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sprinkles08
sprinkles08, ASE Certified Technician
Category: UK Car
Satisfied Customers: 21570
Experience:  ASE Master and Advanced level certified. Factory trained with 15 years dealership experience
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I have a 2008 Smart fortwo coupe. I have fitted a new alternator

Customer Question

I have a 2008 Smart fortwo coupe. I have fitted a new alternator and battery but the battery goes completely flat if the vehicle is left unattended for over 48 hours. All lights switched off and doors locked
JA: Sometimes things that you think will be really complicated end up being easy to fix with Mercedes. The Mercedes Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so he can help you best.
Customer: Have fitted a new alternator and battery but the battery still goes flat after 48 hours if left unattended.No warning lights when engine running so I assume it is charging OK
JA: The mechanics have tons of experience in that area. Do you happen to have the code?
Customer: No
JA: Not a problem. You can work with the Mercedes Mechanic to figure that out. Are you hoping to fix this yourself?
Customer: yes
JA: Great! What have you tried so far?
Customer: Recharged the battery and then left the car unattended
JA: Is there anything else the Mercedes Mechanic should be aware of about your Mercedes?
Customer: 2008 Smart Coupe does this have a diagnostic system and an electronic control unit. If so It is beyond my capability to solve the problem I have only just bought the car and it is under warranty
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: 6 months ago.
Category: UK Car
Expert:  sprinkles08 replied 6 months ago.
Hello and welcome to JustAnswer!If the battery is good yet is dying while the vehicle sits it is because a component is drawing current when all components should be asleep. The problem may be a light coming on or staying on, if you have an aftermarket radio or amp that may be a factor, one of the most common causes of a battery draw is a module staying awake and drawing current.What needs to be done is an ignition off draw test. An ammeter will need to be set up between the disconnected negative battery cable and it's post. After about five minutes if the draw exceeds about 40 mA then the draw is excessive. At that point fuses can be removed one at a time to see when the draw drops, and then components on that circuit can be concentrated on.

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