Mercedes Alternator and Mercedes Starter:
Understanding Your Mercedes' Electrical System

Today's auto electrical systems are getting more intricate and are stressing the limits of current technology but they are basically the same design as 30 years ago. What can today's do-it-yourselfer do to keep from being electrically-challenged in the middle of nowhere? A quick overview of your Mercedes' electrical system would be a good start. The major components of your Mercedes' electrical system are explained below, along with some troubleshooting tips.

Your Mercedes Battery

The battery is a storage device, currently 12 volts, used to start the engine and help operate the electrical accessories installed on your Mercedes. The battery consists of six cells of stacked positive and negative lead plates, separated by insulators and immersed in electrolyte, which is a water and sulfuric acid mixture. Each of the six cells "produces" 2.1 volts for a total of 12.6 volts (although it's actually stored energy, not produced).

The chemical reaction created between the lead plates and the electrolyte creates dangerous, explosive gases that vent through the battery cover vents. Care should always be taken when charging or jump starting low batteries and whenever working under the hood of the car. Always keep the battery clean to allow proper ventilation.

Some batteries use a gel to replace the electrolyte for a somewhat safer battery and the use of "maintenance-free" batteries has eliminated the need for refilling the electrolyte. However, all batteries lose charging capacity over time through the loss of the electrolyte, deterioration of the plates and chemical breakdown of the connections.

Whenever your Mercedes exhibits symptoms of a charging or starting system failure, the most basic test that should be performed first is a visual and voltage test of the battery. Knowing the exact condition of the battery is the best way to know whether or not to suspect other system components and can prevent the unnecessary installation of a starter or alternator, which cannot repair the vehicle until the battery is up to snuff.

To analyze the condition of a battery, very little high-tech equipment is really needed. While there is excellent equipment available for big $$$$, all that is really needed is a good DVOM (Digital Volt Ohm Meter), a battery charger, some patience and a little common sense.

A quick way to tell if a battery needs recharging is by observing the "eye" on the battery and being able to determine what the color of the eye is telling you. Below is a chart of various battery manufacturers that use a colored eye, as well as what the different colors mean:

Before replacing any battery due to a failure, have the battery and the car's electrical system tested for proper functioning. Replacing a dead battery only to have another a few weeks later due to a weak alternator or a parasitic drain on the system is not only frustrating but can be expensive and dangerous as well.

An average battery should last 3-5 years in average climates but those in high heat or extreme cold areas may last only 2-3 years. The replacement battery should always have the same or higher CCA rating (cold cranking amps) as the original and be of the same or compatible "group size" to fit the battery tray and cable connections.

Your Mercedes Alternator

The alternator produces electricity used to maintain battery storage charge and to help run all the electrical accessories, including the ignition and the engine control systems. It is belt-driven by the engine and produces an alternating current (AC), which is converted internally to 12 volts direct current (DC) by the diode bridge or rectifiers.

AC current cannot be stored but is much more efficiently produced, which is why cars no longer use generators but use alternators and convert the electricity to DC. Most alternators now use internal voltage regulators to maintain the proper system voltage, from 12.6-14.5 volts. You should check your Mercedes' repair manual or with your local dealer to obtain the exact proper voltage for your Mercedes.

Contrary to popular belief, an alternator does not constantly produce electricity. It cycles on and off as demand goes up and down. The battery buffers it from the electrical demands of the car, and it only works enough to maintain system voltage. At peak efficiency and for longest life, the alternator should be charging no more than about 50% of the time.

However, with the demands placed on the system by heat and by extraneous electrical devices (i.e., high powered stereos, running lights, etc. - see Power Demand Chart below), the alternator and battery are stressed even further. The average alternator now is lucky to last 3-4 years, which is why a prematurely dead battery may actually be caused by a dead alternator.

Typical Power Demands on a car's charging system are as follows:

And this doesn't include amperage requirements for items like cell phones, laptops, CD players, boom boxes or additional lights.

Understanding the importance of having an operational warning light for the alternator (idiot light) is crucial to catching problems early. In most modern systems, the electrical current passing through the filament of the warning light is what energizes a circuit in the alternator to start charging.

