Note: See my companion articles on Diagnosing Car Electrical Problems and Understanding the Check Engine Light.
In many older cars, people run into mysterious electrical gremlins that cause no end of grief. However, electrical circuitry is not magic, but science. Electromagnetism is magic.
Finding and curing electrical gremlins can be difficult, but if you take a systematic approach, any electrical gremlin can be cured. And taking a proactive approach may prevent such gremlins from coming back.
The biggest problem I see when shade-tree mechanics try to solve electrical problems (or any problem, for that matter) is that they want to isolate the problem to one source, fix that source, and be done with it. Electrical problems can have a host of sources, and multiple sources can contribute to create a "gremlin" problem.
If you attack all of these problems, systematically, you can eliminate electrical problems for good, from any car. Even an MG or a Fiat. You need not live in fear of Lucas Electronics. Any electrical circuit can be made reliable.
The problem with diagnosing these gremlins is that most people look to the least likely sources for trouble first, while pointedly ignoring the most likely sources of trouble. So the average shade-tree mechanic assumes there is "a short somewhere in the wires" (very unlikely) causing his battery to go flat, while assuming the battery itself is good (without testing it). Of course, the most likely cause of a battery going flat is the battery, which is where you should start any electrical gremlin hunt.
You don't need to be an Electrical Engineer to solve most of these problems, either, as most electrical problems in cars are cause by mechanical failure of components. But learning the basics of circuit law and how to use a Volt-Ohm Meter never hurts and can help you immensely in diagnosing circuit problems.
So here is a short list of how to track down and kill electrical gremlins:
1. The Battery: Before starting any hunt, make sure your battery is working properly. Yea, I know, it is only 7 years old. But check anyway. If you are having electrical problems, check the voltage at the battery with the engine off. It should be about 13 volts. Clean and tighten the battery connections. You'd be surprised how a loose connection can cause all sorts of weird problems, and yes, people always discover this last, not first. It never hurts to clean and tighten battery connections, so just do it and stop whining. Apply some type of corrosion inhibitor to the connectors. They make special sprays for this, but any type of dielectric grease may help.
If you are unsure about the battery at all, remove it and have it tested at your local auto parts store, which will do this service for free. If the battery is over 5 years old and in any way marginal, just replace it. Batteries are cheap ($100 or less) and eliminating this as a source of trouble is important. Marginal batteries can do weird things in electrical systems. And of course, they will eventually leave you stranded somewhere.
A note on jump starting: Jump starting causes more problems for amateur mechanics than anything else. The wild swings in system voltage may set off the check engine light (low voltage being one of the CE trip codes) and can cause damage to the car's electronics. So avoid jumping the car whenever possible. That means not jumping a car again and again with a "mysterious battery problem" and instead just breaking down and buying a new battery.
Second, most modern alternators will NOT charge a dead car battery. So if you jump a car, keep it connected to the donor car for a good long time (at least 20 minutes) or connect the car to a battery charger right away. Most modern alternators will overheat and self-destruct if you try to recharge the battery from flat. So a bad battery problem quickly cascades into a blown alternator problem.
Note also that even a brand new battery can be destroyed if allowed to go completely flat several times. So if you leave the lights on overnight, it really damages the battery. Do it several times, and you may find yourself needing a new battery.
2. The Alternator: With the car running, your system voltage should be about 14.5 volts. If is it substantially less than this, chances are, your alternator is shot. Again, your local auto parts store can test the alternator if you are in doubt, including under load, and sometimes while on the car, and again, they may do this for free.
Rebuilt alternators are not too expensive, and there may be a shop in your town that rebuilds and rewinds alternators (these are getting scarcer as more alternators are made in Asia and are disposable). Look in the yellow pages under armature shops, etc.
Yes, you can "rebuild" some alternators yourself. Some have a cartridge that replaces the brushes and voltage regulator in one shot. And you can even break apart the unit and replace bearings and diodes yourself. Once the insulation melts on the windings, however, the unit will need a professional rebuild, or increasingly, be thrown away.
For the cost of a rebuilt or new alternator, however, it is far easier to just replace the unit and return your old one as a "core" and let others do the overhauling.
3. Connectors: Once the charging system (battery and alternator) is found to be sound, you can attack other sources of trouble. But do not do so until you have eliminated items #1 and #2 above. More than half of all electrical problems can be traced to power supply problems, and going after electrical gremlins without checking your power supply (battery and alternator) first is counterproductive.
