Many, if not most, electrical or electronic system faults on modern vehicles and equipment are intermittent in nature. This means that normal test procedures designed for constant (hard) faults will be ineffective. Test measurements made while the problem is not occurring will generally result in normal readings. There are several things one can do to chase these illusive faults down. The following information is some of the strategies that have been found to be effective in real world troubleshooting.

The first thing you need to do for any intermittent fault troubleshooting is to verify the operators complaint. In some cases the operators complaint may be normal operation. It is important that you communicate with the person who actually experienced the fault to get a clear understanding of exactly what his/her concern is. Once you have determined that the complaint is not normal operation, you must operate the vehicle or machine to learn how to make the fault occur. It is very important that this be done before you touch anything. Do not wiggle wires, check connections, take readings, or anything until you have verified the complaint and learned how to duplicate it. The reason is that you could temporarily correct the condition without knowing it. This leads to repeat repairs. Generally, in order to duplicate the fault you should ask the operator questions such as these:

1. How often does the fault occur?
2. How are you operating the vehicle or  machine when it occurs?
3. What is turned ON and what is OFF?
4. What gear is it in?
5. Where is the engine speed?
6. How much engine load?
7. Does it always occur under a given set of conditions or is it completely erratic?
8. Did it start all of a sudden or did it slowly get worse over time?
9. What is the ambient temperature when it occurs?
10. What is the engine temperature when it occurs?
11. What is the weather like when it occurs? Rain, snow, wind, humidity, dust, etc.
12. How long after start-up does the problem begin happening?
13. Are there any other problems with the vehicle or machine?
14. When was the vehicle or machine last worked on and what was done?
15. How is the vehicle or machine oriented when the fault occurs? Are you heading uphill, downhill, slope to the left or right?
16. What time of day does it generally occur? Morning, night?
17. Any other information about conditions during the fault.

Once you have verified the fault and can duplicate it, you are ready to start testing. If the symptoms are related to an electronically controlled system the first thing to do is check for fault codes. If a fault code is now present (after clearing it before the verification process) it is likely associated with the customers complaint. The fault code will give you an indication of what circuit the problem is in. Non-electronic systems can usually be narrowed down to a circuit by the symptoms. For instance, if the flood lamps flicker you already know it is a flood lamp circuit fault.

It is important to make an effort not to disturb anything to make these tests. Use connector probes  (like Caterpillar 7X-1710) to connect into the circuit without disturbing the connection.  These probes slip into the back of the connector to contact the terminal without disconnecting the connection.

It is generally best to start testing at the circuit level and narrow down to the component or harness. For instance, if you have a 3-wire analog sensor circuit, you should start by taking readings at the PCM/ECM connector between signal return and the signal wire. Avoid testing between signal wire and chassis ground. You will also need to check the voltage reference wire to signal return.

Here is where experience will really help you determine where to start testing. If you have had several occurrences of a particular harness or connector fault, check for that first. Since you have already verified and duplicated the fault, you may be able to go right to the suspect area and inspect for the problem. Always check these things before beginning your testing. Keep in mind this does not eliminate the need for verification and duplication!

Having a good understanding of the operation of the system is a must. You don't really need to know what goes on inside the PCM/ECM but you do need a good understanding of what to expect for a signal on a particular wire for a given set of conditions. For PCM/ECM inputs it is also important to understand what symptoms will occur if the signal is not correct. This information will help lead you from a symptom to a possible cause before you begin testing. Most PCM/ECM input faults will cause several output symptoms. If you understand the relationships you can become fast and proficient at intermittent fault troubleshooting.

Keep in mind there are many hydraulic and mechanical systems associated with most machine functions. Just because a fault is intermittent does not mean it is an electrical or electronic fault. Just because a sensor reading goes out of range does not mean it is bad. It could be whatever it is sensing is actually out of range. For instance, if a temperature sensor has a normal range of -40F to 250F, a fault code will appear if it reads 275F. It is very possible whatever it is sensing is actually that hot. Don't change the sensor if it is not at fault. If the problem has gotten progressively worse over time, look for things that could be wear or contamination related. Think about what things would get worse over time and target those areas first.

Since the condition is intermittent you will need to take the readings while the problem is occurring. There are several methods for doing this. The best tool for this is a strip-chart recorder or a data logger that will give you a graph of voltage (y-axis, or vertical axis) and time (x-axis or horizontal axis). Another method is to use a DVOM with a min/max record feature. The Fluke 87 has this capability. Another method is to have the operator duplicate the fault while you are monitoring the voltage levels. For simple electrical circuits (non-PCM/ECM inputs), a test light will actually work even better for the last method. A lab scope with capture capability is ideal for this. Fluke makes the Scopemeter which is 2 channel with capture capability. The more channels the better. If you have a 10 channel scope for instance, you can simultaneously test 10 points in the system. All it would take is one occurrence of the fault to see the 10 different points in the system.

