Advanced Diagnostic's

Circuit-System Testing

There will be 3 basic types of electrical wiring/component faults.
For engine performance faults, you will always have X amount of probable causes. It will be to your advantage to not always use the old age adage that it's usually this that is the problem. It is true that certain vehicles will eventually produce pattern failures, known parts or systems that go bad.

Types of Electrical Problems
• High Resistance Faults
• Low Resistance Faults
• Component Faults
• Intermittent circuit/component failures

Circuit Failure Testing ( Consumer/Function operates intermittently).

If you have a failure that is not always present, intermittent failures can and will be the most difficult to diagnose. If the system is electronically controlled, and its control module is capable of storing a fault code, try to retrieve the fault code with a scan tool. Fault codes will always be a guide to diagnosis and will give you the most probable causes of the fault. There usually is a flow chart tree that will guide you in your diagnosis. There will be times when it is also useful for you to deviate from the test plan and create your own test plan. Think outside of the box.
Remember to that it is vital to gather the following information about any intermittent problem.
• When does the function fail?
• Are any other functions affected?
• Were any other functions in operation at the time of failure?
• Is the failure related to a vibration or bump occurrence?
• Does the failure occur at any specific temperature, time of the day, engine or transmission operating condition?
• Try to recreate the failure by operating the vehicle under the conditions observed at time of problem or described by client.
• If the failure can be replicated, proceed to with general diagnostic tests for the symptoms you observe or feel.
• Warning
• When performing a wire or circuit "wiggle test" make sure that the circuit being tested is not related to the airbag circuits.


Basic Electrical Faults

Basic electrical circuit faults can be categorized as follows:

Open Circuit

An open circuit is a break in the path of current flow. A circuit’s path must be complete from start to end, only then will current flow. An open circuit can be caused by a bad electrical connector or electrical pins with bad crimps, cracked wiring at the crimps, and broken wire strands and faulted components as well. It is essential to understand that if the circuit is powered, it will have voltage potential that will be present in the portion of the circuit that is still connected to the power source. It's also important to know that a bad ground of a circuit or component or connection will increase resistance.

With parallel circuits, an open circuit in one branch of the circuit will stop operation in that branch, but the other branches will continue to operate. An Ohmmeter test can determine if a circuit is open (infinite resistance). Remember that when you are testing for resistance in a circuit to test the integrity of the wiring, you must disconnect the power source and the other end of wire that goes to the component. It all depends on the circuit. As I describe the engine management’s sensor and actuators later in the book, you will see how the wiring tests can be performed.

A voltmeter from a Dvom can also be used to determine an open circuit by measuring the available voltage at various points of the circuit or the voltage drop between two points in the circuit. This procedure also helps to find and determine the location of the open circuit.

High Resistance

A high resistance circuit is a circuit with more resistance than specified. Any circuit resistance should typically in the components that perform work. High resistance will reduce the amount power, current and voltage that should be available to the components connected to the circuit.

High resistance in a circuit can be caused by loose, dirty or corroded connections or internal component failures. Broken strands of wire within the insulation will also increase the circuit’s resistance. When diagnosing a circuit for high resistance, try not to disturb the connections until you have narrowed down the possible area of high resistance. The reason behind this is that you may unintentially clean off corrosion in a connector. This might temporarily correct the fault and make your diagnosis more difficult to isolate.

You can use an ohmmeter to test an unpowered circuit; this will allow you to determine if you have high resistance in the circuit. But the preferred way to test for resistance is using the voltage drop method. We will get into this test method in just a bit.

Low resistance

A short circuit is a low resistance fault that allows too much current to flow to ground due to the low resistance. This will typically blow a fuse or damage wiring or components. Remember that low resistance is always caused by a short to ground or an unwanted ground.

Short circuit to ground

A short circuit to ground occurs when the circuit is grounded or partially grounded where it’s not designed to. Typically with Porsche this is usually caused by radio or accessory installations or body shop wiring repairs or the lack of. If there is a short circuit to ground after the load, the circuit control may be lost, causing operation of components when it is not wanted.

To diagnose a short circuit to ground in a fused circuit, systematically disconnecting circuit components until your voltmeter reads 0 V will identify the area of the short circuit.

Short circuit to power

A short circuit to voltage happens when the insulation fails causing the wire strands to contact the voltage of another circuit. This will cause the circuit or circuits to operate incorrectly. You should carefully observe the symptoms and refer to the appropriate wire diagram to understand the circuit operation flow. Remove fuses until the circuit is isolated, and then measure resistance and voltage to find the problem area.

Component faults

Sensors or Actuators or control units can fail and be faulty even when the part is new. This is definitely the most difficult part of diagnosis, since most of us don’t just have spare parts lying around to plug in and see what happens. But once you have verified that the wiring is ok and that the proper control signals and voltages and grounds to a component are present, then you can start to isolate the failure to a component. I will indicate the proper signals and resistance values and scope patterns that show what a bad and good component look like.

Components can fail from normal wear and tear or an outright defect with the part.