Light bulbs are the most common failure mode in any electrical circuit, but I presume you know how to locate and change a light bulb. Bear in mind that on some foreign cars, even this pedestrian task can be made esoteric by the odd shapes and wattages of euro-spec bulbs. Suffice it to say that you should use the exact same bulb that is specified for the car, or you may have troubles. And since bulbs are sold in pairs, replace them in pairs, as they do dim over time and if you replace one, the car will have a winky effect, with one tail light brighter than the other. Also, if one bulb fails, the other will not be far behind.
Behind light bulbs, though, electrical connectors are the next most likely failure mode in any electrical circuit. And this is the one area most often overlooked by the average shade tree mechanic.
Which connector? Well, all of them. And in many European cars, this means the ground connections as well.
With the battery disconnected, examine each electrical connector you see on the car. Carefully unplug each (many have tangs and clips or bails that hold them together, do not force or break them). Note if there is a gasket on the connector and be sure to replace it when reassembling.
Look for evidence of corrosion from water or air - dull color, green oxidation, or the like. Clean contacts with an emory board or small piece of fine grit sandpaper. For circuit boards, try gentle pressure from a pencil eraser. Blow out any dust. Apply a layer of dielectric grease (available in tubes, pots, or even pressurized cans, from any auto parts store) to the connector and reassemble.
Many European cars have centralized grounding connectors. A bundle of brown wires may attach to some point on a fender or in the trunk. Look for corrosion on these. Disassemble, clean each contact, and then reassemble, applying a generous layer of dielectric grease.
Many cars use aluminum connectors to save money. When oxidized, these connectors create aluminum oxide, a good insulator. If you have aluminum connectors, cleaning them and applying dielectric grease does wonders for electrical gremlins. Such connectors, when oxidized, can create a 3-5 volt drop or even an open circuit.
Note that many "chronic" electrical component problems are in fact, connector problems. For example, one fellow I met went through three engine computers in his Ford truck. The car would run for a year or so and then quit. The mechanic would replace the engine "brain" and the truck would run, for a year or so.
Most electrical connectors are of the "wipe" variety, so when you plug in a component, the connector wipes down the side of its mating part, scraping off a thin layer of oxidation and making electrical contact. So merely replacing a bad computer "brain" often "fixes" the car. But the problem was not the computer, but merely oxidation on the connector.
Cleaning the connector first often solves the problem, without expensive component repair. Many low voltage and low amperage sensors, such as wheel sensors for ABS systems are particularly prone to this, compounded by the fact that the connectors may be located in the wheel well and subject to road spray.
A standard pencil eraser, by the way, can be used to polish up and clean electrical contacts, particularly flat ones, such as on circuit boards and edge connectors.
Note that the ignition switch on many cars has connectors for a whole host of electrical items in a car. I once spend several hours tracing down electrical problems on a car, only to pull the ignition switch connector last and see it blackened with oxidation and corrosion. The combined amperage flowing through this connector, which was slightly loose, caused a lot of arcing, which in turn built up a layer of insulating oxide, eventually leading to circuit failure.
Fuses are connectors in a way, and modern two-prong fuses often have aluminum connectors that can oxidize readily. On my boat, these corrode with regularity and have to be cleaned nearly every season, and then coated with dielectric grease.
A special note on fuses: It is tempting to put in a higher amperage fuse in a circuit as a "fix" to a fuse blowing constantly. Fuses are designed to protect WIRING by providing a weak point in the circuit that will melt before all others. If you put in a higher amperage fuse, the weak point in the circuit becomes the wiring itself, which, when it melts, can cause a car fire. So if you are blowing fuses in a circuit, don't try the dimestore trick of replacing the fuse with one of a higher value - you'll just cause no end of grief, and possibly burn down your house.
If you own an older car, get in the habit of inspecting such connectors and fuses during your regular maintenance on the car. Clean any that have even the slightest corrosion and also use dielectric grease on all connectors. If you do this as a preventative measure, electrical gremlins will be less likely to pop up down the road, and you won't have to "chase" them later on.
For diagnostic purposes, start with the connector closest to the problem element (tail light, turn signal, whatever) and then follow your way back. Be sure to trace ground wires as well! I had a ground wire problem with a fuel pump on a Fiat once. I turned out that the fuel pump (mounted under the car, admidships) grounded to the left tail light housing. When the car was repainted, the body may did not tighten the tail light housing and the ground for the fuel pump was intermittent.
Ground connections are particularly frustrating as they can be located far away from the component of interest, and if all you do is trace the power wiring, you are missing half the circuit. So check those ground connections as well. European cars in particular seem to be fond of grounding several items at one point, which results in odd things happening when the ground goes bad.
For example, in the Fiat example above, since the fuel pump grounded to the tail light, when I applied the brakes or turned on the running lights, it applied 12V to the floating ground, resulting in a zero voltage drop across the fuel pump, shutting it down. While this may seem like "magic" to the uninformed, it is a simple Kirchoff circuit law analysis.
4. Switches and Relays: Switches wear out, just like light bulbs. And after connectors, they are the next most probable source of problems. Every switch, when switching a live load, has a moment when the contacts are not quite together, where an arc may jump between the two contacts. This arcing produces oxidation, which as noted above, results in a layer of insulation being deposited on the contact surface. Over time, this oxidation will prevent the switch from working completely.
Arcing can also literally weld a hole through a switch, which can also lead to failure of the switch. For most switches, it is not possible to clean the contacts on the switch itself, so once the switch fails, you have little choice but to replace it.
You can test a switch a number of ways. One is to merely jumper out the switch and see if the circuit works using a jumper. If it works, then odds are, the switch is bad. Of course, this assumes you know how to jumper the correct leads and you don't electrocute yourself.
Another is to test the switch with a Volt-Ohm meter, measuring resistance across the switch (removed from the circuit, of course) in the open and closed positions. One problem with this approach is that sometimes a switch may appear to be "working" when the milliamps of current are applied by the VOM, but not work in the actual circuit where it is being asked to switch higher amperage.
And again, the ignition switch on many cars carries a large number of loads and thus may be especially prone to failure over time (Hint: Get in the habit of turning off lights, wipers, heater blower, etc before shutting off a car. When you turn on the ignition switch with these on, you are switching a hefty load through that ignition switch!).
One way to limit the load on switches (and thus make them lighter and cheaper) and also limit wiring loads (and make wiring lighter and cheaper) is to use a power relay to switch power loads. Interior switches in the car thus end up switching the relay, which in turn handles the job of switching the high power load.
Over the years, manufacturers have increased the number of relays used, to reduce weight and reduce the amount of copper in a car. Back in 1960, many a car had nary a relay. Today, even the most pedestrian make has five or six. If the car has a number of identical relays, you can easily test a relay by swapping it with one of its mates. But caution: Not all relays are alike, even those that have the same form factor and shape and plug arrangement. Many are wired differently (normally open versus normally closed) or are rated for different amperages. So before trying this route, be sure you have the right relay (same part number).
Again, you really can't fix a relay (although I did disassemble one in a pinch and was able to clean the contacts with a fine piece of sandpaper). Replacement is really the only solution.
5. Water: This arguably falls under the connector problems listed above, but bears special mention by itself. Water ingress can cause immediate short circuits by itself. But in addition, continued exposure to water can cause corrosion of connectors and other components.
Headlight and taillight assemblies can fill with water and short out. Usually, evidence of such water intrusion is readily apparent and fixing such problems is a matter of sealant (which rarely works) or replacing the assembly (which usually ends up solving the problem). Pay careful attention to gaskets and the like on bulb connectors. Often these are lost when replacing bulbs, allowing for water ingress.
A properly functioning car should be able to drive through water a foot deep or more and have no problems. But playing puddle jumper never is a good idea, and if you can avoid deep puddles and downpours, with an older car, it will probably save a lot of grief.
Also, garaging a car will result in a lot less water ingress over time, which is why, if you can, spend money on buying a house with a garage before you spend money on a fancy car. Garaged cars last longer and run better, period.
Keeping dead leaves and debris out of your underhood area and trunk lid are important. Most cars have a complex series of drain holes and tubes to drain off excess water from around door seals, trunk seals, and hood seals. If these drains become clogged, water will overflow and short out electronics (and get inside your car and give it that wet dog smell and grow a mushroom farm).
BMWs have two drain tubes behind the firewall that, if clogged, may flood the engine management computer, causing this expensive item to short out and die.
Rubber boots and connectors, designed to repel water may end up retaining it, as happened to me with one car. The power leads to the Anti-lock brake system were sheathed in rubber boots. The ABS light went on after every rain storm. After removing these rubber boots, a thimble-full of water fell of each one. Packing these with dielectric grease solved the problem, permanently. ABS sensor connectors suffer from a similar problem and dielectric grease often solves this problem as well.
Flood cars are usually junked out because electrical problems from water ingress can be chronic and difficult to repair. If you end up with such a car, I would suggest attacking every connector below the waterline with an Emory board (to remove corrosion) and dielectric grease (to prevent further corrosion).
