GMC G Service Manual page 1027

ALDL CONNECTOR (Figure 12)
The twelve terminal connector Assembly Line Diagnos­
tic Link (ALDL) is wired to the ECM, and is located under
the instrument panel.
This connector has terminals that are used to diagnose
the system. The following terminals are used:
A. This terminal provides a ground circuit to other
terminals.
B. This terminal is the "diagnostic terminal" for the ECM.
When grounded to "A" terminal, the "Service Engine Soon"
lamp will flash codes.
E. This terminal is the serial data line for LH6 and is used
by a "Scan" tool to read various system data information.
T E R M IN A L IDENTIFICATION
[A] GROUND
[B] DIAGNOSTIC T E R M IN A L
[U SERIAL DATA
[f] T.C.C.
Figure 12 — ALDL Connector
"SERVICE ENGINE SOON" LAMP
The ECM performs the diagnostic function of the EGR
and TCC Systems. It can recognize operational problems,
alert the driver through the "Service Engine Soon" lamp on
the instrument panel. The ECM stores a code, which will
identify the problem area to aid the technician in making a
repair. Refer to the Diagnostic section for more information
on how the "Service Engine Soon" lamp is used to identify a
code.
WIRE HARNESS AND CONNECTORS
A wiring harness electrically connects the ECM to var­
ious sensor, solenoid, and relays within the system. Many
connectors in the engine compartment are environmentally
protected, because of the system's low voltages and cur­
rent levels.
INPUT INFORMATION
Coolant Sensor (Figure 13)
The Coolant Sensor is a thermistor (a resistor, which
changes value based on temperature) mounted in the
engine coolant stream. Low coolant temperature produces
a high resistance (100,000 ohms at -4 0 degrees C/-40
degrees F), while high temperature causes low resistance
(70 ohms at 130 degrees C/266 degrees F).
The ECM supplies a 5 volt signal to the Coolant Sensor,
through a resistor in the ECM and measures the voltage.
The voltage will be high, when the engine is cold, and low
when the engine is hot. By measuring the voltage, the ECM
DIESEL EMISSIONS 6E2-7
knows the engine coolant temperature. Engine coolant
temperature affects the Cold Advance and Glow Plug
System.
MAP Sensor (Figure 14)
A Manifold Absolute Pressure (MAP) Sensor, mounted
on the left side of the cowl, is used to monitor the amount of
vacuum in the EGR vacuum line and sends a signal back to
the ECM. The signal is compared to the EGR duty cycle
calculated by the ECM. If there is a difference in the ECM
L00717
command and what is at the EGR valve sensed by the
MAP, the ECM makes minor adjustments to correct. When
a major difference is sensed, the ECM recognizes a fault
and sends a full EGR signal.
Throttle Position Sensor (Figure 15)
The Throttle Position Sensor (TPS), mounted on the
injection pump, is a variable resistor that signals the ECM
the degree of throttle opening. The sensor is connected to a
5 volt reference, and has the highest resistance at closed
throttle. At wide open throttle, the resistance is lowest and
output to the ECM will be near 5 volts.
Figure 13 — Coolant Sensor