Mercury 3.0 Liter Service Manual download pdf (Page 36)

90-898303 2-31

Control Module

The control module provides rev– limit (carb models), bias control, shift

stabilizer, idle stabilizer, injector timing signal (EFI models), and low oil

warning.

On carburetor models, the rev– limiter affects the cylinders in the following

sequence 2– 3– 4– 5– 6– 1. As the engine RPM exceeds the maximum

specification (5900 ± 100), the control module will retard the timing on cylinder

#2. The controller will retard the timing a maximum of 30 degrees and then, if

necessary, stop spark on the cylinder. If the engine rpm are still above the

maximum specification, the controller will begin to retard timing on the next

cylinder, then stop spark, continuing in sequence until the engine rpm drops

below the maximum specification.

Bias Circuit

Bias voltage is Negative (– ) voltage applied to the ignition system to raise the

trigger firing threshold as engine RPM is increased, thus stabilizing ignition

timing and preventing random ignition firing.

Disconnect neutral switch before performing test.

Test Black/White wire to engine ground. Reading is negative (– ) voltage and

performed at 2500 RPM. Normal readings are – 25 to – 40 volts @ 2500

RPM.

If readings are not within specifications, replace control module.

Shift Stabilizer Circuit

The shift stabilizer circuit ( not used on all models is designed to increase the

idle to timing approximately 2 degrees when the engine is shifted into gear.

Check idle timing with engine out– of– gear, activate the switch, timing should

increase approximately 2 degrees.

Shift switch may be tested with a resistance test. Continuity between the back

wires (disconnected) with the engine in gear and No continuity with the engine

in NEUTRAL.

Idle Stabilizer Circuit

The idle stabilizer will electronically advance the ignition timing by as much as

3 degrees if the engine idle speed falls below approximately 550 RPM. This

timing advance raises the idle RPM to an acceptable level (550 RPM). When

the idle stabilizer senses the idle RPM has reached the acceptable level, it

returns the timing to the normal idle timing.

NOTE: Retarding the timing with the spark arm is not an effective method of

checking idle stabilizer.

Check idle timing with engine in– gear, slight movements of timing indicates

idle stabilizer operation.

Notes

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Mercury 3.0 Liter Service Manual Page 36