Alternator voltage goes down 12.6 when rpms go up and goes up 14.3 when at idle
Thread Starter
Joined: Aug 2016
Posts: 60
Likes: 15
ML350, E550 Coupe, 53 F100, 458 Italia, 458 Spyder
Alternator voltage goes down 12.6 when rpms go up and goes up 14.3 when at idle
I have a strange issue with a 2012 E550 coupe AMG package. I have had the alternator / regulator and battery tested all was good. All of this information is with the running lights on the AC with the fan on high, phone charging, obd2 dongle, radar detector and the radio on. I throw no codes.
Yet the alternator /regulator voltage goes down 12.6 when rpms go up and the voltage goes up briefly to 14.3 when I coast or at idle. On a hot day say 90 degrees the alternator only puts out 12.5-12.6 volts as I go down the highway with everything on in the car.. yet after the car cools down in the evening the alternator / regulator will provide 14.5 volts to the system.. All of this completely opposite in the way the system is suppose to work. I read some where that the alternator output is adjusted depending on the temperature is that true and if so is this running as it should? Is it the regulator that adjust for temperature or is there another device?
Yet the alternator /regulator voltage goes down 12.6 when rpms go up and the voltage goes up briefly to 14.3 when I coast or at idle. On a hot day say 90 degrees the alternator only puts out 12.5-12.6 volts as I go down the highway with everything on in the car.. yet after the car cools down in the evening the alternator / regulator will provide 14.5 volts to the system.. All of this completely opposite in the way the system is suppose to work. I read some where that the alternator output is adjusted depending on the temperature is that true and if so is this running as it should? Is it the regulator that adjust for temperature or is there another device?
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Joined: May 2018
Posts: 6,600
Likes: 6,547
From: Jakarta-Indonesia
2014 - W212.065 - E400 ( M276.820, 3 liter Turbo) RWD not Hybrid
Did you put the aircond blower to MAXIMUM high ?
By choosing maximum high, the alternator will go full power and 13.8V to 14.3V charging.
If you are already on max high for aircond blower, the next step is to verify if your alternator can pump out the demand you are asking out of it.
I can tell you how to do that, but you need a 200amps or higher current clamp.
I am sure your alternator is 180amps minimum, however you are in a 4 season country and have lots of heaters.
Me get 200amps alternator but me in tropical country, so car has no heater other than rear windshield one and my alternator on average works under 60% of its rating even with maximum radiator fan speed.
By choosing maximum high, the alternator will go full power and 13.8V to 14.3V charging.
If you are already on max high for aircond blower, the next step is to verify if your alternator can pump out the demand you are asking out of it.
I can tell you how to do that, but you need a 200amps or higher current clamp.
I am sure your alternator is 180amps minimum, however you are in a 4 season country and have lots of heaters.
Me get 200amps alternator but me in tropical country, so car has no heater other than rear windshield one and my alternator on average works under 60% of its rating even with maximum radiator fan speed.
MBWorld Fanatic!
Joined: Jan 2009
Posts: 9,224
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From: V E G A S
1922 Ford Model T / no OBD
Sounds like normally working "smart charging" system, but I thought they come to US in 2014.
What is your location?
Run datacard to confirm what system you have
What is your location?
Run datacard to confirm what system you have
Last edited by kajtek1; May 24, 2022 at 01:09 AM.
Out Of Control!!
Joined: Apr 2019
Posts: 11,921
Likes: 6,775
From: Silicon Valley
W212 MY'14 M276-3.5NA @75kMi
Smart ALT supplies consumers + charges Batts...
You've discovered the ALT voltage regulator can delivers a dynamic voltage.
ALT charges both onboard batteries plus supplies 100% consumers at all time.
ALT output is controlled by the ECU acting upon distributed sensors.
You can set the Instrument Cluster to display the "Maintenance menu" while driving. It will show you battery Amp+Volt data. From that you can guess the ALT output.
> SYSTEM PASS:
What you should see is the voltage will switch from 14.9v all the way down to 12.6V when the battery charge current gets low. This is indicative of a fully charged battery or resting voltage.
At that point the ALT will keep suplying 100% of the load at 12.6v or greater to lower the overall consumer current load.
The ultimate proof your car is charging well is seeing the battery already fully charged will always keep a low charge current (<1Amp) under any stable alternator voltage.
> SYSTEM FAIL:
What you wish not to see while driving are sky-high battery charge (>30... 50... 90Amps!) currents and low alternator voltages (< 12.3... 12.0...11v). Usually you get a combination of both.
That's evidence of bad alternator control by the ECU. Frequent battery replacement and random error messages serve as witness of disrupted electricals - Hopefully not your VIN.
✌️
ALT charges both onboard batteries plus supplies 100% consumers at all time.
ALT output is controlled by the ECU acting upon distributed sensors.
You can set the Instrument Cluster to display the "Maintenance menu" while driving. It will show you battery Amp+Volt data. From that you can guess the ALT output.
> SYSTEM PASS:
What you should see is the voltage will switch from 14.9v all the way down to 12.6V when the battery charge current gets low. This is indicative of a fully charged battery or resting voltage.
At that point the ALT will keep suplying 100% of the load at 12.6v or greater to lower the overall consumer current load.
The ultimate proof your car is charging well is seeing the battery already fully charged will always keep a low charge current (<1Amp) under any stable alternator voltage.
> SYSTEM FAIL:
What you wish not to see while driving are sky-high battery charge (>30... 50... 90Amps!) currents and low alternator voltages (< 12.3... 12.0...11v). Usually you get a combination of both.
That's evidence of bad alternator control by the ECU. Frequent battery replacement and random error messages serve as witness of disrupted electricals - Hopefully not your VIN.
✌️
Last edited by CaliBenzDriver; May 26, 2022 at 12:20 AM.
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Joined: Oct 2010
Posts: 5,141
Likes: 1,293
From: Southern US
2010 E550, 273 Engine: 2012 S550, 278 Engine
I have a strange issue with a 2012 E550 coupe AMG package. I have had the alternator / regulator and battery tested all was good. All of this information is with the running lights on the AC with the fan on high, phone charging, obd2 dongle, radar detector and the radio on. I throw no codes.
Yet the alternator /regulator voltage goes down 12.6 when rpms go up and the voltage goes up briefly to 14.3 when I coast or at idle. On a hot day say 90 degrees the alternator only puts out 12.5-12.6 volts as I go down the highway with everything on in the car.. yet after the car cools down in the evening the alternator / regulator will provide 14.5 volts to the system.. All of this completely opposite in the way the system is suppose to work. I read some where that the alternator output is adjusted depending on the temperature is that true and if so is this running as it should? Is it the regulator that adjust for temperature or is there another device?
Yet the alternator /regulator voltage goes down 12.6 when rpms go up and the voltage goes up briefly to 14.3 when I coast or at idle. On a hot day say 90 degrees the alternator only puts out 12.5-12.6 volts as I go down the highway with everything on in the car.. yet after the car cools down in the evening the alternator / regulator will provide 14.5 volts to the system.. All of this completely opposite in the way the system is suppose to work. I read some where that the alternator output is adjusted depending on the temperature is that true and if so is this running as it should? Is it the regulator that adjust for temperature or is there another device?
