Question about 2001 Mercedes-Benz C-Class

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Seek a advise for undervoltage/alternator or battery

I get this message - 'under voltage/alternator or battery, switch consumers off'. The battery just changed a couple moths ago.

I am not sure the alternator issues or battery. Please advise. Thanks.

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Measure the battery when the car is off, it should read just a little over 12V. With the car running, measure the battery again and it should read between 13V-14V+-.2V.... odds are the voltage is below 13V if that's the warning you get and the alternator is not charging properly

Posted on Sep 08, 2009

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2nd newly rebuilt alternator burning out.

Due to the nature of the battery technology used with vehicles the alternator is mostly incapable of charging the battery. The car alternator is designed to keep a fully charged battery fully charged and to provide all the power for the car equipment.

The alternator charge rate is regulated by a voltage regulator. Because the alternator output is connected to the battery, the alternator and battery voltage will be the same and the voltage regulator monitors that voltage.

The lower the battery voltage the more output the alternator will produce in order to correct the situation but because a lead acid battery has a high internal resistance to accepting a charge the terminal voltage will quickly rise to the alternator regulated voltage and fool the alternator into thinking the battery is fully charged when the output will drop to the order of just a couple of amps.

Switch on the headlights or a similar load that will lower the battery voltage and the alternator will increase it's output again - but only by the amount of current the headlamps or other load is consuming.
It matters not what the alternator rated maximum output is, it is designed to provide only the necessary current and no more.

The only time an alternator should ever need to produce maximum output is when on a dedicated testbed and then only for a short duration to avoid damaging the unit. Testing the current output on a modern vehicle is not recommended except for the regulated voltage testing and a rule-of-thumb output test where all equipment is switched on and the engine speed raised while the battery voltage is monitored.

Most modern alternators use an internal voltage regulator but a few systems use a separate voltage regulator. No alternator rebuild would be complete without a regulator test and probably a new or replacement regulator, which is where the majority of charging system problems are, or the brush gear.
Assuming the wiring is ok, no alternator should suffer any harm if the voltage regulator and auxilliary diodes (if fitted) are in good order though fitting a defective or a discharged battery can cause it to overheat and be damaged.

The alternator usually just about stops producing an output when the battery voltage is in the region of 14.5/14.8 volts.
Your description indicates the voltage regulator is not working correctly - unless 40 amps was being consumed by the car equipment the alternator should not have been producing 40 amps.. I suggest you also have your battery tested

May 12, 2017 | 1988 Acura Legend

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P1447 Diagnostic Module Tank Leakage (DM-TL) Pump Too High During Switching

Hi there:DTC P1447 - Diagnostic Module Tank Leakage (DM-TL) Pump Too High During Switching.

I suspect the main problem is the alternator. When the alternator is failing it can be intermittent (ok some times and then stops). The battery on a vehicle is there to turn the starter and as a buffer for the engines electrical system. When the alternator stops charging with the engine running the overall voltage of the electrical system cannot be handled by the battery and systems will start to shut down like the radio and wipers until the vehicle stalls. Then in some cases you can still restart, the alternator kicks back in and everything looks normal.If you are familiar with checking voltages, to check this with a volt meter, you would want to see 13 volts to 14.5 with the engine running (alternator charging). If you see less than 13 volts the alternator is not charging. If you want to make sure you can have the faults read at the dealer and they should see undervoltage faults that have set when the alternator had stopped charging. Also, to check yourself, when the problem is occuring you can use a volt meter to check the voltage at the jump terminal to see what the output of the alternator is. This would even set the ****** malfunction due to an undervoltage and the system would go into failsafe.

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Mar 05, 2012 | BMW 325 Cars & Trucks

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Alternator put in and the car won't start and dead new battery'

I am taking it you have a new battery. You put on a alternator. Now you need to charge the battery until it is fully charged. That way the car will run for a while before it stalls from undervoltage. With the battery charged check the voltage at the battery. It should be 12 to 13 volts. Now start the car and check the voltage it should be up to almost 14 volts. If that is good check with lights and heater blower on should be at least 13.5 volts. If the voltage does not increase you either have a bad alternator or it is not getting excited by the ignition switch on the vehicle. So you should find the wire that energizes the alternator and turn the key on it should have 12 volts on it. Hope all this helps. please rate my answer so I get points.

Jan 12, 2012 | 1995 Jeep Cherokee

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I got a c-class mercedes and on my dash board today appeared UNDERVOLTAGE CONSUMER DEFECTIVE. what can i do?

Check your battery and voltge when car is running if les than 12 when off than battery is flat, if less than 14 when running then it's not charging like it should.
If its charging ok and battery under 12 you need to replace battery.
If its not charging ok than it's alternator- Hope i helped!

Jan 25, 2011 | 2001 Mercedes-Benz C-Class

1 Answer



Apr 16, 2010 | 2003 Mercedes-Benz C-Class

1 Answer

2003 C320 - 'Battery under voltage, switch consumers off'

when the vehicle voltage supply gets low, the car will shut down high current consumers (seat heaters, climate control fan, etc) until the voltage stabilizes. you might have a battery going dead, or the alternator might be wiggin out and not charging at times or when it gets hot. There have been many issues with the "alternator voltage regulator" mounted on the back of the alternator, failing completely or failing intermittently and under load. How old is your battery? Have you had to jump start the car recently?

Mar 15, 2009 | 2001 Mercedes-Benz C-Class

2 Answers


Sounds to me like a voltage regulator issue. This is attached to the alternator and monitors how much voltage is sent back to the battery. Any decent parts store should be able to hook up a charging system tool-most common is a Vat-40 and check the system. And they will probably do it for free. If there is an issue with the charging system, then try the voltage regulator first, as opposed to the alternator-there is quite a price difference I assure you. Let me know what you find.

Nov 18, 2008 | 2001 Mercedes-Benz C-Class

1 Answer

2001 C240 ''Undervoltage Consumer Defective''

The same thing happened to me and it turned out to be the alternator. Need to replace your alternator. Easy check, start the car and check the DC voltage of the battery, with the car running you should read around 14 volts. If you are reading 12-12.25 volts then the alternator is not generating.

Nov 18, 2008 | 2001 Mercedes-Benz C-Class

3 Answers

Undervoltage consumer defective

Regarding the first problem, there is possibly an oil level sensor issue or vehicle version coding issue(both can be more fully checked at a dealership). The second problem is not uncommon. Obviously, there could be a low voltage if the vehicle sits for a long time or is not driven alot. Sounds like the ac system lost synchronization due to lack of voltage. With vehicle running, you should be able to press and hold recirc/rest or recirc/defrost buttons on ac control panel(can't quite remember which combination). Press and hold until leds start flashing, then the ac system will go through a resynch phase. If it completes this and the led's stop flashing, then it's ok. If it does not and the led's flash very quickly, this indicates an actual malfunction of ac system adjustment motors.

Nov 06, 2008 | 2001 Mercedes-Benz C-Class

1 Answer


i have the same problem with my 2001 C240 and my a/c does not work properly as a result.

Oct 31, 2008 | 2001 Mercedes-Benz C-Class

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