Making The Connection

   The Data Link Connectors on General Motors vehicles are typically located under the dash in the area of the steering column. On Ford Motor vehicles the Data Link Connector is typically located at the RH rear of the engine compartment, mounted on the rear of strut tower. In Chrysler products the Data Link Connector can be found located on the left side of the engine compartment, near the battery. Or under the dash above the hood release cable or is located in the engine compartment next to the Powertrain Control Module (PCM). You will need to check your service manual for the exact location for your particular Chrysler model.

   After connecting the scan tool, powering up and selecting your vehicle you start with a basic scan. This will bring up any Diagnostic Trouble Codes (DTC) that are active or stored including those that do not turn the Check Engine Light, now referred to as the Malfunction Indicator Lamp MIL. If a malfunction occurs that turns the MIL on but corrects itself, the MIL will turn off but the code will remain stored in memory for 50 key starts and then, if it does not reoccur, it will be erased. Checking for trouble codes is a good start, but most malfunctions do not set a code so you will need to do a little digging to find the problem.

   If you have a computer and a scan tool with the ability to store and then transfer the data to a computer, you will have an added advantage. Scan your vehicle when it is in good condition and running perfectly and you can store that data in your computer. You now have a baseline reading that you can use to compare to a later scan when you are having a problem. By checking the “bad” scan with the “good” scan, you will be able to see which system or sensors are “off” and quickly isolate a problem. I have a database of all my regular customers so when they come in with a problem, I know what is “normal” for that particular vehicle.

   The hardest problems to locate are intermittent. They can come and go so quickly that the computer doesn’t set a DTC. In this case you can do what I do, put the scan tool in Freeze Frame Mode and drive the vehicle. For this you will need an assistant so you can focus on operating the vehicle safely and the assistant can watch the scan tool and activate the freeze frame. When the problem manifests itself, have your assistant press the freeze frame button. This will capture readings from 15 seconds before activation to 15 seconds after. This will allow you to see which sensor is causing the problem.

   If you know your vehicle's normal operating condition you can determine if there are deviations, however briefly they occur, that are causing the problem. When you can do this, you will be able to find a bad connection and pinpoint a weak sensor. Now when you replace a part, you will know for sure it was bad.

Using Scan Data For Diagnosis

   You will have to learn how to use scan data for diagnosis, and as you use the scan tool you will learn more about reading this data. For instance, a Throttle Position Sensor (TPS) is measured in volts and usually starts at less than one volt, almost at zero, and goes to almost 5 volts at full throttle. As you step on the gas pedal, the reading should show a gradual increase. Some scan tools will show this as an angle. So if the throttle is ½ open, it will read as 50%.

   Another example is a Coolant Temperature Sensor (CTS). A CTS is measured in ohms but in a data stream as an actual temperature reading in Fahrenheit or Celsius depending on the scan tools setup.

   Here are just some of the things you can check with your scan tool: