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FAQ: Electrical
FAQ (Frequently Asked Questions) >Electrical
Battery keeps running downCan you replace a halogen headlight with an ordinary headlightHow do you know when a sensor needs to be replacedHow does the oxygen sensor work
What size battery is neededWhich engine sensors are the most importantWhy are there no returns on electrical and electronic parts
Battery keeps running down

Is it the Battery, Alternator, or Voltage Regulator?

It could be any one of the three, or an undetected voltage drain caused by a trunk light, underhood light, or glovebox light that does not go out when the lid is closed.

An alternator is based on the rotation of a magnet inside a fixed-loop conductor. The output circuit and the field circuit make up the automotive charging system.

The first thing that should be checked is the battery state of charge. If it has a built-in hydrometer (charge indicator), a green dot means the battery is 65% to 75% charged and okay for use or further testing.

If the charge indicator is dark, the battery is less than 65% charged and needs to be recharged and load tested.

On 1985 and later model Chrysler vehicles, the charge indicator on some batteries also contains a red dot which shows if the battery is less than 50% charged.

If the charge indicator is clear or yellow, the level of electrolyte inside the battery has dropped too far to give a reading. It also means the battery will need to be replaced soon. Once water level drops below the top of cell plates, they dry out and lose their ability to hold a charge.

Never attempt to jump start or charge a battery with a low electrolyte level. It may explode.

The state of charge of a sealed top battery without a built-in charge indicator can be determined by measuring its open circuit (no load) voltage:

Open Circuit Voltage

State Of Charge









11.7 or less


  A low charge level does not mean anything is wrong with the battery or charging system, it simply means the battery is low and needs to be recharged.

Performing a load test would be the next step. This checks the battery's ability to deliver current. The battery must be at least 65% charged before load testing. If not, a good battery may fail the test.

A conventional load test is performed with a carbon pile battery tester. The load created by the carbon pile is adjusted according to the battery's cold cranking amp (or amp/hour) rating. The carbon pile is usually set to one half the battery's CCA rating (or three times its amp/hour rating).

Temperature compensation is also important because a cold battery puts out fewer amps than a warm one. The load is then applied to the battery for 15 seconds while voltage output is observed. If voltage remains above 9.6 volts, the battery is good. If it drops below 9.6 volts, the battery can be recharged and retested, or given a three-minute charge test.

A three-minute charge test checks for a sulfated battery. Slow charge the battery at 40 amps for six minutes, then check voltage across the terminals with the charger on.

If the voltage is above 15.5 volts, the battery is not accepting a charge. Slow charging for 20 hours can sometimes reverse the sulfated condition, otherwise the battery is junk.

If the battery check is okay, the next item to check is the charging system. A properly working system produces a charging voltage around 14 volts at idle with lights and accessories off (refer to a shop manual for exact charging specs).

When the engine is first started, charging voltage should rise quickly to about two volts above base battery voltage, then taper off and level out at the specified voltage.

Exact charging voltage will vary according to battery state of charge, load on vehicle electrical system, and temperature. The lower the temperature, the higher the charging voltage. The higher the temperature, the lower the charging voltage.

On a GM application, for example, accepted voltage charging range is 13.9 to 14.4 volts at 80 degrees F. At 20 degrees F below zero, charging range is 14.9 to 15.8 volts. At 140 degrees F, the charging voltage is 13.0 to 13.6 volts.

Charging output can also be checked with an adjustable carbon pile, voltmeter and ammeter. The carbon pile is attached to the battery and adjusted to obtain maximum output while the engine is running at 2,000 rpm.

If charging voltage is low, the alternator or voltage regulator could be faulty. To find out which component is bad, a procedure called "full fielding" can be used to bypass the regulator.

If the alternator produces the specified voltage or current output after full fielding, the problem is in the regulator (or wiring) not the alternator.

The exact procedure for full fielding an alternator varies from vehicle to vehicle depending on how the alternator is wired. Basically, the regulator is bypassed by connecting a jumper wire between the field (FLD or "F" terminal) and battery positive (BAT) terminal on the alternator.

On older GM applications with Delco integral regulator alternators, inserting the tip of a screwdriver through the D-shaped hole in the back of the alternator full fields the unit.

