Auto Repair Help

by Lance Wright

The distributor is used to deliver secondary ignition current, in sequence, to the individual spark plugs. The components consists of a housing, central shaft and drive gear. The distributor drive gear is pressed onto the distributor shaft and driven by the camshaft. The distributor shaft is placed in the distributor housing and supported using bushings. Engine oil is used to lubricate the distributor shaft. If a magnetic pulse generator is used to signal engine position, the reluctor wheel is generally placed on the distributor shaft. The distributor housing contains a mounting surface for the signal generator. The distributor housing is also used to provide a mounting surface for the distributor cap. The mounting surface is designed to only allow the cap to mount in the correct position. The distributor is held in the engine using either a mounting clamp or a slotted flange. This design allows the distributor to be rotated for timing adjustment. Most distributors use an “O” ring or gasket to prevent oil leakage. Repairing an automotive distributor can be a frustrating experience and thus typically they are swapped for new or rebuilt units.

The pick-up is a signal generator mounted in the distributor. It is used by the ignition module to determine ignition timing and engine speed. There are two basic types of distributor pick-up assemblies in use, the magnetic pulse generator and the hall effect switch. Both are operated by the rotation of the distributor shaft. The magnetic pulse generator produces an AC voltage signal induced by the rotation of a toothed wheel past a permanent magnet. The toothed wheel, known as a reluctor, is mounted on the distributor shaft. The number of teeth or points on the reluctor, corresponds with the number of cylinders of the engine. A pick-up assembly containing a permanent magnet is placed in close relationship to the reluctor wheel. The pick-up is positioned to provide a small air gap between the magnet and the highest point of the teeth of the reluctor. As the distributor shaft rotates, an AC voltage current is induced as each tooth of the reluctor moves past the magnet of the pick-up. The AC voltage signal is sent to the ignition module where it is converted to a digital on/off signal. This digital signal is used by the ignition module to trigger the ignition primary circuit and determine engine speed.

The Hall effect switch consists of a permanent magnet, a semiconductor and a vaned wheel that is either mounted on the distributor shaft or manufactured as part of the distributor rotor. The permanent magnet is positioned facing the semiconductor, leaving a small gap between the two. The vaned wheel is designed to pass between the magnet and the semiconductor as the distributor shaft rotates. As each vane of the wheel, one per cylinder, passes between the magnet and the semiconductor, the magnetic field between them is broken. This causes the semiconductor to output a digital on/off voltage signal, that is used by the ignition module to control ignition timing and determine engine speed.

The distributor cap and rotor are used to provide a rotating current path to the spark plugs. They are manufactured out of a special plastic insulating material, designed to withstand both heat and high secondary ignition voltage. The inside surface of the distributor cap contains ridges to prevent the high voltage secondary current from crossing over to the wrong cylinder and resist carbon tracking. Secondary ignition current enters the distributor cap through an electrode placed in the center of the cap. Inside the distributor cap is a contact constructed of carbon material and used to provide a current path between the center electrode and the distributor rotor. Electrodes are placed around the perimeter of the distributor cap, corresponding with the number of cylinders the engine contains. The electrodes are spaced far enough apart to prevent arc over and reduce carbon tracking. As the automotive distributor shaft rotates, the rotor moves past each electrode, in sequence, providing a current path to each spark plug. The rotor does not touch the electrode but passes by a specified distance from the electrode. The secondary current is required to bridge the gap between the rotor and the electrode. Deterioration of the distributor cap electrodes or the rotor tip, can cause the rotor gap to become excessive. Troubleshooting hard start problems can be a simple auto repair job only involving replacing the rotor. This could result in high secondary voltage requirements and can affect engine performance.

(Lance owned his own auto repair shop for 30 years before retiring in 2006.)

2 responses to “Distributor Overview”

  1. Frank says:

    I am replacing the reluctor wheel (magnet) on a gm 4.3 liter distributor. The old reluctor is already removed. I looked at another 4.3 distributor and the reluctor wheel magnet appears to be indexed so that two of the teeth are in the middle of the rotor’s notch in the distributor shaft.
    Is the indexing of the wheel very important or can I just press the new wheel onto the distributor shaft very important?

    • Austin says:

      Yes, it is absolutely crucial to index the rotor and the reluctor correctly, or the reluctor will not trigger at the same time the rotor is in synch with the pickups on the cap. They must be correct or there is no way the distributor will ever function

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