Tech Stuff

PCM module diagnostics: How can I know if it’s really the module?

Figure 1. Early Jeep Libertys can burn the PCM as a result of simply filling the gas tank due to possible static. A ground strap needs to be added to the fill neck to prevent static electricity.
<p>Figure 1. Early Jeep Libertys can burn the PCM as a result of simply filling the gas tank due to possible static. A ground strap needs to be added to the fill neck to prevent static electricity.</p>

Vehicle diagnosed by Kevin Quinlan.

How long do most diagnostics take you? Maybe an hour? A diagnostic procedure that can be completed that quickly is usually pretty cut and dried, such as a bad sensor or vacuum leak. When diagnostics start taking longer and the problem seems to continue after we change common failure components, we start suspecting a bad module.

When a module arouses suspicion, you start to doubt yourself. Is it really the module or did you miss something? Modules are expensive, they cannot be returned, and on the late model vehicles they cannot be swapped without being reprogrammed or sometimes not swapped at all!

The stakes are very high when diagnosing a module, but there is a way you can diagnose a module with 100% confidence. There are not that many steps, because diagnosing a module employs simple logic.

Here are the steps to diagnose any module when you have a DTC:

1. Check TSBs to see if there are any software updates, extended warranties for defective modules, or other good diagnostic information that indicates you need to replace a module. (DO NOT SKIP THIS STEP! Follow Step 1 first before wasting your time taking needless measurements or replacing any components.)

2. Diagnose and repair any bad sensors or actuators that may be responsible for the DTC. (This is very time consuming, but necessary.)

3. When all measurements and components are good, look for illogical PID data or vehicle conditions given the sensor feedback.

You don’t need to be Socrates or Spock to understand logic. Logic is simple and absolutely necessary when diagnosing modules. Why? Because when the car does things that are the opposite of what you expect, it is only natural to become overwhelmed and want to start guessing. Don’t start guessing! Let yourself be informed by your diagnostics.

On a vehicle, we must make our decisions based upon solid evidence that tests on a vehicle will provide us. We’re going to cover our three step module diagnostic process using a case study so we can learn how to be confident of how to diagnose modules.

2002 Jeep Liberty 3.7L with wrong fuel level reading

We had a 2002 Jeep Liberty in our shop and the problem with the vehicle was that its fuel gauge went onto the empty indicator remarkably soon (see Figure 1).

Also, the instrument cluster displays for the vehicle’s four-wheel-drive system worked backwards. For example, two-wheel-drive was engaged when 4-HI was illuminated and vice versa.

Tags: Jeep  Jeep Liberty 
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