Not that I recommend installing used PCMs, but I
occasionally need a rebuildable core to replace a missing or badly damaged
powertrain control module (PCM). But lately, I’ve noticed that it’s becoming
increasingly difficult to find a used PCM for an older import at a local auto
salvage yard. While it’s sheer speculation on my part, this apparent shortage
of PCM cores might be caused by too many poorly trained technicians replacing
PCMs to solve sensor-based problems. In most of these cases, the problem
remains unsolved even as the original and fully functional PCM is casually
tossed into the trash when the tech moves on to another job.
Photo 1: PCM connectors can be sensitive to moisture intrusion. If in doubt, remove the connector and brush on a drop of a contact-enhancing agent like Stabilant 22.
BASIC PCM AND SENSOR WIRING
Understanding PCM wiring circuits is essential to
understanding the relationship between a PCM and its sensors. For example, the
keep-alive memory in a PCM requires constant battery voltage (B+) to prevent
stored diagnostic trouble codes (DTCs) and adaptive data associated with fuel
trim values and transmission shift points from being lost.
The PCM also requires key-on voltage to activate the
computer’s sensor and actuator circuits. To ensure a complete electrical
circuit, most PCMs attach redundant ground wires to the engine block.
Practically all PCMs power the various sensors with a 5-volt reference voltage and some might also supply an
8-volt reference to one or more sensors.
WHY PCMS FAIL
While the modern PCM is a very reliable component, water
intrusion from a leaking heater core or windshield seal can corrode PCM
connectors or short-circuit various internal PCM components. Older PCMs can
also develop a temperature-sensitive crack in their circuit boards. In other
cases, the PCM can be ruined by an over-voltage condition caused by faulty
alternator voltage regulation or over-boosting the battery with a battery
charger. Although most modern PCMs have built-in voltage spike protection,
voltage spiking caused by something as simple as arc welding on the chassis can
degrade or ruin various PCM components. See Photo 1.
In more normal circumstances, PCM actuator drivers are
usually the most common failure points in modern PCMs. While some PCMs
automatically disable actuator drivers when their amperage draw exceeds a
critical level, others don’t have that protection. If, for example, an ignition
coil driver were burned out on a PCM, it would be a good preventive maintenance
practice to also replace the ignition coil assembly. PCMs with more
sophisticated on-board diagnostics can test actuator circuits and store
actuator trouble codes in the same manner as with sensors.
Photo 2: Erratic sensor performance on this vehicle was eventually traced to a bad battery. The B1318 DTC confirmed the diagnosis.
BASIC PCM DIAGNOSIS
Before diagnosing any PCM or sensor-related problem, it’s
absolutely essential to test the battery for an adequate state-of-charge (SOC)
and for clean terminal connections. In many cases, erratic cold driveability
symptoms and other symptoms related to sensor performance can be traced to low
SOC or a bad battery cell. See Photo 2.
The simplest method for testing communications with any OBD
II PCM is to connect a “generic” or “global” scan tool or code reader. If the
scan tool won’t communicate, check the PCM fuses with a digital voltmeter. If
the fuses display battery voltage at both pins, use a wiring schematic to
locate and test the PCM’s key-on and B+ terminals for battery voltage.
Remember also that high internal resistance in a PCM relay can reduce B+
voltage at the PCM’s key-on terminal, which can cause unusual performance
problems and intermittent cranking, no-start complaints that resemble a bad
crankshaft position sensor (CKP). To ensure circuit integrity, always clean
and tighten the PCM ground connections.
When diagnosing any sensor-related complaint, the PCM ground
wires should display less than 0.050 volts in the key-on, engine off or key-on,
engine running position. While a few specific vehicle applications might
display as much as 0.080 volts, most ground terminals will display close to
zero voltage. If voltage is excessive, clean and reinstall the terminals.
In some cases, a sensor shorting to ground may cause a loss
of the five- or eight-volt reference voltages. If the vehicle won’t start or
the PCM won’t communicate with a scan tool, the most convenient place to begin
testing for reference voltage is at the throttle position sensor.
Photo 3: Because the TP sensor incorporates a 5-volt reference, signal return and ground wire, it can be used for verifying PCM reference voltage and ground circuit condition.
In most throttle sensor configurations, the reference
voltage should be five volts and the input to the PCM should be under one volt
at closed throttle. If no reference voltage is present, begin disconnecting
sensors, including the vehicle speed sensor, to determine which one is shorting
the reference voltage to ground. See Photo 3.
When the ignition is first turned on, the PCM checks for
sensor circuit failures by looking for electrical inputs. If the PCM detects a
sensor with an open, grounded or shorted circuit, it usually stores a DTC and
illuminates the “check engine” (CE) warning light.
As the engine is cranked, the PCM looks for a signal from
the CKP and camshaft (CMP) position sensors. If the PCM doesn’t detect a signal
from the CKP, it will not activate the drivers that, in turn, actuate the fuel
pump, fuel injection and ignition systems. As the engine starts, the PCM scans
the incoming data for increasing engine speed from the CKP and CMP sensors.
