While mostly phased out due to electronically controlled ignition systems, the distributor has been the main source of timed ignition for several decades. The Distributor is a device that turns with a camshaft, and sends electrical current from the ignition coil to the spark plugs at the precisely correct moment.

Sending spark to the proper spark plug at the proper time is accomplished the same way as opening and closing the engine cylinder head valves. Mechanical timing is used as a highly reliable source of timing because it never changes as long as the gears stay in mesh. The distributor drive gear stays in mesh with the camshaft, and that turns the distributor drive shaft and rotor at one-half of engine speed. This allows the distributor rotor, powered by the ignition coil, to revolve and distribute power through individual electrodes on the distributor cap. These electrodes are connected to one spark plug wire each, and that spark plug receives this high energy current from the ignition coil repetitively, on time, and in sequence with the other spark plugs.

A failed or failing distributor will either not allow the engine to run, or the engine will run poorly. Often, these symptoms can be corrected by replacing the maintenance items for the ignition system, the ignition coil, or ignition module, but if more economical, the distributor can be replaced as an assembly to alleviate ignition distribution issues. Also, there may be a grinding or whirring noise from the distributor, or the engine may misfire rapidly when the accelerator is pressed. This would result from internal failure of the distributor bearing or vacuum advance canister respectively, and replacement of the distributor may become necessary. Lastly, the distributor should be replaced if the driveshaft has been damaged, especially if the distributor is no longer contacting the drive gear or the shaft has become warped or eccentric.

Upon diagnosis of distributor failure, the distributor will need to be replaced immediately. Not only will the vehicle likely fail to start, run, or produce power, but it may also create debris inside of the engine crankcase if there is physical damage.

Some makes and models have constant issues with distributors, and require replacement before the first 100,000 miles is reached. However, in most cases, distributor replacement will likely occur after 150,000 miles. The distributor assembly does require frequent maintenance to ensure proper operation, so the components within the distributor are much more likely to fail than the distributor assembly as a whole.

Diagnostics will take one of two paths for a distributor, mechanical or electrical. If the distributor is suspected of mechanical failure, including failure of the vacuum canister, mechanical advance, bearing, or drive shaft, the distributor may be inspected in or out of the engine. The vacuum advance canister can be tested with a vacuum gauge to ensure there are no leaks in the diaphragm. If damage to the ignition distributor drive shaft is suspected, it must be removed and inspected. Electrical diagnosis begins by testing the ignition coil in most cases, and if the ignition coil and primary ignition wire are working correctly, the distributor cap and rotor will be inspected, as well the ignition module. Finally, the pickup coil will be tested for proper resistance across the coil, or proper voltage production when signaling the ignition module to fire. Note, the majority of these components can be replaced individually, but when multiple components are found to be faulty, it may be more economical to replace the ignition distributor assembly.

The ignition distributor will likely be left in the vehicle during inspection and testing, so it must be removed for replacement. In preparation for removal, the engine is turned by hand to top dead center on cylinder one on the power stroke. This will point the distributor rotor towards the electrode on the distributor cap corresponding to cylinder one. At that point, the retaining bolt(s) is removed, and the distributor will twist out of the engine block or cylinder head. Installing the new ignition distributor will involve rotating the distributor backwards to ensure that the forward rotation of the distributor will result in the distributor rotor pointing to the same location as before it was removed. All electrical connections are replaced, including the spark plug wires, and, if equipped, the vacuum advance canister is connected to engine vacuum.

Installing aftermarket distributors should be done after much research. Some of the aftermarket parts available, especially for American classic cars and trucks, are not applicable to the vehicles without modification of the engine wiring harness. Any time a modification is done to an engine, research and consultation should be conducted prior.

The ignition coil transforms battery voltage to thousands of volts, and many times is housed by the distributor assembly. If the coil is not housed in the assembly, the center spark plug wire is for the ignition coil and can cause electrical shock if handled improperly.

Replacing the distributor assembly requires knowledge of engine timing. If the ignition distributor is replaced in an incorrect position, the result could be a non-running engine, or burnt cylinder head valves. It is imperative to understand how to time a distributor prior to beginning this job, but otherwise the well-skilled DIYer can handle the task.