How many volts in the 94 M104 Waste Spark system? - Page 2 - PeachParts Mercedes-Benz Forum

Chas H · #16 03-06-2010, 11:52 AM

Quote:

Originally Posted by Matt L

There is no condenser in the ignition circuit?

Any ignition circuit that I have ever touched in the past has had a condenser in parallel with the primary coil. The breakdown of the field causes a reverse EMF (i.e., voltage) which charges the capacitor. Then, true to form, the current reverses and the process repeats - a few times.

If you've ever seen an o-scope hooked to an ignition circuit, you would never again claim that the circuit was DC. It is an AC spark. The first half-wave is the strongest, yes, but it's still AC.

Condenser haven't been used since points went away. There may be small condensers in the igniter circuit, but you won't see any discrete condensers hanging about.
The current in a spark plug circuit does not reverse. Does the voltage on your oscilliscope ever drop below the zero line?

Matt L · #17 03-06-2010, 11:57 AM

Quote:

Originally Posted by Chas H

This is not true of non-waste spark systems. The basics are the same for both. Voltage flowing through the primary windings of the coil creates a magnetic field. When this voltage is abruptly interupted, the magnetic field collapses and induces a much higher voltage in the secondary windings. This is the spark plug voltage and it is a circuit that flows through the coil, the spark plug(s) the engine and back to the coil. It does not flow to or through the battery.
Here's a schematic to help.
http://www.sentex.net/~mwandel/cannon/sparky.html

The positive side is broken? Well, I've never seen that one, but I will admit that it could exist. On any system that I have seen with an external coil (where you can actually discern the connections), there was no ground connection at the ignition coil. The high-tension current went through the battery, which won't affect anything given its extremely low impedance.

As for the condenser, I didn't specifically say "external" or "discrete" condenser. I imagine that there's a condenser in there.

I do know how well a points system runs with a bad condenser, as I've had a few go bad on me over the years.

Chas H · #18 03-06-2010, 12:01 PM

Quote:

Originally Posted by Matt L

The positive side is broken? Well, I've never seen that one, but I will admit that it could exist. On any system that I have seen with an external coil (where you can actually discern the connections), there was no ground connection at the ignition coil. The high-tension current went through the battery, which won't affect anything given its extremely low impedance.

As for the condenser, I didn't specifically say "external" or "discrete" condenser. I imagine that there's a condenser in there.

I do know how well a points system runs with a bad condenser, as I've had a few go bad on me over the years.

The points typically ground the primary side of the coil. This is how it's been done forever.

Matt L · #19 03-06-2010, 12:15 PM

Quote:

Originally Posted by Chas H

The points typically ground the primary side of the coil. This is how it's been done forever.

Right. That means when the points are open, the only path between the secondary side and ground is through the battery. But 12V one way or the other isn't going to mean much to a secondary ignition circuit.

Matt L · #20 03-06-2010, 12:30 PM

Quote:

Originally Posted by Chas H

The current in a spark plug circuit does not reverse. Does the voltage on your oscilliscope ever drop below the zero line?

Fakes:

http://stixpix.com/t%2022.html

http://www.highperformancepontiac.com/tech/hppp_0612_pontiac_ignition_systems/photo_06.html

http://www.mainlineauto.com.au/support/downloads/ca9000/training/IgnitionSystems.pdf

I can't find any online images of an actual scope. But this is what they look like.

Chas H · #21 03-06-2010, 12:34 PM

Quote:

Originally Posted by Matt L

Right. That means when the points are open, the only path between the secondary side and ground is through the battery. But 12V one way or the other isn't going to mean much to a secondary ignition circuit.

Wrong. When the points open the flow of current, which does come from the battery, through the primary windings stops. This causes the magnetic field created by that current to collapse. This induces a secondary current in the seconary windings. This is what makes the spark. The voltage causing the spark does not flow to or through the battery. Study the schematic I recently posted again, 'cause class for ignition systems 101 is over.

Matt L · #22 03-06-2010, 12:58 PM

The schematic that you posted matches no ignition system that I've seen. Certainly not a points system. You did agree that the points switched the negative lead. Take a coil and an ohm meter and you'll see that there is no internal connection to the ground. The secondary current needs a complete circuit to function, and the circuit is completed through the battery to the ground.

Chas H · #23 03-06-2010, 03:05 PM

Quote:

Originally Posted by Matt L

The schematic that you posted matches no ignition system that I've seen. Certainly not a points system. You did agree that the points switched the negative lead. Take a coil and an ohm meter and you'll see that there is no internal connection to the ground. The secondary current needs a complete circuit to function, and the circuit is completed through the battery to the ground.

Whoops, that schematic is indeed incorrect.

Here's one that is correct.

Matt L · #24 03-06-2010, 03:45 PM

That's almost right. It shows a box between the condenser and ground. If that's the ballast, it's in the wrong place. It's also my understanding that the common point is usually the + connection and not the points connection. This can be verified with an ohm meter.

When those points open, the ringing occurs between the primary and condenser (as in any LC circuit). That causes an alternating magnetic field in the core, thus the AC at the plug wire.