Think Like the PCM - Use your best diagnostic tool to repair OBD II powertrain diagnostic trouble codes — your head. - Search Auto Parts | Automotive News
The individual tests that make up each monitor can be broken down into three basic types. The PCM can test a component's electrical
circuit for opens and for shorts to power or ground. It can also test input devices by comparing values to another component
to see if the information from the first makes any sense.
These are "rationality" tests. An example would be the comparison of throttle position sensor (TPS) values to mass airflow
(MAF) sensor values to check the TPS.
Finally, it tests the function of output devices and whole systems by operating certain components and looking for the desired
result. For instance, the PCM on a Chrysler minivan will command the EGR off when normally on and look for the difference
in short-term fuel trim to see if it indeed is flowing properly when open.
Here is where some knowledge and understanding of Mode $06 comes into play. The tests and their results are reported in this
mode, and can help in diagnostics by telling you what test(s) failed and by how much.
WHAT'S THE FIRST STEP?
Figure 3
After verifying the complaint, it's time to hook up the scan tool to the PCM. Because many of us have to rely on aftermarket
scan tools, we're going to do the same to show that there are very few powertrain DTCs you can't fix with these tools. To
get the most out of your aftermarket scanner, though, make sure it's updated to access Global OBD II modes. The rest of this
article is going to be based on using this function to access the information we want to look at.
Before you even connect, look to see if the MIL is on. How many times have you had the complaint of a MIL on, only to see
the light is off when you get it in your bay? Remember, think like the PCM.
If the monitor that is responsible for that DTC runs three times in a row without seeing the same failure, the PCM thinks
the problem is gone and will turn off the MIL. The offending DTC, however, will remain in memory for a while longer, until
a specified number of warm-up cycles has been completed. A warm-up cycle is generally the time it takes to start the car,
run it long enough for engine temperature to increase to a certain level and then be shut off. In addition, the PCM will confirm
misfire and fuel trim-related codes only under the same conditions as when the fault first triggered a DTC. If the misfire
originally occurred under wide open throttle (WOT) at 65 mph, you can't verify your fix at idle in the bay.
The first step after connecting to the vehicle and establishing communication is to check the status of the monitors. Some
scan tools will report this information after connection, on others you will need to access Mode $01. What we want to know
is if all the monitors have run to completion. Here is where some more understanding of how the PCM thinks is in order.
Figure 4 The PCM is even telling us when it saw the failure.
Monitor status terms vary a bit from tool to tool, but what we are looking for is a "complete" or "ready" indication on all
of them. Monitors listed as "NA" or "not available" are not in use by the vehicle you are testing and can be ignored. Monitors
listed as "not ready" or "incomplete" can indicate a few things.
Peter Meier is an ASE-certified Master Technician, member of iATN and full-time tech in Tampa, Fla. His experience reaches back over 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model.
Articles by Peter F. Meier
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