Keep in mind that the CKP identifies top-dead-center (TDC)
on all cylinders while the CMP identifies the compression stroke on number-one
cylinder. If the CMP signal is missing or if the cranking rpm doesn’t
immediately increase, some PCMs are programmed to use a default strategy that
locates the compression stroke on number-one cylinder by changing the ignition
timing 180 degrees. If a default strategy isn’t programmed into the PCM, a
missing CMP signal might not allow the engine to start.
Photo 4: If the coolant and intake temperature sensors are correctly calibrated, they should display approximately the same voltage after a cold soak.
The throttle position, coolant temperature and air flow
sensors relay input data to the PCM. The PCM uses that data to calculate
actuator outputs like spark advance, fuel injector pulse width and idle speed.
Once the engine starts, the PCM looks for changes in engine speed, coolant temperature,
throttle position and oxygen sensor activity on your scan tool’s data stream.
See Photo 4.
If, for example, the oxygen sensor voltage remains at 0.458
volts, you might be looking at a default rather than a real-time voltage value
simply because the oxygen sensor has been left disconnected. If the coolant
temperature doesn’t reach the specified value, check the thermostat housing
outlet temperature with a non-contact pyrometer. A significant difference
between data stream and real-time temperatures might indicate that the coolant
temperature sensor is mis-calibrated.
The PCM also scans the incoming data for an increase in
airflow through the mass airflow (MAF) sensor. Keep in mind that older imports
equipped with mechanical airflow sensors used a minimum airflow value to
activate the fuel pump relay. A leaking or missing duct between the MAF sensor
and engine can cause a cranking, no-start condition in any engine because the
PCM can’t sense the presence of incoming air.
As the vehicle is driven, the PCM processes data inputs from
all of the engine’s sensors to adjust actuator outputs like air/fuel mixture
ratio, spark advance and idle speed. At that point, the oxygen or air/fuel
ratio (AFR) sensors are used to trim air/fuel ratios to the desired values.
THE EFFECTS OF SENSOR FAILURES ON THE PCM
The old “garbage-in, garbage-out” theory of computer
operation very well describes the importance of accurate sensor inputs.
Sensors usually fail because of circuit problems, calibration errors and data errors.
To illustrate, a circuit failure like a broken ground wire
at the throttle position sensor might cause the sensor to indicate a wide-open
throttle condition to the PCM, which could result in a false clear-flood mode
situation that results in a cranking, no-start complaint. A voltage-biased
oxygen sensor might cause a calibration error that drives the air/fuel ratios
lean or rich. A data error could be caused by a MAF sensor duct leaking
un-metered or “false” air into the engine’s air intake. A data error like this
might cause the MAF to under-report the amount of air flowing into the engine,
which will cause the engine to run too lean and result in a P0171 DTC to be
stored in the PCM’s diagnostic memory.
Oxygen Sensor R&R Tips And Safety Precautions
The oxygen sensor (O2) operates in an extremely hostile
environment, where age, contamination and extreme heat can affect oxygen sensor
response characteristics. Of the many possible HO2s DTCs, “Slow Response” is
the most frequent code developed. Degradation of the signal can be in the form
of an extended response time or a shift in the sensor voltage curve. Both
conditions reduce the oxygen window, thereby reducing the catalyst capacity for
exhaust gas conversion.
Normally, the O2 sensor is supplied with anti-seize
compound on the threads so it can be more easily removed at the specified
change interval. Over time, the anti-seize compound loses its effectiveness and
the sensor can become “welded” into its location, making it nearly impossible
to remove using normal tools. Using excessive force to remove the oxygen
sensor may damage the sensor and surrounding components.
If the sensor becomes seized in its mounting location, a
simple 15-minute replacement job can become a much more complex and difficult
task. Replacing the O2 sensor within the specified change interval will
minimize the possibility of this problem and additional component damage.
When replacing an oxygen sensor, keep these tips in mind:
1. The primary sensor is on the manifold or the exhaust
pipe; late-model vehicles also have sensors farther downstream. Be sure you
know which one needs to be replaced.
2. Unplug the wire connection, then spray a penetrating
lubricant onto the threaded connection.
3. Use an appropriate oxygen sensor tool.
4. Most oxygen sensors come with a special electrically
conductive anti-seize compound applied to the threads, so it’s merely a matter
of threading the new sensor into the void left by the old one.
5. Use anti-seize compound to coat the sensor’s threads
(some oxygen sensors have the anti-seize compound applied at the factory).
6. Always check the appropriate reference material for the
required torque specification. When there’s no torque value given for
tightening the new oxygen sensor, treat it much like you would a spark plug. In
other words, less is probably better here. Once it’s snug, plug the connector
into your car’s factory wiring to finish the task.
Courtesy of Delphi Product & Service Solutions
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