Even cars that are not in a flood can have water problems. For example, the previous owner spilled water in the back of my X5. Since the battery and electronics modules are under the spare tire, this resulted in some corrosion of connectors. When installing the trailer hitch wiring harness, I noticed this problem and corrected it - by cleaning the contacts and applying dielectric grease.
6. Sensors: Again, most sensor problems, such as ABS sensors, end up being connector problems. ABS sensors are just coils and rarely "wear out". But many a dealer mechanic has replaced three or more under warranty, when the root cause was a bad connector.
Modern fuel injected engines have a number of sensors that control various aspects of the engine. These may include temperature sensors, mass air flow sensors, throttle position switch, camshaft position sensor, crankshaft position sensor, knock sensors, and the like.
For the most part, you can diagnose failures in these sensors from the OBD II error codes (using a code reader). But note that just because a code reader mentions a part by name does not mean the part in question is in need of replacement.
For example, "oxygen sensor out of range" errors are often caused by intake leaks, such as at the intake elbow. A "catalytic converter efficiency below threshold" error, on the other hand, may indicate a poorly operating oxygen sensor.
Before you run off replacing parts, Google the error code, make and model, and see if someone else had the same problem. Chances are, you may find the solution from those input criteria.
Other types of sensors, such as a throttle position switch, can be tested using a volt-ohm meter. Your service manual will detail which sensors can be tested and how to go about testing them, by measuring resistance or some other parameter.
Sensors can be cheap enough that in some instances, it makes sense to just replace them, if they are at all suspect. I had an old fuel-injected Fiat that I bought secondhand, and never ran right. I realize that replacing all the sensors for the fuel injection system (other than the flow meter, which had already been replaced) would cost about $250. So I just replaced them all and the car ran great after that. But usually, the "throwing parts at it" solution is not a good idea, particularly if the parts are expensive.
Oxygen sensors bear special mention. These are, in a way, like light bulbs, in that eventually they wear out. Most cars have scheduled replacement intervals (50,000 miles, 100,000 miles), and it makes sense to replace these if they are beyond their scheduled replacement interval, particularly if you are having any engine issues at all. Note that age, in addition to mileage, may also wear on these. So even a lower mile car, which is older, may need a new oxygen sensor or sensors. And also, just because the book says they should be replaced at 100,000 miles does not mean they won't fail before then.
7. Actuators: Anything with a moving part is prone to failure, and actuators usually are inclined to be less reliable than sensors. However, since most Engineers know this, actuators tend to go through more testing and analysis and I think tend to last longer. Power window motors and door lock actuators do wear out over time, but not as often as you'd think, given the amount of use they get. Replacing these is not a big issue. When they fail, and you've eliminated the switch, fuse, connectors, and relays from the problem, chances are, it is the actuator.
Electronic fuel pumps bear special mention. Many of these are mounted in the tank and are cooled by the fuel pumping through them. Running a car out of fuel may overheat the pump, which will shut it off or cause it to fail. Also note that many newer cars have impact switches that shut off electronic fuel pumps after an accident, to reduce the likelihood of fire. So before you yank a fuel pump, check to see if there is such a switch and a reset for it.
And again, many diagnosed actuator problems may in fact be connector problems, so check connectors first. But motors eventually wear out, usually because the contact bushes (in a DC motor) wear out and cause the motor to die. Since armature shops are getting harder to find, replacing the motor is usually the only option.
I have had an interesting fan blower problem on two cars, where water ingress causes the blower to stop working. The blower "froze" in position, most likely because the carbon brushes got wet and stuck to the armature. In both cases, opening up the plenum and rotating the fan by hand fixed the problem - at least temporarily. But be careful if the fan is switched on, as it may ding your fingers once it starts up.
7. Wiring: This is usually the last place to look and the first place most amateur mechanics think about. I cannot tell you how many postings I see online where someone with a decade-old car (and decade old car battery) posts a query, wondering why their battery keeps going flat. Their first impulse is "It must be a short somewhere, I'll have to tear into the wiring".
Wires just don’t fail, period. A piece of copper, lying undisturbed, doesn't just suddenly decide not to conduct, or break in half on its own accord.
If a wire does fail, it is likely to be at a door or trunk lid, where wiring flexes over time, causing work hardening, which in turn can lead to brittle failure. But you know what? Automotive Engineers know all about this, and design their wiring harnesses to take such abuse. So even such failures are very, very rare (I've owned a mountain of junkers over the years, and rarely has the door wiring ever failed).