This signal light is linked directly to the alternator through its terminal (#1, I, L, D+, etc. depending on the brand) and functions slightly differently on different manufacturers.

To check the warning light circuit, turn the ignition switch to the "on" position without cranking or starting the engine; if the idiot light does not come on, remove the plug from the alternator and ground the wire that terminates to the #1, I, L or D+ terminal (depending on manufacturer). If the light comes on, the wiring is okay but the alternator is defective; if the light still does not come on, the wiring to the light circuit and the bulb should be checked.

Don't forget to check the fuse that controls the light circuit, too. This fuse could be labeled differently in various cars. It could be labeled "charging", "regulator", "meters", "gauges" or "engine". In some cars, if the fuse is out, the idiot light will come on but may not go off. In others (like GM), a burned out fuse may make the warning light work in reverse order; that is, when the key is on, the light is off but as soon as the engine starts and the alternator starts charging, the light will come on.

Checking out these simple circuits first can greatly reduce your troubleshooting time and unnecessary replacement of your alternator.

As we delve further into charge light indicators, we find that in some cases it is normal for the charge indicator light to come on when nothing is wrong with the alternator.

According to information published by GM, any car may have a low voltage reading or lights that dim when electrical loads are heavy at idle. Furthermore, this condition is normal and no repairs should be attempted unless a fault has been found.

For clarification, as a car idles for extended periods of time during high heat conditions, a number of things happen that contribute to "lowered" alternator output that coincides with physics and the design of the alternator:

1.  As heat within the alternator increases, the electrical resistance in the alternator also increases, which reduces the alternator's charging capacity.

2.  As temperature rises, the voltage setpoint of the regulator is lowered to reduce the chance of overcharging or "boiling" the battery.

3.  Newer designed alternators have a "delay/soft start" built into the regulator circuit. This delays the load being placed upon the engine when starting up from a stop, so that the smaller engines in use today are not loaded down upon acceleration due to charging demands. This can delay the charging by up to 15 seconds.

With the alternator's capacity for charging reduced by heat and other factors, an alternator may only be able to produce up to 70% of its rated output under these conditions. So an alternator rated for 100 amps may only be able to produce 70 amps when hot at idle when there is 77 or more amps of demand on it.

If it can be considered normal for warning lights to glow while a healthy alternator is running, how do you know if the alternator is really good or if there are other problems lurking around?

A thorough diagnosis is always the best route to determining whether or not the alternator is at fault, but there are times when diagnosis time is short and you still need a positive identification of the problem. Cases like this require a foolproof tool to speed things up. In the case of Delco CS series alternators, there is a tool available from Kent-Moore tools (J-41450-B), which isolates the alternator from the car's wiring harness and lets you see if the alternator is at fault or if there is a wiring problem elsewhere within the car's wiring harness. The best thing about this tool, besides being compact and handheld, is that it doesn't require any interpretation of data by the operator. The little light on the unit lights or doesn't light depending upon whether the alternator is good or not.

Unfortunately, while this is a great little piece of equipment and there are other similar tools available, once you determine that the alternator is not the culprit, you still have to fix the electrical problem.

Your Mercedes Starter

The other major electrical component in your Mercedes' electrical system is used only a few times a day but is the single largest power user and most critical to your Mercedes' operation - its starter. The starter is simply a DC motor that turns the engine crankshaft through the flywheel, starting the combustion process by creating compression within the cylinders. Voltage to the starter is supplied directly from the battery and is controlled by a relay and/or solenoid operated from the key switch inside your Mercedes.

Starters can be of varying types and designs - gear-reduction types for higher torque, permanent-magnet types to reduce size and weight, or just plain, old-fashioned heavy starters. But whatever the type, they all function in the same basic way.

A slow cranking engine may be a sign of a bad starter and with age, that's more and more likely. But on most cars today, it's due to low battery voltage, poor electrical connections at the battery or a failed relay or fusible link.

