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Lou P. master tech
Lou P. master tech, Mercedes Technician
Category: Mercedes
Satisfied Customers: 4954
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 Mercedes SL500, which I owned years now. When I

Customer Question

I have a Mercedes SL500, which I owned for two years now. When I bought it, there were evident signs that the front battery and possibly the rear too, had previously exploded before I bought the car, indicating gross overcharging. I was always suspiious about the charging system until recently, I had to replace the rear battery which my electrician said I "boiled over". A few days later, the smell of burning was evident and the dash reading told me to go to the nearest electrician. Result completely burnt regulator. A new one was installed and system checked. Less than a week later, while driving, the smell of boiling sulphuric acid indicated to me the battery was over-charging and very hot to the touch. For me this could be faulty earths, but it could mean much more. Other Merc 500 owners have had similar faults. I don't want to burn my alternator, or battery, or sell the car. Could weak earth connections (as these cars are notorious for rusting) be the cause of the problems? If I drive very slowly, nothing much happens which I notice, but as soon as I raise the revs for a VERY SHORY WHILE, maybe 30 seconds, I already feel the smell of overcharging. When these happened, I had nothing on, no radio, no headlamps, but the AC was on in both cases. Can you help please? ANTON Malta.
JA: Battery issues can be tough to diagnose - lots of things to consider. Luckily, the mechanic I'm going to connect you with knows all the tricks and shortcuts. Are you hoping to fix this yourself? No, I use streaming services like Pandora.
Customer: I have worked on cars for thirty years or so, mostly preparation for racing, that is after first qualifying in electronics. Unfortunately now I am too old to "get out and get under" as in the old days. I have tipped my electricians about this problem of weak earths, but they seem to want to think that they are wiser than me.
JA: Is there anything else the Mercedes Mechanic should be aware of about your Mercedes?
Customer: No, not really. Once it was caught in a flood, but since complete dismantling and drying out literally every nut and bolt, it has worked fine, only that as I mentioned earlier, on being driven on the highway, the starting battery literally exploded, but that was before the flood. No problems after until a few months ago. This happened as it was driven overland from England to Malta.
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. My name is ***** ***** I think I can help you with this. Before we go looking for grounds (earths) I would like to know if you have a meter that we can check the voltage of the battery with. Also there is a way to see the voltage in the dash. If you close all the doors and make sure your dash screen is set to the odometer screen you can turn the ignition to the first click and quickly press the center dashboard reset knob 3 times fast. It will show you the voltage of the system. Then take it for a drive and watch the voltage and tell me how high it gets. That would be the first thing I want to know. Also what year is your SL? Thanks ! Lou P