Intermittent electrical faults are the bane of the auto repair business. If an electrical component has failed, or there is an open or a short in a circuit, you can usually find it fairly quickly because it isn’t hiding from you. It’s a persistent fault that can be isolated by a systematic process of elimination.
Not so with intermittents. Chasing intermittents is like chasing ghosts. One minute the fault appears, the next minute it vanishes into thin air. Worse yet, the problem may occur at random intervals with no apparent connection to driving conditions, temperature or humidity. And if you can’t get the fault to appear long enough for you to catch it, you can’t fix it.
So how do you deal with intermittent electrical faults?
One approach that’s often used is to wait it out. If you’ve had no luck getting an intermittent problem to recur in the shop or by test-driving the vehicle, you may tell your customer to keep driving his vehicle until it happens again — or until whatever is going wrong with it eventually fails completely.
Sometimes that’s all you can do. But most customers don’t see that as a very good solution to their problem — especially if an intermittent has been affecting the reliability or safety of their vehicle (things like hard starting, stalling, loss of power, lights going out, etc.). The best approach to dealing with intermittent electrical faults is a logical approach that will help you find and fix the fault now, so here are some tips on how to proceed.
First, go online or search your service database for any vehicle manufacturer TSBs that may shed some light on the problem. If the intermittent is a pattern failure that’s occurring in other vehicles of the same make, model and year, chances are the vehicle manufacturer may have published a TSB that identifies the fault and tells you how to diagnose and repair it.
TSBs are often the “magic bullet” you need to nail an elusive ghost. Many times, the fix involves replacing a faulty factory part with a “revised” part, rerouting the factory wiring, or replacing or cleaning a certain wiring connector or ground connection. Best of all, TSBs will usually show you which components or wires to check so you don’t have to search through pages of wiring diagrams in an attempt to figure out which circuits are which and what goes where.
Another source that can often provide insight and answers for hard-to-diagnose faults like intermittents is the International Automotive Technicians Network (www.iatn.net). Thousands of technicians worldwide belong to this group and share their knowledge and experience in the form of posts and follow-ups. Chances are if you’re having trouble fixing a particular fault, somebody else has already run into the same problem.
Let’s say you want to fix cars the old-fashioned way and figure out everything for yourself. With intermittent electrical faults, you’re going to need a couple of things. First, you’ll need wiring diagrams for the vehicle. The diagrams will show you how the components are connected, how the power to a particular circuit is routed and how the circuit is grounded. If an intermittent is affecting the operation of several different components (say the interior lights, wipers and horn), you may discover that they all share a common power supply or ground. This can save you the time of trying to disconnect and check each component individually. You can zero in on the power supply circuit (fuse, relays, wiring) or the ground connection.
The next thing you need is a good DVOM (digital volt ohmmeter), a 12-volt test light, some fused jumper wires and possibly a logic probe (for detecting digital signals in low-voltage circuits).
You’ll also need a solid understanding of basic electricity (how voltage and resistance affect each other), and know how to do a voltage drop test to check for bad connections.
Every electrical device also requires a certain amount of voltage to operate. A light bulb will glow with reduced brilliance as the voltage drops. But, for some components, there is a threshold voltage, below which it won’t operate. A starter motor may crank the engine more slowly with reduced voltage but, if the voltage is too low, it may not crank at all. Minimum threshold voltage is especially critical for such components as solenoids (which need a certain amount of voltage to overcome spring resistance), relays, timers, buzzers, horns, fuel injectors (which are solenoids, too) and most electronics (the ignition module, computer and radio).
Checking the load point for full battery voltage will tell you whether or not sufficient voltage is getting through, and to do that you need a voltmeter. The battery itself should be at least 70% charged and read 12.43 volts or higher (12.66 volts is fully charged). If the battery is low, it should be recharged and tested. The output of the charging system should also be checked and be about 1.5 to 2.0 volts higher than the battery’s base voltage. If the battery is OK, your voltmeter should read within 0.4 volts (or less) of battery voltage at the circuit load point in any given circuit. With some components such as relays, the relay may not close reliably if the supply voltage is low.
Low circuit voltage is usually caused by excessive resistance at some point in the wiring. Usually this means a loose or corroded connector, a faulty switch, bad relay or poor ground. To find the point of high resistance, use your voltmeter to do a voltage drop test at various points throughout the circuit. If the voltmeter shows a drop of more than a 0.1 volts across a connection, it means trouble.
If low voltage is detected in a number of circuits, do a voltage drop test across the battery terminals, and engine/body ground straps. Loose or corroded battery cables and ground straps are a common cause of voltage-related problems. Clean and tighten the battery cables and/or ground straps as needed.
Every electrical circuit requires a complete circuit to operate. Voltage to the load won’t do any good unless there is also a complete ground path to the battery. The ground path in the case of all metal-bodied cars is the body itself. In plastic-bodied cars, a separate ground wire is needed to link the load to the chassis. In either case, a poor ground connection has the same effect as an open switch. The circuit isn’t complete so current doesn’t flow. Road vibration, or changes in temperature and/or moisture, may cause resistance in the connection to change, creating an intermittent drop in voltage.
To check wiring continuity, you’ll need an ohmmeter, DVOM or self-powered test light. An ohmmeter or DVOM is the better choice because both display the exact amount of resistance between any two test points. A test light, on the other hand, will glow when there’s continuity, but the intensity of the bulb may vary depending on the amount of resistance in the circuit. But it’s OK for making quick checks.
Never use an ohmmeter to check resistance in a live circuit. Make sure there’s no voltage in the circuit by disconnecting it from its power source, by pulling the fuse or by testing downstream from the circuit switch or relay. Ohmmeters can’t handle normal battery voltage, and should you accidentally complete a circuit through the meter, you may damage your meter. Ohmmeters are great for measuring circuit resistance, but you have to use care when checking electronic components. An ohmmeter works by applying a small voltage through its test leads, and this voltage can be enough to damage some electronic components (such as an oxygen sensor). Special high impedance 10,000 mega ohmmeters should be used for electronics testing.
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