Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
103
the scan tool could actually be a default value that the
ECM substitutes to compensate for possible circuit fail-
ures. Also, serial data transmitted by the ECM to the
scan tool is an interpretation of what the ECM thinks it
is seeing. The true readings may be different. You can
confirm actual signal values by testing the circuit live
with a DVOM, breakout box, or lab scope, depending on
what you need to test. False data stream values may be
caused by an internal ECM fault or a ECM ground cir-
cuit problem. The following are examples of using seri-
al data to test and diagnosis driveability and intermittent
problems:
• Thermistors: disconnect or short across thermistor
circuit to check the maximum range of the sensor.
For example, disconnect the ECT to create an open
circuit. Temperature reading should drop to - 40°F
(-40°C). Install a jumper wire across the connector
to create a short circuit. Temperature should go to a
maximum reading, about 266°F (130°C)
• Create the opposite circuit problem to see if a DTC
sets. For example, a P0117 code in memory tells
you an ECT sensor circuit voltage went low, indi-
cating a short. To create an open circuit, disconnect
the ECT sensor and see if the ECM sets a P0118
(circuit high). If it does, then the circuit and ECM is
operational and the problem is probably in the
sensor
• Intermittent problem testing: Wiggle, tap, heat up,
or cool down a component or circuit to see if the se-
rial data for that circuit changes
• Testing the effect one circuit on another by manip-
ulating the input signal. Manipulate the signal by
disconnecting circuits or substituting values. Here-
are some examples:
• IAT, ECT, TP sensor, MAP, MAF, and HO2S sig-
nals' effect on injector pulse width.
• ECT, ACT, TP sensor signals' effect on Idle
speed control.
• IAT, ECT, TP, MAP, and MAF signals' effect on
ignition timing control.
• ECT, TP sensor, and EVP signals' effect on EGR
control
• ECT and TP sensor signals' effect on canister
Purge
• VSS, TP sensor, ECT, and MAP signals' effect
on torque convert clutch operation.
Circuit Testing Operations
While scan tools are an important part of any diagnosis,
once you locate a problem you must use either a DVOM
or lab scope to accurately check a circuit. The following
section covers circuit testing procedures and guidelines
for using the proper test equipment.
Voltage
When using a DVOM to check voltage in and out of sen-
sors, always check the voltage using the signal ground
return at the sensor, rather than using an engine or bat-
tery ground, figure 5-14. Sensors are grounded directly
through the ECM, rather than being connected directly
to a chassis ground. This way sensors avoid noise in-
terference. Sensors need a "clean" ground for reliable
operation.
An open signal ground return will cause the ECM to see
a high voltage on the sensor signal line. An example
would be a TP sensor that always sends a wide open
throttle (high voltage) signal to the ECM.
Resistance
Ohms law says that even very low resistance in an au-
tomotive computer circuit will cause sensors and actu-
ators to work improperly because of low voltage. For ex-
ample, an on-board ECM ignition feed circuit drawing
365 "milli" amps with a resistance in the ignition feed
wire of 2.5 ohms, will cause a voltage supply drop of 1.5
volts. This voltage drop will cause severe driveability
M
POWER
CIRCUIT
RELAY
COIL
CONTROL
CIRCUIT
CONTROL SWITCH
(COMPUTER FUNCTION)
+
-
Fig. 5-20.
Applying power to the low current control circuit of a relay
closes the contacts to allow high current through the power circuit.
V Hz~
~
10A
RPM+
COM
A
C F
RPM
V
%
OFF
V Hz
GROUND RETURN
LAMP
Fig. 5-21.
Voltmeters are connected in parallel to the circuit being
tested.
Summary of Contents for ASE-A8
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