Either voltage or current output can be compared against manufacturer specs to determine if the alternator is functioning at full capacity. Generally speaking, alternator output should fall within 10 amps or 10% of its rated capacity at 2,000 rpm.

For several reasons, it is important to follow full fielding test procedures exactly. If only one diode or stator winding is bad, for example, the alternator may still make enough electricity at high rpm to keep the battery charged, but not at idle or low speed. The alternator and/or regulator can also be damaged if the wrong test procedure is used.

On Chrysler externally regulated alternators, for example, you do not apply voltage to the "F" terminal. This system is full fielded by grounding the green wire at the regulator connector. On externally regulated Ford alternators, the alternator is full fielded by disconnecting the four-wire connector from the regulator and jumping across the "A" and "F" terminals.

If charging output goes up when the regulator is bypassed by full fielding, but otherwise fails to produce voltage, check the regulator for a poor ground. This is especially important on Ford and Chrysler systems. Poor or open wiring connections between alternator and regulator can also cause a charging problem.

A slipping fan belt is one of the most common causes of under charging. A fan belt that holds at idle or low rpm may slip when the alternator is under load. Glazed or burned streaks on the belt are an indication of slipping.

If the battery and charging system are okay and the battery keeps running down, check for a voltage drain somewhere in the electrical system. To isolate the cause, remove one of the battery cables and connect a volt meter or amp meter between it and the battery.

A voltage drain will cause a reading on the meter. Disconnect fuses one by one until the circuit is found that causes the reading to disappear.

On-board electronics such as the computer, an electronic clock, etc., will draw a few milliamps all the time, but should not be enough to run the battery down unless the vehicle is not driven for long periods of time.

Can you replace a halogen headlight with an ordinary headlight

It is possible to replace a halogen headlight with an ordinary headlight, but only if the halogen headlamp is a sealed beam. Halogen headlights with interchangeable bulbs only accept halogen bulbs. There is a trade-off to consider; ordinary sealed beam headlights cost less than halogen sealed beams, but they are not as bright.

Halogen lights are about 40% brighter than conventional incandescent lights. Light output is measured in units called lumens, which is a more accurate measure than candlepower. Conventional incandescent bulbs give off 15 to 18 lumens per watt. Halogens produce 20 to 25 lumens per watt, extending average headlight range 200 feet further down the road. Halogen light is whiter, which also aids visibility.

The filament in a halogen bulb is thinner and burns hotter. They are called halogen bulbs because of the gas mixture used to fill the glass; halogen plus krypton, argon and/or nitrogen. The gas mixture conducts heat away from the filament to prevent it from burning out. Halogen helps redeposit microscopic particles of tungsten that boil off the filament back onto the filament. This extends filament life and prevents bulb darkening with age.

How do you know when a sensor needs to be replaced

The vehicle will usually exhibit a driveability problem (hard starting, stalling, hesitation, poor mileage, high emissions, etc.) and/or an illuminated check engine light. Many things other than a bad sensor can cause driveability problems, but a check engine light is a good indication that the problem is in the electronics.

Mileage is another consideration. The oxygen sensor should go 50,000 miles or more, but some fail in as little as 30,000 miles. Other sensors should last the life of the vehicle. All are covered under the vehicle manufacturer's five year/50,000 miles emissions warranty.

Troubleshooting sensor problems requires checking the on-board diagnostics to see if the computer has set a trouble code (see chart) corresponding to one of the sensor circuits. This is done by either putting the computer into a special diagnostic mode and counting check engine "flashes" or special diagnostic LEDs on the computer itself (many import applications), or by plugging a special scan tool into the diagnostic connector to access on-board diagnostics.

The latter is the preferred technique because it also allows you to read sensor voltages and inputs directly on most GM and some Ford and Chrysler systems.

A trouble code does not necessarily mean a sensor is bad. However. It only means a problem has been detected in a particular sensor circuit. It could be the sensor, the wiring, or a connector somewhere in the wiring harness.

To isolate the fault, a series of diagnostic tests usually have to be performed, following a step-by-step procedure. Tests may require the use of a breakout box that allows individual circuits to be tested. By checking continuity, resistance and/or voltage readings, the faulty component can be isolated.

Another approach is to use a tester that simulates voltage, resistance or frequency inputs from various sensors. The tester is used in place of a sensor to produce a substitute signal. If the on-board computer then responds properly, the sensor is assumed to be faulty.

