See full version: Dwell and Timing Explained
Although many ignition systems on new cars today are computer controlled, your classic or late model car likely has a points ignition system. And if you enjoy working on the car, there are a few things you need to understand before setting its timing, including the importance of setting the dwell.
The dwell is adjusted by setting the points gap to a specified distance at maximum opening. A narrower gap gives more dwell and a wider gap gives less. Taking it to extremes, excessive dwell means that the points close too soon after opening, cutting off the magnetic field collapse before it delivers all its energy. Too little dwell gives the magnetic flux insufficient time to build up to the maximum. [links]
Ignition points are a set of electrical contacts that switch the coil on and off at the proper time. The points are opened and closed by the mechanical action of the distributor shaft lobes pushing on them. Getting the best gap between the points is essential for proper engine performance and reliability. Set the points too wide and the spark plugs don't get enough juice. Set them too close and the engine stops working after a few miles.
Many mechanics like to check the dwell measurement with a dwell meter after setting the points. There are some who say you don't have to. But it is a good way to check the point gap and make sure it is right.
The basic ignition system consists of the ignition coil, points, condenser, distributor, and spark plugs. A ballast resistor may also be included in this system. When all of these parts are connected and working properly, we will get the spark the engine needs to run. Now, what are these parts and what do they do?
Once you have the rubbing block on top of a lobe, you can measure the point gap. Loosen the screw that holds the stationary point bracket to the base plate. Not completely, just enough so that you can move the bracket by inserting a screwdriver tip and twisting it Adjustment is a matter of trial and error. Move the stationary point out a bit if it was too close, tighten the holding screw (not too tight), and measure the gap. If it still isn't right, try again. The feeler gauge should have a light drag when the points are properly adjusted. This is where practice and patience come in handy. [links]
All cars up until 1975 or so used a point type ignition system. After 1975, most cars went to electronic ignition systems. Basically, electronic ignitions were "improved points." The principles were the same and it simplified the ignition system.
Now that we know what the parts are and what they do, let's talk about replacing them. Replacing the points and condenser are very easy. All you need to replace the points are some basic tools, a magnetic screwdriver, feeler gauges, and a dwell meter. more
There are many mechanics that set the points by dwell alone. It is a perfectly acceptable and accurate way of adjusting the points. In fact, most all GM distributor caps have a little door that allows access to the points so the dwell can be adjusted while the engine is running. On engines that don't have that access, you need to be a little more creative. What we do is remove all the spark plugs from the engine, set up the points, turn the key on and crank the engine while adjusting the point dwell. Once it's set, we lock them down and finish the tune-up. [links]
You'll go to your doctor's office to get a blood test. Before your appointment, your doctor will tell you if you need to avoid food and certain drinks. Let them know if you're taking any medicines -- especially an antibiotic -- because that can affect your test results.
If your doctor diagnoses you with a condition based on a high or low anion gap result, they'll come up with a treatment plan that helps you take charge of it. [links]
Once you get your test result, ask the doctor to explain exactly what it means. Keep in mind that even if your anion gap is high or low, that doesn't necessarily mean you have a serious health problem.
Modern engines use computer-controlled factory-preset self-adjusting ignition systems that never change their timing, have no moving parts and never need maintenance. Yay! A generation ago, every teenager, every mechanic and a lot of vehicle owners understood the theory and practice of changing points and setting the timing. Nonetheless, there are still plenty of older vehicles, outdoor power equipment, boats and tractors that need periodic adjustment or replacement.
The distributor on these older vehicles performs two related tasks. The first uses a simple on/off switch, the ignition points, to provide properly timed pulses of 12-volt electricity to the ignition coil. In the coil, essentially a transformer, it's stepped up to 10,000 to 20,000 volts. Then, the high-voltage electricity from the coil returns to the distributor, where the rotor inside parcels it out to the correct spark plug to ignite the fuel/air mix.
There's a lobed cam on the distributor shaft that pushes on a small rubbing block on the movable side of the points. As the cam and distributor rotate, the points open and close constantly. As they close, current from the ignition switch flows through the contacts into the coil's primary windings and then off to ground. This current generates a magnetic field in the coil's iron core. When the points open a few degrees of crankshaft rotation later, the current is interrupted, causing the magnetic field to collapse. This induces electrical current into the secondary windings of the coil, where the current is raised to 20,000 volts or more. The high voltage now travels over to the distributor, where the rotor metes the high-voltage pulses out to the correct spark plug. more