Mercury 305 CID, Service Manual, Page: 326 / 967

THUNDERBOLT IV AND V IGNITION SYSTEM - 4B-17 90-823225--1 1096
Circuit Description
Refer to the circuit wiring diagram on the following
page for reference to this circuit description.
Ignition Control Module
The ignition module receives its power (+) thru
the PURPLE wire “9”.
Ignition module ground (–) is accomplished thru
the BLACK wire “10”.
There is also a Case Ground (–) wire “12” that is
connected to one of the ignition module attaching
screws.
The 12 volt signal from the ignition module to the
distributor is carried thru the WHITE/RED wire
“8”, to the distributor sensor and back to the
ignition module thru the WHITE/GREEN wire “7”.
The tachometer signal is carried to the
instrument panel thru the GRAY wire “11”.
The PURPLE/WHITE wire “3” carries the signal
from the knock control module to the ignition
control module.
There are two BLACK wires “5” that have bullet
connectors. This circuit is reserved for future
options. On current models, the two BLACK
wires must be connected for the system to
function properly.
The TAN/BLU wire “6” carries a signal from the
Audio Warning circuit to the ignition module.
Knock Control Module
The knock control module receives it’s power (+)
from the PURPLE wire “4”.
Knock module ground (–) is accomplished thru
the BLACK wire “2”.
The PURPLE/WHITE wire “3” carries the signal
from the knock control module to the ignition
control module.
The BLUE wire “1” carries the signal from the
knock sensor to the knock module.
Ignition Control System Timing Lead
The ignition control system has a lead with bullet con-
nector “11” that is connected into the PURPLE/
WHITE wire “3”. This lead is used for performing the
following tests and procedures:
Setting “Base Ignition Timing”
Setting “Engine Idle Speed”
Setting “Idle Mixture”
Testing Knock Control Circuit
This lead, when connected to an engine ground (–),
locks the ignition control module into the “Base Tim-
ing” mode.