MSD 6421 6AL-2 Ignition Control Installation - Manual (Page 8)

8 INSTALLATION INSTRUCTIONS
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from the battery, it does not require much current to keep the unit energized. If you are experiencing run-
on, it is due to a small amount of voltage going through the charging lamp indicator and feeding the small
Red wire even if the key is turned off.
Early Ford and GM: To solve the Run-On problem, a Diode is supplied with the MSD in the parts
bag. By installing this Diode in-line of the wire that goes to the Charging indicator, the voltage is kept
from entering the MSD. Figure 6 shows the proper installation for
early Ford and GM vehicles.
Note: Diodes are used to allow voltage to flow only one way. Make
sure the Diode is installed facing the proper direction (as
shown in Figure 6).
Ford: Install the Diode in-line to the wire going to the “1” terminal.
GM: Install the Diode in-line to the wire going to terminal #4.
GM: 1973 - 1983 with Delcotron Alternators.
GM: Delcotron Alternators use an internal voltage regulator. Install
the Diode in-line on the smallest wire exiting the alternator (Figure
7). It is usually a Brown wire.
SMALLER OF THE 2-WIRES
DIODE
WIRE LOOM
GM 2-WIRE ALTERNATORS
Figure 7 Installing the Diode to a
1973-1983 GM Vehicle.
MISSES AND INTERMITTENT PROBLEMS
Experience at the races has shown that if your engine is experiencing a miss or hesitation at higher
rpm, it is usually not directly ignition. Most probable causes include faulty wiring, a coil or plug wire
failure, arcing from the cap or boot plug to ground or spark ionization inside the cap. Several items
to inspect are:
• Always inspect the plug wires at the cap and at the plug for a tight connection and visually inspect
for cuts, abrasions or burns.
• Inspect the Primary Coil Wire connections. Because the MSD is a Capacitive Discharge ignition and it
receives a direct 12 volt source from the battery, there will not be any voltage at the Coil Positive (+)
terminal even with the key turned On. During cranking or while the engine is running, very high voltage
will be present and no test equipment should be connected.
WARNING: Do not touch the coil terminals during cranking or while the engine is running.
• Make sure that the battery is fully charged and the connections are clean and tight. If you are not
running an alternator this is an imperative check. If the battery voltage falls below 9 volts during a
race, the MSD output voltage will drop and the current draw will increase.
• Is the engine running lean? Inspect the spark plugs and complete fuel system.
• Inspect all wiring connections for corrosion or damage. Remember to always use proper connections
followed by soldering and seal the connections completely.
Most other applications: On
other applications where engine
Run-On is experienced, a
Resistor can be put in-line
to the MSD's small Red wire
(Figure 8). This resistor will keep
voltage from leaking through to
the MSD unit.
Figure 8 Wiring the Dual Ballast Resistor for Run-On.
MSD