MerCruiser 305 CID (5.0L) Service Manual page 472

SERVICE MANUAL NUMBER 24
KNOCK SENSOR CIRCUITRY (MEFI-3 MODELS ONLY)
The MEFI-3 ECM is used with the knock sensor to control spark knock. The KS module cir-
cuitry is within the MEFI-3 ECM. When spark knock is present, a small AC voltage signal
is sent from the knock sensor to the ECM through pin connector J1-30. (If the engine has
a second KS, its voltage signal goes through pin connector J1-14). An AC voltage monitor
inside the ECM will detect the spark knock and start retarding spark timing. A Code will be
set only if the ECM does not see any activity on the KS signal circuit(s).
THROTTLE POSITION (TP) SENSOR
The Throttle Position (TP) Sensor is a potentiometer connected to the throttle shaft on the
throttle body. The TP has one end connected to 5 volts from the ECM and the other to ECM
ground. A third wire is connected to the ECM to measure the voltage from the TP. As the
throttle valve angle is changed, the voltage output of the TP also changes. At a closed
throttle position, the voltage output of the TP is low (approximately .5 volt). As the throttle
valve opens, the output increases so that at wide-open-throttle (W.O.T.), the output voltage
should be near 4.5 volts. By monitoring the output voltage from the TP, the ECM can deter-
mine fuel delivery based on throttle valve angle (driver demand). A broken or loose TP can
cause intermittent bursts of fuel from the injector and an unstable idle, because the ECM
thinks the throttle is moving.
If the TP circuit is open, the ECM will set a Code 21. If the TP circuit is shorted, and a trouble
Code 21 will be set. A problem in any of the TP circuits will set a Code 21. Once a trouble
code is set, the ECM will use a default value for TP.
DISTRIBUTOR REFERENCE (DIST REF)
The distributor reference (engine speed signal) is supplied to the ECM by way of the "Dist
Ref Hi" line from the High Energy Ignition (HEI). This pulse counter type input creates the
timing signal for the pulsing of the fuel injectors, as well as the Ignition Control (IC) functions.
This signal is used for a number of control and testing functions within the ECM.
Index
90-861327 OCTOBER 1999
FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
73049
Page 5D-19