Mercury MerCruiser MCM 454 Mag MPI, Service Manual

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GENERAL SYSTEM DIAGNOSTICS
SERVICE MANUAL NUMBER 23
90-861326--1 MARCH 1999 Page 5E-99
Discrete Input Circuit Check - Non-Scan 7.4L MPI Only (2 of 5)
NOTE: The ECM should only be replaced after all switches and circuits have been tested
and found to be functioning properly.
TESTING BUZZER
STEP ACTION VALUE YES NO
PROCEED TO
1 1. Key “ON”, Engine “OFF”.
Does buzzer sound? Step 2 Step 3
2 1. Start engine.
Does buzzer sound? Step 6 Step 4
3
1. Disconnect TAN/BLU wire at buzzer.
2. Key “ON”, Engine “OFF”.
3. Touch TAN/BLU wire to ground (–).
Does buzzer sound? Step 4 Step 5
4
Buzzer is working properly. Proceed to “Testing
Switches” or “Testing Circuits”.
– –
5
Check to ensure that there is battery power (+)
to the PUR wire going to buzzer. If there is,
replace buzzer and verify repair
(Starting at Step 1).
– –
6
Discrete switches may all be functioning prop-
erly.
Proceed to check all discrete circuits to verify
each works properly.
– –
TESTING OIL PRESSURE SWITCH
STEP ACTION VALUE YES NO
PROCEED TO
1
1. Disconnect PUR wire from oil pressure
switch.
2. Engine “OFF”.
3. Check for continuity between terminal on
switch and ground (–).
Is there continuity?
0 Ohms Step 2 Step 3
2
1. Start engine.
2. Check for continuity between terminal on
switch and ground (–).
Is there continuity? 0 Ohms Step 3 Step 4
3 Replace oil pressure switch. Verify repair. – –
4 Oil pressure switch is working properly. – –
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