Mercury MerCruiser MCM 454 Mag MPI Service Manual page 709

TROUBLE CODE DIAGNOSTICS - 454/502 MAG MPI & 8.2L MPI
Code 33 (2 of 3): Manifold Absolute Pressure (MAP) Sensor Circuit (Non-Scan)
TEST DESCRIPTION:
Number(s) below refer to step number(s) on the diagnostic table.
Step 2. This step will determine if there is an adequate vacuum supply to the MAP sensor.
If the gauge reading is erratic, refer to "Stalling, Rough, Unstable or Incorrect Idle" in "Trou-
bleshooting."
Step 3. This step checks for a voltage from terminal "C" (5 volt reference) to terminal "A"
(sensor ground).
Step 4. This step determines if the MAP signal circuit to the ECM is open.
Step 5. This step completes the test for the ECM and wiring. If the test light is not ON, the
MAP sensor has an internal problem. To confirm an internal MAP sensor problem, use the
MAP output voltage check chart.
Step 7. This step will identify if the problem is in the supply 5 Volt reference or ground circuit.
Step 16. Low manifold vacuum may result from vacuum leaks in the engine induction sys-
tem.
NOTE: Using a test light with 100 mA or less rating may show a faint glow when test actually
states no light.
STEP
1
2
3
4
Index
Page 5F-12
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ACTION
Was the "On-Board Diagnostic" (OBD) System
Check Performed?
1. Ignition OFF.
2. Disconnect vacuum plug located under
plenum at the front and install vacuum gauge
in the vacuum port.
3. Start engine and raise rpm to approxi-
mate 1000 rpm.
Vacuum gauge reading should be 14 In. Hg
(45.5 kPa) or more and steady, is it?
1. Ignition OFF.
2. Remove vacuum gauge and reinstall vac-
uum plug in vacuum port.
3. Disconnect MAP sensor electrical
connector.
4. Ignition ON.
5. Connect a DVOM from the harness ter-
minal "C" (CKT 416, 5 volt volt reference) to
harness terminal "A" (CKT 814, sensor
ground).
Is voltage reading over 4 volts?
Connect DVOM from harness terminal "C"
(CKT 416) to harness terminal "B" (MAP sen-
sor signal CKT 432).
SERVICE MANUAL NUMBER 23
VALUE YES NO
PROCEED TO
Go to
OBD
– Step 2
System
Check
– Step 3 Step 17
– Step 4 Step 7
90-861326--1 MARCH 1999