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Discussion Starter · #1 ·
Hi all,

As the title says, the battery in my 09 300C awd will hold a charge, but there doesnt appear to be any power power getting to it, and it gradually loses charge and dies. I went ahead and threw in a brand new alternator, but the battery voltage still declines until dead when the car is running. Is there a stand alone voltage regulator in this car? Any other ideas?
 

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2006 CRD Touring
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If your battery is the original, then it could be due for replacement. Typically, batteries have a life of 5 to 6 yrs. Although you have replaced the alternator, this may not have been correctly installed; it has been known for the control field connector to be left unconnected after replacement, so I would check both control and output terminals are OK.

If your battery is flat, then try re-charging using an external charger. When fully charged and with a light load you should see 12.6V on a test meter. Anything less could indicate a faulty battery.

Once you have checked the battery charge is OK, switch on the ignition, but before cranking check the battery volts again using the EVIC hidden menu: http://www.300cforums.com/forums/tu...instructions-screens-meanings.html#post647558

Check that your battery volts is 12.6V and then crank the engine. The volts will then drop to 10 or less until the engine fires.

Once the engine is running. the volts should climb to about 14.5 and the battery warning light should extinguish if the alternator is working correctly.

If you have an OBD2 code reader, scan for fault codes. There are specific codes relating to battery/alternator faults.
 

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Discussion Starter · #3 ·
Just swapped in a new battery, car reads 11.8 when running, not the 14.6 it usually does. Double checked that the alternator had all three wires plugged in. Oddly enough, I am getting no warning lights. Is there any way to test the feed current to the alternator before dropping the cash on a new PCM?
 

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It does seem that your alternator isn't working, but it should it should bring up the warning light. You could try measuring the alternator output voltage directly with the engine running, if that can be done safely and also the volts at the feed (control) terminal. I don't know if your power system is similar to that in my Diesel, but my system incorporates two regulators; one in the alternator and another in the ECU. Before throwing any more money at the problem it might be worth having a diagnosis by an auto electrician.
 

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Discussion Starter · #5 ·
Turns out it was the PCM. I wonder what caused the failure. I use a diablosport and a HUD that plugs into the OBD2 port, maybe a transient voltage from one of those damaged the PCM. One of those things I guess I'll never know for sure. Those PCMs are way pricey, if this happens again Im just going to convert it to a stand alone voltage regulator
 
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I don't think it would be a good idea to remove the PCM control function from the alternator. The PCM will try to balance alternator output against varying electrical load conditions and might throw up a fault code if the control loop is broken. It also looks as if the PCM is performing "intelligent" alternator management; for instance, when accelerating under maximum power, the alternator output could be reduced to minimise engine braking effects.
 

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Battery light been coming on and off over the last few months but volts on the dash didn’t change but finally the other day it did it all again battery light on the dash come on and the volts jumped up to 18-19volts on the dash and everything went haywire dash lights went out amps for the back stereo and lights went all funny now the limo stuck at the work shop with a new alternator and ECU saying it’s charging 14.8 volts battery lights on the dash and the alternator reading 0volts
 

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Turns out it was the PCM. I wonder what caused the failure. I use a diablosport and a HUD that plugs into the OBD2 port, maybe a transient voltage from one of those damaged the PCM. One of those things I guess I'll never know for sure. Those PCMs are way pricey, if this happens again Im just going to convert it to a stand alone voltage regulator
I just swapped my pcm after replacing battery and the alternator still getting 11.8 while car is running. Checked all the grounds as well. I got a plug and play PCM. Did you buy a new one from a dealer ?
 

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I am having the same problem. Tried a new alternator, still getting 11 volts with the motor running. Just did a motor overhaul, so not sure if something else is off. Tried a new aftermarket PCM, didn't help. Running out of ideas. Technician has check codes and doesn't know what to try next. 2006 SRT8. Also have a new battery, checked fuses. What else can I try? NAPA tested the alternator and the battery and both seem good.
 
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