Bulkheads are another area where failure may occur, if a wire rubs against a raw metal opening. But again, Automotive Engineers know this and use rubber grommets and fittings to prevent such problems. Of course, Buddy down at the local car stereo shop may not be so well informed, which is why aftermarket installations often have problems. Inspect such installations carefully to be sure that the wiring is not compromised, and more importantly, that they have not compromised the existing wiring in the car.
A special note on car storage: Be sure to rodent-proof your storage area if you plan on storing a car for a month or more. Hire an exterminator or place rat poison or other devices near or on the car (be sure to remove afterwords!). Mice love to eat the insulation on some cars, and have been known to destroy wiring harnesses, which are costly to replace. Also, in the South, so-called "palmetto bugs" (cockroaches) will eat vinyl insulation with similar results.
The general goal of any electrical system is to minimize the number of connectors and breaks in the system, as each is a source of potential trouble. In this regard, cutting and splicing wires should be avoided. Some folks, to try to save a few dollars, will try to cut and splice a "generic" oxygen sensor into the existing harness. This may result in a savings of maybe $60 per sensor, but involves quite a bit of work, to do the solder job right. And each connection is a potential problem (cold solder joint, water ingress, corrosion) that could make the job go horribly wrong.
Now granted, if no other part is available, then you have no choice. But if you have a plug and play part available for not a lot more money, I don't see the "savings" in going against general Electrical Engineering principles. And usually, it turns out, the fellow soldering his connections to save $60 is the same guy who spend $250 on clear turn signals. Get your priorities straight!
Wiring works great - if left alone. This is one reason why I shy away from any aftermarket products that are not "plug and play". Tearing into the wiring harness of a car to install an optional, an unnecessary accessory is creating a risk of problems. And the message boards are full of sob stories from folks who try to install aftermarket remote starters (or alarm systems or boom-boom stereos) and end up with a car that won't start and now has to be flatbedded to a dealer for repair - expensive repair at that.
Even those cheesy "tap off" electrical fittings can break a wire or cause corrosion to intrude. Unless you have a real compelling need, avoid cutting wiring on your car.
And increasingly, the aftermarket is offering "plug and play" solutions. Aftermarket plug and play alarms are available, even for BMWs, that require no cutting of wires. These are not only easier to install, they are more reliable as well.
And for car stereos, you can buy wiring harnesses in most catalogs which will plug into the factory harness. You can solder and shrink tube this harness to the stereo you buy and then plug in the entire thing into the factory wiring. But before you try even this, ask yourself if it is really worthwhile.
8. Accessories: In this vein, many aftermarket accessories can also cause problems for car owners. People try to "upgrade" their car using Chinese-made junk, and then end up in trouble and upset.
Putting in higher wattage bulbs for headlights for example, may make your lights brighter, but also make them blind oncoming drivers. And yes, they will likely blow a fuse. And as I noted above, replace the fuse with a higher amperage one is not a good idea.
If you want to install non-functional cosmetic modifications like "angel eyes" and the like, knock yourself out. Just do a good job of it, because no one will have much sympathy for you when you post your tale of woe on a message board about how they don't work right. And even if they do work right, bear in mind that everyone is still laughing at you, anyway. Leave that sort of junk to teenagers.
Taillights under glass are another example of a "mod" which causes electrical problems down the road. In BMWs, the resistance of the rear bulbs is measured by the OBC and an error message generated when it is off. LED and other "racey" (ricey) looking aftermarket taillights often set off the OBC and cause a permanent error message to appear on the dashboard. Many of these self-styled boy racers "Fix" the problem by covering over the OBC and/or removing the light bulb from the dashboard display. This sort of thing is jury-rigging at its worst.
The best way to avoid electrical problems with aftermarket accessories is just to avoid aftermarket accessories entirely. They add no value to the car, and in fact, devalue it. People shy away from buying a used car that reeks of boy-racer mods, as it speaks volumes about the maturity of the owner and the way the car was treated. And as I noted above, ironically, the same folks who think nothing of spending thousands of dollars on "mods" often neglect basic maintenance items like fluid changes.
The only upside to these nimrods is that they usually sell their take-off parts for cheap on Craigslist. So if you need a set of new tires or a headlight, chances are, some boy-racer will sell you them for cheap, as he "upgrades" to badly made Chinese replacements. His loss, your gain. 18-year-olds were born to be taken advantage of.
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So there you have it. Solving electrical problems is not that hard, if you take a systematic approach, investigating the high-probability failure items first, before you attack the least-likely failure items. And shying away from creating problems is also part of the deal as well.
The best general advice I can offer anyone is to clean connectors and apply dielectric grease. It will prevent a whole host of electrical problems, and fix many electrical problems that are incorrectly diagnosed as component error.