High Signal Voltage - Mercury MerCruiser MCM 454 Mag MPI Service Manual

TROUBLE CODE DIAGNOSTICS - 7.4L MPI
Code 33 (1 of 3): Manifold Absolute Pressure (MAP) Sensor Circuit (Scan)

High Signal Voltage

CIRCUIT DESCRIPTION:
The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure
(vacuum). The ECM receives this information as a signal voltage that will vary from about
1.0-1.5 volts at idle to about 4.0-4.5 volts at Wide Open Throttle (WOT).
If the MAP sensor fails, the ECM will substitute a default MAP value that will vary with RPM.
The MAP sensor voltage of 5 volts is delivered to the MAP sensor through CKT 416E. CKT
814 is the ground circuit for the MAP sensor. The MAP signal CKT 432 will send a voltage
signal back to the ECM according to what the manifold pressure is.
DIAGNOSTIC AIDS:
Check for the following conditions:
Poor connection at ECM. Inspect harness connectors for backed out terminals,
improper mating, broken locks, improperly formed or damaged terminals, and poor ter-
minal to wire connection.
Damaged harness. Inspect the wiring harness for damage. If the harness appears to be
OK, observe the MAP sensor display on the scan tool while moving connectors and wir-
ing harnesses related to the MAP sensor. A change in the MAP sensor display will indi-
cate the location of the fault.
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. Comparison of this BARO reading, with
a known good MAP sensor, is a good way to check the accuracy of a "suspect" sensor.
Reading should be the same, plus or minus 0.4 volt.
If DTC 14 is also set, check for open in ground CKT 814.
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.
Index
Page 5G-52
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a
a - Manifold Absolute Pressure (Vacuum)
b - To ECT Sensor
c - To TP sensor
d - Map Signal
e - Map And ECT Ground
c
b
SERVICE MANUAL NUMBER 23
d
814 BLK
e
90-861326--1 MARCH 1999