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Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 53941
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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Marty, my 2004 S430 has a bad ignition. The "accessory"

Customer Question

Marty, my 2004 S430 has a bad ignition. The "accessory" settings stay on all the time, draining the battery (over 24-48 hours, depending on outside temperatures). This started about a year ago. I installed a "kill" switch behind the fuel filler door. I switch it to "off" every night, and to "on" when I want to leave. Everything has worked fine until yesterday.
(Yesterday, everything worked fine except the heater blower. It did not come on at all. Now, this car has glitches once in awhile so I figured it would come on later, which it did.)
I have to have my car emission tested (I'm in Ontario) to renew registration. So I have to leave the computer on for about 48 hours. So last night I did NOT throw the "kill" switch, leaving the circuit to the computer open. But in order not to run the battery down, I connected a trickle charger. It ran all night.
This morning there was no life at all. No dash or interior lights, no locks, etc. The key wouldn't turn in the ignition switch. I opened the trunk with the stub and jump started with a booster battery. That worked fine. (Even the heater blower came on again)
I put the multimeter on the car battery with the engine running. It read 12.85V. I don't think that's enough, is it? It used to read at least 13.50V. Is there a problem with the alternator? The nearest MB shop is 60 miles away, either in London, ON or Troy, MI. I have most of my repair work done by Michael's in Fort Gratiot. They're excellent and fair, but they admit to not knowing everything about MB and won't touch some things.
Right now, everything runs fine, except that there's a malfunction with the ESP which comes on a twice a year and which I ignore because "it just goes away again." But what bothers me is the 12.65-12.85V output reading (at the battery terminals).
Do I have a charging problem?
Submitted: 7 months ago.
Category: Mercedes
Expert:  Marty replied 7 months ago.

Hi James, I don't think you have a problem with the alternator. Connect your voltmeter to the battery with the car off and measure the voltage then start the car and hold the rpm at 2000 rpm and measure the voltage. If the voltage increases when you start the motor your alternator is probably fine. Recharging the battery will probably improve your voltage reading. With the battery in the trunk you often read lower voltage at the battery than the alternator is actually producing.

Expert:  Marty replied 7 months ago.

Just checking in. Do you need anymore help with this?

Expert:  Marty replied 7 months ago.

Looks like you are not going to reply or rate my service.