Mercury 305 CID - Testing Thunderbolt IV HEI System

IMPORTANT: Use a voltmeter when mak-
ing these tests. DO NOT use a test light.
1C-8 - TROUBLESHOOTING 90-823225--1 1096
Testing Thunderbolt IV HEI System
!
WARNING
Be sure that engine compartment is well venti-
lated and that there are no gasoline vapors pres-
ent during the following test to prevent a poten-
tial fire hazard.
Remove High-Tension Lead
from Distributor to Coil. Insert
a Spark Gap Tester from Coil
Tower to Ground. Remove
WHT/GRN Lead from Distribu-
tor Terminal - Ignition Key in
RUN Position. Strike the Termi-
nal on the WHT/GRN Lead
against Ground
0
Volts
Spark
at Coil
12 Volts
0
Volts
No Spark
No Spark at Coil
No Spark at Coil
Spark
at Coil
1 to 12 Volts
0 Volts
0 Volts
Voltage
Unplug WHT/
RED bullet con-
nector from Dist.
Term., then
Check for Voltage
on This Lead
Replace Ignition
Sensor in
Distributor
Replace Ignition
Module Harness
Replace
Ignition
Module
With Key in RUN Position,
Check for 12 Volts at Positive
(+) Terminal on Ignition Coil
Check for 1-12 Volts at WHT/
RED Terminal on Distributor
Replace Ignition Module
Substitute a New Ignition Coil.
Repeat Above Test
Check Engine
and Instrument
Wiring Harness,
Battery Cables,
Key Switch
Replace Ignition
Sensor in
Distributor
Install New
Ignition Coil
Check all Terminal Connections
at Distributor, Ignition Module
and Ignition Coil
Battery OK?
Distributor Clamping Screw
Tight?
Note 1 : Early , large ignition modules did not have a replaceable wiring harness.