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Lou P. master tech
Lou P. master tech, Mercedes Technician
Category: Mercedes
Satisfied Customers: 4959
Experience:  15 years master certified 100% fix rate and have access to factory radio codes, wiring, and instructions.
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I have a 2002 c240. After driving the car an hour the

Customer Question

i have a 2002 c240. After driving the car for about an hour the battery light comes on. I checked the output from the alternator and it is 14.2.
JA: Have you tested the battery? Have you tried testing the alternator?
Customer: Yes the battery and alternator are in perfect condition.
JA: Are you hoping to fix this yourself? What have you tried so far?
Customer: I changed the alternator and serpentine belt and tensioner.
JA: Anything else you think the mechanic should know?
Customer: Heat seems to play a factor.
JA: OK. Got it. I'm sending you to a secure page on JustAnswer so you can place the $5 fully-refundable deposit now. While you're filling out that form, I'll tell the Mercedes Mechanic about your situation and then connect you two.
Submitted: 1 year ago.
Category: Mercedes
Expert:  Lou P. master tech replied 1 year ago.
Hello. Lou P here. When you checked the voltage did you have the meter hooked up to watch the voltage as the fault occurred ?
Customer: replied 1 year ago.
I checked when the battery light was on. It was 14.2.
Expert:  Lou P. master tech replied 1 year ago.
Ok I meant hook up the meter at start and watch the voltage closely and see if it goes under 13 or over 14.5. You can also watch the voltage through the service menu in the cluster.
Customer: replied 1 year ago.
It takes about an hour before the service light goes on. That is why I think it is heat related. I can check it tomorrow.
Expert:  Lou P. master tech replied 1 year ago.

ok, so here is how you can actually access the voltage screen in the dash

the video has a 211 e class but its the same method for yours. you have to start on the main mileage odometer screen and make sure the doors are closed.