Intermittent faults are the hardest to find, and some sensor problems may not generate a trouble code at all. A technician may have to test drive the car with a portable "flight recorder" plugged into the on-board computer system in order to locate the problem. When the problem occurs, pressing a button records the various sensor readings for later analysis.

How does the oxygen sensor work




This is the only sensor that makes its own voltage. The voltage signal is proportional to the amount of unburned oxygen in the exhaust. When hot (at least 600 degrees |F), the zirconium dioxide element in the sensor's tip produces a voltage signal that varies according to the difference in oxygen content between exhaust and outside air.

The higher the concentration of unburned oxygen in the exhaust, the lower the differential across the sensor tip and the lower the sensor's voltage output. The sensor's output ranges from 0.1 volts (lean) to 0.9 volts (rich). A perfectly balanced (stoichiometric) fuel mixture of 14.7:1, gives a reading of around 0.5 volts.

Some O2 sensors have three wires and an internal heating element to help the sensor reach operating temperature more quickly. The heater also keeps the sensor from cooling off when the engine is idling.

An O2 sensor's normal life span is about 30,000 to 50,000 miles. Sensors can fail prematurely if they become clogged with carbon, or are contaminated by lead from leaded gasoline or solvents from the wrong type of RTV silicone sealer.

As the sensor ages, it becomes sluggish. When the signal starts to lag behind changes in the exhaust, or becomes static, the engine experiences driveability problems (loss of power, rough idle, poor fuel mileage, or excessive emissions).

Sensor accuracy can also be affected by air leaks in the intake or exhaust manifold, or even a fouled spark plug. A misfiring plug allows unburned oxygen to pass through into the exhaust, causing the O2 sensor to give a false lean indication.

Most aftermarket replacement oxygen sensors are of a universal design which means some wire splicing may be necessary during installation. Graphite anti-seize compound should be used on sensor threads unless they are precoated. The rubber boot that fits over the sensor should not be pushed down further than half an inch from the sensor's base.

Some vehicles are equipped with a different type of O2 sensor that has a titania rather than zirconia element. Instead of generating its own voltage signal, a titania O2 sensor changes resistance as the air/fuel ratio goes from rich to lean. Instead of a gradual change, it switches from low resistance (less than 1,000 ohms) when the mixture is rich, to high resistance (over 20,000 ohms) when the mixture is lean.

The engine computer supplies a base reference voltage of approximately one volt to the titania O2 sensor, and then reads the voltage flowing through the sensor to monitor the air/fuel ratio.

When the fuel mixture is rich, resistance in a titania sensor will be low so the voltage signal will be high (close to 1.0 volt). When the fuel mixture is lean, resistance increases and the voltage signal drops down to about 0.1 volt.

Compared to the more common zirconia O2 sensors, titania sensors have three advantages: (1) they don't need an air reference (there is no internal venting to the outside atmosphere to plug up); (2) they have a fast warm-up time (about 15 seconds); and (3) they work at lower exhaust temperatures (they won't cool off at idle and they can be located further downstream from the engine or used with turbochargers).

You'll find titania O2 sensors in '86 and later Nissan 300ZX and Stanza 4WD wagons, '87 and up Nissan Maxima and Sentra models, and 1986-1/2 and up Nissan D21 trucks. Chrysler also uses them on the Jeep Cherokee and Wrangler (because of the sensor's ability to handle off-road driving through water), and the Eagle Summit.

What size battery is needed

A battery should be big enough to allow reliable cold starting. The standard recommendation is a battery

with at least one Cold Cranking Amp (CCA) for every cubic inch of engine displacement (two for diesels). CCA rating is an indication of a battery's ability to deliver a sustained amp output at a specified temperature.

Specifically, it is how many amps a new, fully-charged battery can deliver at 0 degrees F for 30 seconds and still maintain a minimum voltage of 1.2 volts per cell.

A rule of thumb says a vehicle's battery should have a CCA rating equal to or greater than engine displacement in cubic inches. A battery with a 280 CCA rating would be more than adequate for a 135 cubic inch four-cylinder engine, but not big enough for a 350 cubic inch V-8.