Short Circuits

Now that you have narrowed down the problem to a particular portion of the circuit, you can begin more detailed testing to find the root cause. Say for instance you have an oil temperature fault code and you found that the signal wire intermittently drops to almost 0 volts while the sensor power supply and ground circuits are OK. At this point you would suspect a harness short to ground or a faulty sensor. Connect the DVOM to the signal wire at the sensor or ECM and carefully wiggle and pull on the wiring harness. If the harness passes over a metal surface, push it against that surface to see if the voltage drops while you are pushing. Once you do get the voltage to drop, you can inspect that area for damaged wiring.

For non-PCM/ECM input circuits, shorts to ground will generally cause a fuse to blow or breaker to trip. In some cases it will just switch a device ON. You can install a small breaker in the circuit in place of the fuse for testing. Wiggle and pull on the wiring in the suspect harness until the breaker trips. When the breaker trips, you are in the area of the fault. Inspect that area for damaged wiring. Sometimes it is helpful to have a test light connector to the wire in question. The light will flicker when the short occurs.

Shorts to power or other circuits can be handled somewhat similar except the problem will be between 2 or more wires rather than between a wire and ground. Look for damaged or overheated wiring. Look for wires where the insulation has had fluid on it.

There are several short finders available from most tool distributors such as Mack, Matco, and Snap-On. These short finders have a magnetic field sensor in them. When current passes through a wire, a magnetic field is generated. The short finder has a needle that will indicate the strength of this magnetic field. When a short causes additional current to flow, the needle will deflect. You can follow the harness while making the short occur and watch for needle deflection. When you pass the shorted area the needle will stop deflecting. Using this method you can narrow down the problem to a particular area.

Snap-On Short Finder

Open Circuits

Open circuit faults can be the most challenging. They don't blow fuses or trip breakers and they can't be traced with a tool like the short finder. The trick here is to narrow down the fault to the point where the open exists. Poor connections at connectors are often the culprit however wires can fatigue and break in the insulation as well. Be particularly suspicious of areas in the harness that move or vibrate in operation. For instance, the wiring harness between the engine and the chassis will have movement.

Start by monitoring the signal at the very end of the wire. For instance, if the fault is in a temperature sensor signal circuit, monitor the signal wire at the sensor. Carefully wiggle and move the wiring harness and connections until the signal is interrupted. Once the signal is interrupted and you figure out how to make it happen, move the DVOM to the next connection before the sensor and wiggle again. Eventually you will pass the faulty area and the signal will not interrupt. The fault will be between the last 2 points. If the wires are exposed in this area (no braid or harness covering) you can pull on them to see if the wire is broken in the insulation or not properly crimped in the connector. Also look for connector terminals that are not fully inserted into the connector body.

Other Intermittent Faults

There are other faults that can cause strange behavior of the machines electrical and electronic systems. Most of these are associated with aftermarket equipment. If 2 wires are places parallel to each other one wire can induce a voltage in the other. The more distance the two wires travel together, the worse it will be. Generally this is only a problem when a very high impedance circuit is placed parallel to a very high current circuit. For instance, a starter cable running parallel to a sensor circuit. Circuits that power inductive loads are even more likely to cause a problem. Examples of inductive loads are starters, fan motors, supplemental steering motors, horns, large relays (or magnetic switches), and starter solenoids.

Large inductors can also cause problems in other electrical circuits even when the wiring and connections are all good and the wires are not run parallel to other wires. What happens is that when voltage is removed from a large inductor, the magnetic field collapses causing a voltage spike in the electrical system. This voltage spike can jump across switch and relay contacts. In some cases, a clamping diode is needed to stop the voltage spike from affecting other circuits. The AC compressor clutch is a good example of a large inductor where a clamping diode is generally used. Again, these problems are usually associated with aftermarket components that cannot handle voltage spikes.

Moisture can cause an intermittent path for current flow as well. This is especially true for water-based fluids and even more so for acids (like battery acid). Battery acid can soak through the insulation on a wire and allow current flow to ground or to other nearby wires. This may only happen when the wires are wet and the current may stop when they dry out. This current flow will generate heat. The more current the more heat. Occasionally you can actually feel the heat build up in such areas. A non-contacting thermometer (148-2400) can help trace these problems down by finding the hot spots. If the problems only occur in wet or very humid weather, or only first thing in the morning, this is a likely cause.

Repair and Verify

Once you have narrowed down the problem to a harness, connector, or component, make the repair and verify it has corrected the initial complaint.

There are some faults that occur so infrequently that it is almost impossible to verify or duplicate them. If these faults do not cause a safety concern., it is best to let them get bad enough so that they can be duplicated before trying to troubleshoot them.