Mercury MerCruiser MCM 454 Mag MPI Service Manual page 765

TROUBLE CODE DIAGNOSTICS - 7.4L MPI
Code 33 (2 of 3): Manifold Absolute Pressure (MAP) Sensor Circuit (Non-Scan)
High Signal Voltage
TEST DESCRIPTION:
STEP 2. This step will determine if there is an adequate vacuum supply to the MAP sen-
sor. If the vacuum gauge reading is erratic, refer to the "Rough or Unstable Idle" symp-
tom.
STEP 3. This step simulates a DTC 34. If the ECM recognizes the low signal voltage
and sets a DTC 34, the ECM and wiring are OK.
STEP 4. This step checks to see if CKT 814 is open.
STEP 5. Low manifold vacuum may result from a restriction in the MAP sensor hose or
from vacuum leaks in the engine induction system.
STEP
1
2
3
4
Index
Page 5G-20
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ACTION
Was the "On-Board Diagnostic" (OBD) System
Check Performed?
1. Ignition "OFF."
2. Disconnect vacuum at MAP sensor and
install a vacuum gauge.
3. Start engine and increase RPM to 1000 in
neutral.
4. The vacuum reading should be steady.
Is the vacuum reading steady and above
the specified value?
1. Marine Diagnostic Code Tool switched to the
"Normal Mode."
2. Ignition "OFF".
3. Disconnect MAP sensor harness connector.
4. Start engine and idle for 2 minutes or until
Marine Diagnostic Code Tool indicates a
stored trouble code.
5. Ignition "ON," engine "OFF".
6. Switch Marine Diagnostic Code Tool to the
"Service Mode" and note trouble code.
Is trouble code 34 present?
1. MAP sensor harness connector discon-
nected.
2. Ignition "ON" engine "OFF."
3. Connect DVOM from harness terminal "A"
(CKT 814) to harness terminal "C" (CKT
416E).
Is voltage reading above the specified val-
ue?
SERVICE MANUAL NUMBER 23
VALUE YES NO
PROCEED TO
Go to
OBD
– Step 2
System
Check
14 In.
Step 3 Step 5
Hg (45.5
kPa)
– Step 6 Step 4
4 Volts Step 7 Step 8
90-861326--1 MARCH 1999