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Ask Michael Warfield Your Own Question
Michael Warfield
Michael Warfield, Factory Mercedes-Benz Technician
Category: Mercedes
Satisfied Customers: 3130
Experience:  Mercedes-Benz Master Certified technician
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I have a 2005 mercedes E 320.I get the service light on with

Customer Question

I have a 2005 mercedes E 320.I get the service light on with the picture of the battery and sometimes when I drive the rpms go high and the transmission does not shift, I turn the engine off and I restart the car and everything is normal again. I had the transmission checked and they said it is the engine, the message shows low voltage I replace the battery and still the same.someone said it might be the voltage regulator or the battery control unit.would you please help me diagnose this problem
Submitted: 23 days ago.
Category: Mercedes
Expert:  Michael Warfield replied 23 days ago.

Hi, my name is ***** ***** will be happy to assist you today. The only reason the red battery warning would come on while driving is because the battery voltage fell below it's threshold. Thus the battery control unit sends a signal to the engine computer to raise the engine rpm. A faulty voltage regulator will cause this problem but usually by the time the regulator is failing, the whole alternator needs to be replaced. The alternator can be tested but it would only test bad when the problem was actually happening. This could be related to the transmission shifting problem if the voltage falls very low but typically it is a separate problem. What is common with this model are for speed sensors inside the transmission to go bad and cause "limp" mode which won't allow the transmission to upshift. The person that checked the transmission may not have had a more sophisticated diagnostic scanner that can read all of the control units related to this problem. Cheap lower level scanners usually only read the engine. I suggest taking it somewhere or to someone that can actually read fault codes out of the transmission and battery control unit computers. Let me know what other information you may need before you decide to rate me, thank you!

Customer: replied 22 days ago.
I had my car checked with Aamco transmissions do yo think they are sophisticated enough to diagnose this problem?
bot***** *****ne shold I change the alternator or Shold I go to another place for a better diagnosis.
Customer: replied 22 days ago.
If he is not available for sooner call the list he can do is tell me in case the transmissions censors have to be replaced
Is it going to be as costly as rebuilding the transmission. Thanks for a response back.
Customer: replied 22 days ago.
Please answer my question I paid for it.
Expert:  Michael Warfield replied 22 days ago.

Hello, sorry I am not available weekday mornings or afternoons because I run a Mercedes shop and it is how I maintain my expertise in the field. I don't think Aamco can diagnose the alternator and/or battery issue. Aamco notoriously tries to sell transmission rebuilds, I would not let them if they offer. I would only have them replace the sensor on the conductor plate. They usually can diagnose transmission issues like these because they are so common. I was suggesting you get the battery warning diagnosed first because there is a small chance that it is related. If Aamco has found transmission sensor fault codes, these are completely separate from any battery/alternator issue though. I would trust Aamco with the sensor repair, but not the battery warning or a trans. rebuild for that matter.