Tech Feature: Solving Intermittent Electrical Problems – UnderhoodService

Tech Feature: Solving Intermittent Electrical Problems

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 ( 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.

Pattern intermittents are those that occur under certain operating conditions. The underlying cause in many instances is vibration, heat, cold or moisture causing an increase in resistance in a connector or component, or an intermittent short or open to occur in the wiring. If an electrical fault occurs at certain speeds or when driving on rough roads, hitting bumps, etc., you’re dealing with a vibration-induced intermittent. Inspect the wiring for loose connectors, unsupported wires, wires rubbing against sharp corners or edges, wire grommets that are missing, etc. Sometimes shaking or rocking the vehicle may create enough movement to duplicate the fault in the shop. Or, you may have to test-drive the vehicle on a rough road to get the fault to act up. The fix, in most cases, is to repair, support or insulate the wires or connectors that are shaking or rubbing and causing the fault.

If something stops working after a vehicle has been driven a certain distance, or only when idling in traffic, or only on hot days, or only after a certain component has been on for a period if time, it usually indicates a heat-related electrical problem. When the wiring gets hot, either from its own heat or from an external source, thermal expansion causes something in the circuit to open up. This could be a loose-fitting connector, a crack in a soldered wiring connection or a crack in a printed circuit board.

If the circuit starts working again after the vehicle has been shut off for a while or allowed to sit, that would be another clue that the problem is heat-related. Likely trouble-spots for heat-related intermittents include connectors and relays located near sources of engine heat such as the exhaust manifolds, exhaust pipes, radiator or heater core. To find this kind of problem, you can use a heat gun (low setting only) or blow comb to direct heat at suspicious connections or components. If the device stops working when you apply heat, you’ve found the problem.

Environmental factors often play havoc with electrical systems, too. Road splash or water leaking through a crack in the cowl, under the windshield or around a grommet can sometimes short out a circuit. Flood damage can also cause corrosion to develop in connectors, wiring and modules long after the flood waters have receded. That’s why most insurance companies consider flood-damaged cars to be junk. Sooner or later, they’ll develop electrical problems. If you think moisture may be causing an intermittent problem, look for corroded wiring and connectors, and look for water stains, streaks or damp areas under the cowl, instrument panel or other areas where wiring may be affected (under the rear seat, in the trunk, etc.). Mud packed into crevices where wiring is located can retain moisture and become a source of corrosion. If you find a water leak or damp spots inside the vehicle, determine where the water is coming from and fix the leak. Then inspect and clean the wiring and connectors in the affected area. Hopefully, this will solve the problem.

Another possible cause of an intermittent electrical fault may be one circuit stealing voltage from another. For example, if you turn on the air conditioner and the engine starts to overheat because the electric cooling fan stops working, the high-amp load created by the compressor clutch may be robbing power from a power circuit shared with the fan relay. In some cases, the fan relay may have to be rewired with a separate power supply to eliminate the problem.

Also, watch out for improperly installed aftermarket accessories such as sound systems, alarms and lights. The installer may have tapped into another circuit to power the device.

If an intermittent causes a fuse to fail, replacing the fuse won’t fix the problem. A new fuse may restore power temporarily but, unless the underlying cause of the circuit overload is found and corrected, the fuse won’t last and will likely blow again. Whatever you do, don’t substitute a fuse of greater capacity in an attempt to fix the fault. A larger fuse may be able to handle a greater load, but the wiring and the rest of the circuit can’t. A circuit designed for a 20-amp fuse is designed to handle a maximum of 20 amps. Period.

Using a higher capacity fuse may cause the circuit to run dangerously hot, increasing the risk of wiring damage or an electrical fire. A faulty circuit breaker can also cause intermittent faults. Breakers contain a heat-sensitive bimetal contact arm and contact points to open a circuit if it overloads. After the circuit breaker cools down, the contacts re-close and current is restored. If the breaker is bad, it may be opening prematurely. If the problem goes away when you bypass the breaker with a fused jumper wire, it means the breaker is bad and needs to be replaced. A relay can also be checked with an ohmmeter to see if the coil is within specifications, and the contact points provide continuity when the coil is energized. As a rule, most relay coils should measure 40 to 80 ohms — but always look up the specifications to be sure.

Another quick check is to remove and shake the relay. If you hear anything rattling inside, it means the armature is broken and the relay needs to be replaced. Substituting a “known good relay” for one that is in question is yet another method of testing a relay.

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