Diagnosis - GMC G Service Manual

6E2-8 DIESEL EMISSIONS
Engine Speed Sensor
The Engine Speed Sensor is a camshaft driven pickup,
mounted at the center rear of the engine.
It is sourced by 5 volt reference and allows the ECM to
measure engine rpm by the number of times the voltage is
pulsed. The Engine Speed Sensor pulses 4 times per
revolution.
Vehicle Speed Sensor
The Vehicle Speed Sensor (VSS) is installed on the
transmission. The ECM will calculate vehicle speed based
on a series of pulses provided by the Vehicle Speed Sen­
sor. This calculation is used to apply the Transmission Con­
verter Clutch.

DIAGNOSIS

GENERAL INFORMATION
The Diesel Electronic Control system has a diagnostic
system built into the ECM to indicate a failed circuit. An
amber "Service Engine Soon" lamp, on the instrument
panel, will illuminate if a problem has been detected when
the engine and vehicle are running. This lamp is also used
for a bulb and system check.
The system requires a tachometer, test light, ohmmeter,
digital voltmeter with 10 megohms impedance (J 29125-A),
vacuum gage, and jumper wires for diagnosis.
The diagnosis of the Diesel Electronic Control System
should always start with the Diesel Diagnostic Circuit
Check. This will determine if the DEC System and ECM are
working properly.
One of the most important checks that must be done,
before any diagnostic procedure, is a careful visual under­
hood inspection. This can often lead to fixing a problem
without further steps. Inspect all vacuum hoses for correct
routing, pinches, cuts, or disconnects. Be sure to inspect
hoses that are difficult to see beneath the air cleaner, com­
pressor, generator, etc. Inspect all the wires in the engine
compartment for correct and good connections, burned, or
chaffed spots, pinched wires, or contact with sharp edges
or hot exhaust manifolds. This visual inspection is very
important. It must be done carefully and thoroughly.
Before diagnosing the DEC system, there should be the
following basic knowledge:
Basic Electrical Circuits
You should understand the basic theory of electricity,
and know the meaning of voltage, amps, and ohms. You
should understand what happens in a circuit with an open
or a shorted wire. You should be able to read and under­
stand a wiring diagram.
Use of Circuit Testing Tools
You should know how to use a test light, how to connect
and use a tachometer, and how to use jumper wires to
bypass components to test circuits.
Use of Digital Volt-Ohms Meter (DVM)
You should be familiar with a Digital Volt-Ohm Meter.
You should be able to measure voltage, resistance, and
current. You should be familiar with the controls of the
meter and how to use it correctly.
"Scan" Tool
The tool manufacturers have developed "Scan" tools to
interface with the Diesel Electronic Control System, through
the ALDL. It supplies a visual reading of most inputs to the
ECM and some outputs. Review the tool instruction manual
to understand its operation and limitations.
When the tool is connected to the ALDL and
cigarette/cigar lighter connector to 12 volts, there should be
a visual instruction displayed. If there is no display, or tool
reads "No DATA" or "No ALDL", refer to "ECM Check" diag­
nosis and tool instructions to be sure that 6.2L diesel
engine is available in the tool.
With the tool in the code position, the display window will
indicate any code stored in the ECM memory. Referring to
the applicable code chart, the tool will "Scan" an input to
determine if a specific circuit is operating properly.
Diagnostic Mode
With the key "ON" and engine "OFF", jumper ALDL ter­
minal "B" (Diagnostic Terminal) to "A" (ground). The Diesel
Electronic Control system will enter the Diagnostic Mode
Code system (Figure 16). In this mode, the ECM will display
a Code 12, by flashing the "Service Engine Soon" lamp
(indicating the system is operating). A Code 12 consists of
one flash, followed by a short pause, then two flashes in
quick succession. This code will be flashed three times. If
no other codes are stored, Code 12 will continue to flash
until the diagnostic terminals are ungrounded.
If Code 12 does not display, refer to "ECM Check" chart.
A "hard" code is one which is present when you are working
on the vehicle and the condition still exists while working on
the vehicle. The chart with the stored code number will lead
you to the cause of the problem.
An "intermittent" code is one which does not reset itself,
and is not present while you are working on the vehicle.
This is often caused by a loose connection.
Clearing Codes
When the ECM sets a code, the "Service Engine Soon"
lamp will come "ON" and a code will be stored in memory. If