Battery manufacturers have been trying to outdo one another by introducing batteries with higher and higher cold cranking amp ratings. There was a time when a battery with a 550 CCA rating was considered a powerful battery. Now there are batteries with 650, 750, 850, and even up to 1,000 CCA available.

One reason for the "amp wars" between battery manufacturers is that bigger is definitely better. How much overkill is really necessary to assure reliable cold weather starting? Two amps per cubic inch of engine displacement? Three, four or five amps? The bottom line is bigger sells better.

The difference between a group 23 battery and a group 24 battery is 1/2" in length, 1/16" in width and 7/16" in height. It does not sound like much, but it is enough of a difference that the longer battery might not fit the space provided for the shorter battery if a swap were attempted.

Since there is little or no effort on the part of vehicle manufacturers to standardize original equipment battery dimensions, aftermarket battery suppliers are faced with the task of trying to cram as many amps as they can into the smallest battery case that will fit the most applications.

Consolidation reduces the number of different batteries a jobber has to stock to cover the various vehicle applications. It also simplifies manufacturing by building fewer basic battery sizes.

The most powerful battery in the world will not be able to do its job properly if battery cables are not up to the job. One often overlooked source of cranking trouble is undersized battery cables. If the original equipment cables have been replaced with cheap ones with undersized wires, the cables may not be able to deliver the battery's full amp load to the starter.

Which engine sensors are the most important

All sensors are important. The computer is the brains of a computerized engine control system and sensors are its link to what's happening under the hood.

Some sensors have more influence on engine performance than others. These include the coolant temperature sensor, oxygen sensor, throttle position sensor, and manifold absolute pressure sensor.

The coolant sensor is often called the master sensor because the computer uses its input to regulate many other functions, including:


  • Activating and deactivating the Early Fuel Evaporation (EFE) system such as the electric heating grid under carburetor or the thermactor air system.

  • Open/closed loop feedback control of the air/fuel mixture. The system won't go into closed loop until the engine is warm.

  • Start up fuel enrichment on fuel-injected engines, which the computer varies according to whether the engine is warm or cold.

  • Spark advance and retard. Spark advance is often limited until the engine reaches normal operating temperature.

  • EGR flow, which is blocked while the engine is cold to improve driveability.

  • Canister purge, which does not occur until the engine is warm.

  • Throttle kicker or idle speed.

  • Transmission torque converter clutch lockup.

The coolant sensor is usually located on the head or intake manifold where it screws into the water jacket. Sensors come in two basic varieties: variable resistor sensors called thermistors because their electrical resistance changes with temperature, and on/off switches, which work like a conventional temperature sending unit or electric cooling fan thermostat by closing or opening at a preset temperature.

Variable resistor coolant sensors provide the computer with a more accurate indication of actual engine temperature than a simple temperature switch. The computer feeds the sensor a fixed reference voltage of about five volts when the key is on.

The resistance in the sensor is high when cold and drops about 300 ohms for every degree Fahrenheit as the sensor warms up. This alters the return voltage signal back to the computer which the computer then reads to determine engine temperature.

The switch-type sensor may be designed to remain closed within a certain temperature range, or to open only when the engine is warm. Switch-type coolant sensors can be found on GM "T" car minimum function systems, Ford MCU, and Chrysler Lean Burn systems.

Because of the coolant sensor's central role in triggering many engine functions, a faulty sensor (or sensor circuit) can cause a variety of cold performance problems. The most common symptom is failure of the system to go into closed loop once the engine is warm. Other symptoms include poor cold idle, stalling, cold hesitation or stumble, and/or poor fuel mileage.

The oxygen sensor (O2) measures how much unburned oxygen is in the exhaust. The computer uses this as an indication of how rich or lean the fuel mixture is so adjustments can be made to keep it properly balanced.

A problem with the O2 sensor will prevent the computer from keeping the fuel mixture balanced under changing driving conditions, allowing the mixture to run rich or lean.

The throttle position sensor (TPS) is used with feedback carburetion and electronic fuel injection (EFI) to inform the computer about the rate of throttle opening and relative throttle position. A separate idle switch and/or wide open throttle (WOT) switch may also be used to signal the computer when these throttle positions exist.

The throttle position sensor may be mounted externally on the throttle shaft (the case on most fuel injection throttle bodies), or internally in the carburetor (as in Rochester Varajet, Dualjet and Quadrajet).

