Mercury MerCruiser MCM 454 Mag MPI Service Manual page 801

TROUBLE CODE DIAGNOSTICS - 7.4L MPI
Code 34 (2 of 3): Manifold Absolute Pressure (MAP) Sensor Circuit (Scan) -
Low Signal Voltage
TEST DESCRIPTION:
Step 2. This step will determine if there is an adequate vacuum supply to the MAP
sensor. If the vacuum gauge reading is erratic, refer to the "Rough or Unstable Idle" in
"Troubleshooting" section.
Step 3. This step determines if DTC 34 is the result of a hard failure or an intermittent
condition. A DTC will set when MAP signal voltage is too low with engine running.
Step 4. This step simulates a DTC 33. If the ECM recognizes the high signal voltage,
the ECM and wiring are OK.
Step 5. This step checks for the 5 volt reference on CKT 416E.
STEP
1
2
3
4
5
Index
Page 5G-56
Downloaded from https://needmanual.com/
ACTION
Was the "On-Board Diagnostic" (OBD) System
Check Performed?
1. Ignition "OFF."
2. Install a vacuum gauge.
3. Start engine and raise RPM to 1000.
4. The vacuum reading should be steady.
Is the vacuum reading steady and above
the specified value?
Allow the engine to idle.
Does the scan tool indicate MAP sensor
voltage less than the specified value?
1. Ignition "OFF."
2. Disconnect MAP sensor harness connec-
tor.
3. Connect a jumper wire from harness ter-
minal "B" (CKT 432) to harness terminal "C"
(CKT 416E).
3. Ignition "ON," engine "OFF."
Does the scan tool indicate MAP sensor
voltage greater than the specified value?
1. Ignition "OFF."
2. Connect DVOM from harness terminal
"C" (CKT 416E) to a known good ground.
3. Ignition "ON," engine "OFF."
Does the scan tool indicate MAP sensor
voltage greater than the specified value?
SERVICE MANUAL NUMBER 23
VALUE YES NO
PROCEED TO
Go to
OBD
– Step 2
System
Check
14 in.
HG
(45 5 (45.5
kPa) kPa)
Step 3 Step 6
1 Volt Step 4 Step 7
4 Volts Step 10 Step 5
4 Volts Step 9 Step 8
90-861326--1 MARCH 1999