The entire ignition system is self-contained within the distributor. GM intended for you to buy the entire assembly, but the aftermarket industry came up with all the individual repair parts. There are three common failures with this system.
The first one is the pickup coil around the distributor shaft. You can unplug its two-wire connector from the ignition module, then check its continuity with an ohm meter. As I recall, you can expect to find around 450 to 850 ohms. Usually it is not the coil itself that becomes open. Instead, look where those two wires are connected for signs of corrosion, and if they broke inside their insulation from repeated flexing. Those wires bend each time the vacuum advance kicks in.
The second suspect is the ignition module itself. There are two versions of this. The standard one has two terminals on one end that connect to the pickup coil, and two terminals on the other end to fire the ignition coil. This distributor has mechanical advance weights under the rotor. The second version also has the two terminals on the end for the pickup coil, but it has five or six terminals on the other end. This one splits the system into two parts, then inserts an engine computer in between them to adjust spark timing. Those computers had a reputation of developing broken solder connections and corroded connector terminals, both of which caused intermittent operation or intermittent stalling. You could often get them working for a while by banging on them. Those are inside the car. I think they were behind the right kick panel or in front of the glove box. Today, we install software into computers depending on the car's year, model, and options. With your computer, there is an integrated circuit to remove and transfer to the replacement computer when you have to replace it. They called that the "PROM", (programmable read-only memory), and it contains the fuel mapping and spark timing curves for your application. The engine will run with the wrong PROM, but emissions and engine performance will not be at their best.
A lot of auto parts stores used to have a test bench set up to test these GM ignition modules. You may still be able to get yours tested. If you have to replace it, the new one should come with a small tube of heat-sink grease. Be sure to use that underneath where the metal housing contacts the base of the distributor. Failing to draw the heat out of the module will lead to an early failure.
The third problem is less common, but it is something you need to be aware of. The ignition coils in these distributors were one of the first designs that were capable of developing up to 45,000 volts. All ignition coils only develop exactly what it takes to make the spark jump the gap in the spark plug, and no more, but when checking for spark, we customarily remove a spark plug wire, then hold it near a metal part of the engine. The problem is when that wire is pulled too far away, the spark has to jump an ever-increasing gap. The voltage developed will rise more and more, and at 45,000 volts, that spark is going to find a path to ground somewhere. Once the spark plug wire is pulled too far from the engine, it becomes easier for the voltage to find a path to ground somewhere else, and that is through the rotor, then through the distributor shaft to ground. That is called "punch through", and once that happens, the rotor is shorted. The current flowing through the rotor leaves a carbon track behind, and since carbon conducts electrical current, that becomes a very much easier path for current to flow than across the spark plug's gap. That is why it is considered shorted. For over forty years I worked on tv's that use a high voltage transformer. Most in color tv's develop real close to 25,000 volts. When the insulation breaks down, the arc will burn through the plastic housing and can jump over two inch to a metal bracket. It is a real loud machine gun-like sound that you do not want to listen to very long. GM's HEI coil develops almost twice that much voltage, so it is easy to see why that spark voltage is going somewhere.
Also, check the spring-loaded carbon button in the center of the underside of the distributor cap. Sometimes that falls out and we do not notice it is missing.
The red wire that plugs into the distributor cap must have twelve volts on it when the ignition switch is in the "run" position. That is the only wire needed to run the system when it uses the four-terminal ignition module. This "high energy Ignition", (HEI), system was popular as an upgrade from the older breaker-point distributor as it bolted right into the engine, but be aware the points systems will always have a ballast resistor to limit current through the points and ignition coil to prevent burning the contacts. Other car brands used a white ceramic resistor on the firewall, about 1/2" square and 2 1/2" long. Some GM applications used a resistor wire hidden within the wiring harness. Regardless of the type of resistor, it has to be removed or bypassed when using an HEI distributor in place of a points distributor. The HEI system has to have full battery voltage to operate.
You can test an HEI distributor off the engine. Connect a small jumper wire from the metal housing to the battery's negative post. Connect another jumper wire from the battery's positive post to the terminal in the cap where the red wire connects. If the cap is off, connect a third jumper wire from the ignition coil to the engine, but leave it with a gap to view the spark. If the cap is on, fashion a way for the spark to jump from all eight spark plug wire terminals. You can even use seven jumper wires to ground seven of those terminals to the engine. Shorting them to ground will not hurt the ignition coil. Leave a gap in one of those wires so you can see if spark occurs. All you have to do is spin the shaft by its drive gear. Remember that if any of these gaps is too large, the rotor could become shorted if spark voltage decides that is the easier path to ground.
Friday, January 26th, 2018 AT 5:40 PM