The TPS is essentially a variable resistor that changes resistance as the throttle opens. It is the electronic equivalent of a mechanical accelerator pump. By signaling the computer when the throttle opens, the computer enriches the fuel mixture to maintain proper air/fuel ratio.

Initial TPS setting is critical because the voltage signal the computer receives tells it the exact position of the throttle. Initial adjustment must be set as close as possible to factory specs. Most specs are given to the nearest hundredth of a volt.

The classic symptom of a defective or misadjusted TPS is hesitation or stumble during acceleration. The fuel mixture leans out because the computer doesn't receive the right signal telling it to add fuel as the throttle opens. The oxygen sensor feedback circuit will eventually provide the necessary information, but not quickly enough to prevent the engine from stumbling.

When the sensor is replaced, it must be adjusted to the specified reference voltage. The TPS on most remanufactured carburetors is preset at the factory to an average setting for the majority of applications the carburetor fits. Even so, the TPS should be reset to the specific application upon which it is installed.

MAP sensor function is to sense air pressure or vacuum in the intake manifold. The computer uses this input as an indication of engine load when adjusting air/fuel mixture and spark timing. Computerized engine control systems that do not use a MAP sensor rely on throttle position and air sensor input to determine engine load.

Under low-load, high-vacuum conditions, the computer leans the fuel mixture and advances spark timing for better fuel economy. Under high-load, low-vacuum conditions (turbo boost, for example), the computer enriches the fuel mixture and retards timing to prevent detonation.

The MAP sensor serves as the electronic equivalent of both a distributor vacuum advance diaphragm and a carburetor power valve.

The MAP sensor reads vacuum and pressure through a hose connected to the intake manifold. A pressure sensitive ceramic or silicon element and electronic circuit in the sensor generates a voltage signal that changes in direct proportion to pressure.

MAP sensors should not be confused with VAC (Vacuum) sensors, DPS (Differential Pressure sensors), or BARO or BP (Barometric Pressure) sensors. A vacuum sensor (same as a differential pressure sensor) reads the difference between manifold vacuum and atmospheric pressure (the difference in air pressure above and below the throttle plate). A VAC sensor is sometimes used instead of a MAP sensor to sense engine load.

A MAP sensor measures manifold air pressure against a precalibrated absolute (reference) pressure. What's the difference? A vacuum sensor only reads the difference in pressure, not absolute pressure, so it doesn't take into account changes in barometric (atmospheric) pressure.

A separate BARO sensor is usually needed with a vacuum sensor to compensate for changes in altitude and barometric pressure. Some early Ford EEC-III and EEC-IV systems have a combination barometric pressure/MAP sensor called a BMAP sensor, combining both functions.

Anything interfering with accurate sensor input can upset both fuel mixture and ignition timing. Problems with the MAP sensor itself, grounds or opens in the sensor wiring circuit, and/or vacuum leaks in the intake manifold.

Typical driveability symptoms include detonation due to too much spark advance and a lean fuel ratio, and loss of power and/or fuel economy due to retarded timing and an excessively rich fuel ratio.

A vacuum leak can cause a MAP sensor to indicate low manifold vacuum, causing the computer to think the engine is under more load than it really is. Consequently, timing is retarded and the fuel mixture is enriched.

Why are there no returns on electrical and electronic parts

Too many people have tried to take advantage of the system. Instead of using proper diagnostic procedures, some people (mostly do-it-yourselfers, but also some so-called professionals) resort to trial-and-error parts swapping when they don't know how else to fix an electrical problem. When parts they have installed do not fix their problem, they want to return them and try something else.

Electrical/electronic parts are easily damaged by improper installation or testing. Because electronics are very sensitive to voltage overloads, it does not take much of a voltage spike to ruin a component.

Unplugging a wiring connector while the key is still on can create a momentary voltage surge of hundreds of volts. Crossing up the wrong wires or using the wrong test procedures can also damage sensitive electronics. You have no way of knowing whether or not the part has been used or damaged.

Because of such risks, many jobbers refuse to allow returns on any electronic components. This may seem unfair to some customers, but it protects the next customer who might get a bad part that had been returned.

Most jobbers will allow returns or exchanges on rebuilt starters and alternators if there is a problem with the unit.



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