The distributor is the heart of the ignition system. Its three basic functions are to trigger the firing of the ignition coil, to control timing advance, and to route or distribute the coil's high voltage output to the individual spark plugs.
On older applications with breaker point ignition systems, the contact points switch the ignition coil's primary voltage on and off. The points are mounted on a movable plate in the distributor housing. Cam lobes on the distributor shaft rotate against a plastic block on the points to open and close the points. On newer applications with electronic ignition systems, the distributor contains a magnetic pickup or Hall effect switch instead of points to generate a signal for the ignition module (which switches the coil on and off). On many applications, the ignition module itself is also located inside the distributor (GM HEI) or mounted on the distributor housing (Ford TFI).
Wear in the distributor drive gear can introduce play that can retard timing, while worn distributor shaft bushings (often the result of infrequent oil changes and varnish buildup on the shaft) can result in erratic timing. Both conditions can affect engine performance, fuel economy and emissions. If the distributor is worn, replacement is the only repair option.
With respect to timing, the distributor on older applications has both a centrifugal advance mechanism and vacuum advance diaphragm. Flyweights and springs in the centrifugal advance mechanism add timing advance as engine speed increases. Bushing wear, rust, or weak or broken springs may cause the centrifugal advance mechanism to overadvance or retard timing. Lubricating the mechanism with high temperature dielectric grease and/or replacing weak or broken springs may solve a sticking problem, but worn bushings call for distributor replacement.
The vacuum advance diaphragm rotates the breaker plate to advance timing when intake vacuum is high to improve fuel economy, and retards timing when the engine is under heavy load to prevent detonation (spark knock). If the diaphragm can't hold vacuum, the diaphragm must be replaced (if available separately).
DISTRIBUTOR ROTOR & CAP
The distributor rotor directs the ignition coil's high voltage output to the various spark plug terminals in the distributor cap. The condition of the cap and rotor are very important, as is the "air gap" or distance between the tip of the rotor and cap terminals. If the cap or rotor are cracked, worn, eroded or have carbon tracks, either or both should be replaced.
Sometimes a distributor will "eat" caps. Often this is due to excessive end play in the distributor shaft. Too much end play allows the rotor to push up against the cap and wear down the center button. The rotor may even contact the terminals and damage or break the cap. Shimming the distributor shaft may reduce the end play, but replacement may also be necessary.
REPLACING A DISTRIBUTOR
If your engine needs a distributor, buying a remanufactured unit can usually save you money compared to buying a new one.
A used distributor from a low mileage vehicle in a salvage yard should be okay, but you may have to buy a new cap and rotor for it. Watch out for used distributors at swap meets because they may have been modified for a performance engine and will have the wrong spark advance curves for a stock engine.
Older breaker point distributors usually come complete with new points, condenser, rotor and cap. The points are usually preadjusted so the unit can be dropped right in and the timing set. Some of the newer electronic distributors come complete with a module, rotor and cap while others do not.
One very important point to note when installing GM HEI and Ford TFI ignition module in or on a distributor is to apply a coating of dielectric grease to the underside of the module. Grease is needed to conduct heat away from the module. Forget the grease and the module will likely fail within a few thousand miles.
The index position of the distributor is very important because it must be aligned correctly with the camshaft for proper ignition timing. Before you remove the old distributor, rotate the engine until the #1 piston is at top dead center (use the timing marks on the crankshaft pulley). Remove the cap and see if the rotor is lined up with the cap terminal for the #1 piston. If it is not, rotate the crankshaft 180 degrees so it is lined up with the #1 spark plug terminal. Then remove the distributor hold down nut and clamp, note where the vacuum advance is pointing, then pull the distributor out of the engine.
When you install the new distributor, line up the vacuum advance in the same direction as the old distributor was pointing, rotate the shaft so the rotor will be pointing approximately in the same location as the #1 spark plug terminal, then slide the distributor into the engine. The angle of the drive gears will cause the rotor to rotate slightly and the distributor slides all the way into the engine. Replace the distributor hold down clamp and nut, but do not tighten.
Start the engine and use a timing light to adjust the timing to specifications (typically 6 to 10 degrees of advance with the vacuum advance hose disconnected and plugged at 550 to 650 rpm idle speed). Refer to the emissions decal under the hood, a service manual or online service information for the timing specifications and procedure for your engine, because they do vary quite a bit. If the engine has computer controlled spark timing, no adjustment is possible. Just lock down the distributor.
Here's a short video on how to install a distributor: