A distributor is a device in the sense
parts in the ignition system of an internal combustion engine that
routes high voltage from the ignition coil to the spark plugs in the
correct firing order. The first high-tension distributor was developed
around 1904, independently by Napier, Boston, and Winton.
Distributor consists of a rotating arm or rotor inside the distributor cap, on
top of the distributor shaft, but insulated from it and the body of the
vehicle. The distributor shaft is driven by a gear on the camshaft. The
metal part of the rotor contacts the central high voltage cable from the
coil via a spring loaded carbon brush. As the rotor spins within the
distributor, electrical current is able to jump the small gaps created
between the rotor arm and the contacts due to the high voltage created
by the ignition coil. The metal part of the rotor arm passes close to
the output contacts which connect via high tension cables to the spark
plug of each cylinder.
The distributor shaft has a cam that operates the contact breaker.
Opening the points causes a high induction voltage in the system's
Around the 1970s the primary breaker points were largely replaced with
Hall Effect sensors. As this is a non-contacting device and the primary
circuit is controlled by solid state electronics, a great amount of
maintenance in point adjustment and replacement was eliminated. This
also eliminates any problem with breaker follower or cam wear, and by
eliminating a side load extends distributor shaft bearing life. The
remaining secondary (high voltage) circuit was as described above, using
a single coil and a rotary distributor.
The distributor also houses the centrifugal advance unit: a set of
hinged weights attached to the distributor shaft, that cause the breaker
points mounting plate to slightly rotate and advance the spark timing
with higher engine rpm. The capacitor is connected parallel to the
breaker points, to suppress sparking and prevent wear of the points.
A distributor caps are used in an automobile's engine auto parts to
cover the distributor and its internal rotor.
The distributor caps are prime example of a component that eventually
succumbs to heat and vibration. It is a relatively easy and inexpensive
part to replace if its bakelite housing does not break or crack first.
Carbon deposit accumulation or erosion of its metal terminals may also
cause distributor-cap failure.
The distributor caps have one post for each cylinder, and in points
ignition systems there is a central post for the coil voltage coming
into the distributor. In HEI (High Energy Ignition) systems, there is
not a central post, and the ignition coil sits on top of the
Cars that use a mechanical distributor often fail if they run into deep
puddles because the water that leaks into the distributor shorts the
electric current that should go through the spark plug, rerouting it
directly to the body of the vehicle. This, in turn, causes the engine to
stop as the fuel is not ignited in the cylinders. This problem can be
fixed by opening the distributor's cap and wiping the cam and the
contacts dry with tissue paper or by blowing hot air on them.
The "rotor" head is attached to the top of the distributor shaft which
is driven by a gear on the engine's camshaft and thus synchronized to
it. This rotor is pressed against a carbon brush on the center terminal
of the distributor cap which connects to the ignition coil either
through the top and wired directly to the coil in HEI systems. The rotor
is constructed such that the center tab is electrically connected to its
outer edge so that voltage coming in to the center post travels through
the carbon point to the outer edge of the rotor. As the camshaft
rotates, the rotor spins and its outer edge passes each of the internal
plug terminals to fire each spark plug.