SERVICE MANUAL NUMBER 23
TROUBLE CODE DIAGNOSTICS - 7.4L MPI
Code 34 (1 of 3): Manifold Absolute Pressure (MAP) Sensor Circuit (Scan) -
Low Signal Voltage
c
a
d
814
BLK
e
b
a - Manifold Absolute Pressure (Vacuum)
b - To ECT Sensor
c - To TP Sensor
d - Map Signal
e - Map And ECT Ground
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.
After repairs clear DTC’s following “Clearing Trouble Codes” procedure at the front of this
section. Failure to do so may reset in DTC’s not properly being cleared.
90-861326--1
MARCH 1999
Page 5G-55
| Categories | Mercury MerCruiser Manuals |
|---|---|
| Tags | Mercury MerCruiser 454 CID, Mercury MerCruiser 502 CID |
| Model Year | 1998, 1999, 2000, 2001 |
| Download File |
|
| Document File Type | |
| Copyright | Attribution Non-commercial |
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