Mercury MerCruiser MCM 454 Mag MPI Service Manual page 768

SERVICE MANUAL NUMBER 23
Code 34 (2 of 3): Manifold Absolute Pressure (MAP) Sensor Circuit (Non-Scan)
Low Signal Voltage
After repairs, clear DTC's following "Clearing Trouble Codes" procedure at the front of this
section. Failure to do so may result in DTC's not properly being cleared.
TEST DESCRIPTION:
STEP 2. This step simulates a DTC 33. If the ECM recognizes the high signal voltage and
sets a DTC 33, the ECM and wiring are OK.
STEP 3. This step checks CKT 416E for the 5 volt reference.
Index
90-861326--1 MARCH 1999
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If the idle is rough or unstable, refer to "Troubleshooting" section for items which may
cause an unstable idle.
With the ignition "ON," engine "OFF," the manifold pressure is equal to atmospheric
pressure and the signal voltage will be high. This information is used by the ECM as an
indication of altitude and is referred to as BARO.
STEP
Was the "On-Board Diagnostic" (OBD) System
1
Check Performed?
1. Marine Diagnostic Code Tool switched to
2
"Normal Mode".
2. Ignition "OFF".
3. Disconnect MAP sensor harness connec-
tor.
4. Connect a jumper wire from terminal "B"
(CKT 432) to harness terminal "C" (CKT
416E).
5. Start engine and idle for 2 minutes or Ma-
rine Diagnostic Code Tool indicates a stored
trouble code.
6. Ignition "ON", engine "OFF".
7. Switch Marine Diagnostic Code Tool to
"Service Mode" and watch for a trouble code.
Is trouble code 33 present?
1. Remove the jumper wire from CKT416E
3
and 432.
2. Connect DVOM from harness terminal
"A" (CKT 814) to harness terminal "C" (CKT
416E).
Is voltage reading above the specified val-
ue?
ACTION
TROUBLE CODE DIAGNOSTICS - 7.4L MPI
VALUE YES
PROCEED TO
– Step 2
– Step 4
4 Volts Step 5
Page 5G-23
NO
Go to
OBD
System
Check
Step 3
Step 6