Most starters will easily outlast a new vehicle warranty if it's not overused, if good connections are maintained and if it's not overheated through dirt and grime buildup.

Starting your Mercedes with the major components turned off (like the AC compressor, blower motor and high-powered stereos) will greatly ease the load on the starter. In fact, most new cars have "lock out" relays that will not allow the AC compressor and alternator to turn on until after the vehicle has been started. But turning these power-hogs off before shutting off your Mercedes is always a good precaution.

Although the starter drive, or "Bendix" as it was commonly referred to, can be replaced separately from the starter assembly, it's rarely recommended anymore. Failure of any part is due to age, usage and heat stress, to which the entire starter has also been subjected, so that other parts are just as old and stressed. It's quite common to replace the starter drive only to have to buy another starter in a few months because the brushes wore out, a magnet broke, the solenoid failed, among other common mishaps. Replace the starter as a unit and have the electrical system checked at the same time to prevent further problems.

Other Troubleshooting & Maintenance Tips
For Your Mercedes' Electrical System

Your Mercedes' electrical system should be completely checked and tested every two years or whenever serviced for any type of driveability problem. Many problems associated with day-to-day driveability are caused by voltage variations and must be the first step in troubleshooting any problem. This is due to the use of computerized controls in most cars these days and even quite minor voltage changes can alter the controls.

Your Mercedes' electrical system must be load tested to certain standards, which can be simulated by turning on all the accessories and lights for simple voltage drain but that is not an all-inclusive test. Measuring circuit loads with an ammeter, circuit voltage drops with a DVOM, variable circuit load testing, etc. is the only way to fully check function. With electrical systems operating at 80%-100% of capacity nowadays (see Power Demand Chart), it is crucial that it be up to standards.

The average do-it-yourselfer would have little need to purchase the more critical test equipment, so if voltmeter testing doesn't pinpoint the problem, get a thorough checkup done from an auto electrical technician who knows your Mercedes' system.

A complete and thorough test involves much more than sticking a voltmeter on the battery and the average do-it-yourselfer does not have the test equipment nor does he/she need it. You should allow a professional to do this test. As a good starting point, though, most major chain auto parts stores offer a free service test (in the hopes of selling you a battery or alternator) that should be good enough to notify you of any major problems. Most early problems start from poor electrical connections due to loose connections and/or buildup of corrosion, especially at the battery posts. Keep that battery clean!

A Few Important Things to Remember

Heed these tips and you're well on your way to extending the life of your Mercedes' electrical system components:

Tip #1: Always keep your battery and its connections clean to avoid clogged battery cover vents and overtaxing your starter. This will also allow for proper ventilation of dangerous, explosive gases from your battery.

Tip #2: When replacing your battery, always buy one of the same or higher CCA rating (cold cranking amps) as the original battery and make sure it's the same or compatible "group size" to fit your battery tray and cable connections.

Tip #3: Due to the varying nature of car electrical systems, never jump start your Mercedes using another car that is running. Use the other vehicle's battery power alone to start it because a 14.5 volt running system (i.e., GM) can seriously damage a 12.6 volt system (i.e., BMW) due to the overvoltage.

Tip #4: Start your Mercedes with the major electrical hogs turned off - A/C, stereo, etc. - to ease the load on your battery and starter and extend their lives.

Tip #5: Have your Mercedes' electrical system completely checked and tested every two years or whenever you have it serviced for any type of driveability problem.

Your Mercedes Electrical System Maintenance Shopping List

Here is a list of electrical system parts to check and maintain, plus some common tools, testers and spare parts to keep in your trunk or garage:

You'll find everything you need for Mercedes Benz models like:

Mercedes 190E parts - Mercedes 240D parts - Mercedes 300E parts - Mercedes SL parts - Mercedes 450SL parts - Mercedes 350SL parts - Mercedes ML320 parts - SLK 230 parts - C230 parts - and More

Don't Forget:

Repair articles are added regularly. Come back often to check for new maintenance topics.

These repair tips are designed only as a starting point. Please seek the assistance of a professional Mercedes mechanic for all repair problems beyond your capabilities.

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