell said:
1. Will you guys who have been around forever please take pity on me and tell me how you used to use an old-fashioned, free-standing oscilloscope to examine the spark on an engine? What kind of attenuator would you use, and what sweep rate?

2. What do you think of those "paint-can" smoke machines that are sold on eBay for around $65 or $70, and which one is your favorite?

When I go into the shop ill try to find the diagram that shows a good wave form for you
Click to expand...
 
Since I have "been around forever", will offer some input on the scope. First this is a scope that is designed to be used for auto diagnostics, not for use in a lab or medical use. My understanding is that scopes for automotive use were developed by Snap-On in the late 1940's. Automotive scopes are plug and play and need a feed to the low voltage and high voltage from the engine. Scopes are often used in combination with other instruments hence there are often many different connections for various tests. A classic scope pattern for one cylinder (going from left to right) will display initial spark firing- the length in time of the spark- the closing of the points- the firing of the condenser. The scope is able to see any single cylinder, all eight in a row or display all eight on top of each to compare uniformity. So a scope will tell you the condition of the spark plugs/wires, coil, points and condenser and to some degree the condition of the cylinder. The results are further enhanced by the experience and knowledge of the operator. Wow! That is a long answer.
 
I had a new 68 Roadrunner 383 4 speed that after about 7000 miles ran bad. I took it to a dealer in Delaware where I was stationed in the Air Force, and he put the scope on the Engine and said you have a bad plug. He replaced it and the car ran great again. That was the only time I saw the Scope in action and gained a lot of respect for it. That was 52 years ago. In later years I couldn't find any body that had one to analyze engine operation.
 
The automotive scopes all used a capacitive coupled connection, that is a sheet metal clamp that clamps around the coil wire. This is not inductive, it forms a coupling capacitor. So you can experiment with that and build one, easy. It is two curved pieces maybe 1 1/2--2" long, like splitting a piece of tubing. The two curved pieces clamp around the coil wire. This gives you your full secondary pattern of all cylinders

Trigger or sync the scope off the coil NEG if you have points or a switching type ignition. If you have MSD I'm not too sure.

The (coil) primary pattern is extremely useful. It shows up dwell from which you can calculate the figure, as well as showing up worn bushings or bent shaft in the dist or bad points or condenser.

Lookee here LOL:

https://lunardiagnostics.weebly.com/uploads/1/0/4/4/10449663/eti_feb77_scopetheignition_1b__1_.pdf
 
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You can take a clothes pin and glue some copper foil to make a pick up. Solder a 0.01uF 1KVAC capacitor in series into a coax, and another capacitor from the center to the shield. This builds a AC Voltage Divider such that your scope will be see approximately 1/2 of the kilovolts. Long jumper on the coax shield for the GND. Set your scope for 1M ohm input if selectable. Depending on your scope, triggering can be temperamental. Place on the coil wire and try to see 8 (for a V8) ringing pulses. If you have MSD it will be just a big mess of ringing. If the ringing is not symmetrical then you have a bad plug, wire, distributor port, etc.
 
McGyver aside- scopes are for point ignition. Other ignitions throw a different pattern.
 
While I have yet to see a pattern due to non-working pickups, I'm pretty certain that my scope is made to read electronic ignition (early 90s Snap On Counselor II)...I may have the coil wire pickup repaired, but the #1 trigger doesn't seem to work. Maybe it needs the other pickup to work. As I get my Barracuda closer to road ready I will hook it up again and see what all it tells me.
 
Regardless on how you ring a coil whether with a point contact, or transistor, or CDI connected to a MOSFET the scope can analyze the signal.
 
^^I screwed the pooch earlier^^ For sync you need the no1 plug otherwise the pattern won't know "where to start." The old Allen we had used what MAY have been an inductive device for the no1 plug but it used the forementioned capacitive device for the n1 sync. It also had a "points" (dist neg) connection to see the primary pattern.

I'm not sure whether inductive would work in this case or not. "It might be" that differences in plug wires, resistive or not, wires or plugs, etc that a capacitive pickup works better. I've only worked with about 3 old scopes years ago and they all used a cap pickup for the secondary pattern.
 
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