I replaced the Battery/alternator in my 93 300SE in January. The car has not been driven much since that time because of the new W210. Last week I got her out to prepare her for the market block. The day after detailing her, I come out and the battery is completely dead. I charged it over night, and started it up to check the alternator. At idle it was 13.8, thats solid. I drove it around town all day with no trouble. The problem only occurs on the highway. After cruising for about ten minutes, the idiot light comes on, Radio/AC/Cruise control cut out and the Battery light flickers off and on until I park. It starts right back up, and seems fine around town at low speed.

My guess is either a short in the battery, or the Alternator over charging at high RPM. Any ideas?

Over charging would be more in the range of 14.5 to 14.8, I think 13.8 is fine. Unless, like you said, it's overcharging at higher RPM. I would try to hook up cables and measure the voltage while cruising.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

How would I do that? Especially with the battery in the trunk? All I've got is a little voltmeter.

Last time I felt an overcharged battery, it was quite warm.

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

The first time I noticed the idiot lights coming on while driving down the highway I pulled over and shut it off so I could check the battery with a voltmeter. The battery was hissing, and I noticed it had leaked a bit of acid out of the top.

Sorry, I should have mentioned that in the 1st post.

a bad voltage regulator?

No, I haven't even considered the voltage regulator. How would I determine if it's gone bad?

You can measure your voltage through the cig lighter socket. You just need a plug to plug into it and a volt meter. But I am skeptical that you have a overcharging problem. I don't have that car so I don't have the tribal knowledge on it. Does it have an OVP? Those cause a lot of trouble. It is interesting that you say the radio cuts out. That could suggest a bad connection somewhere. As for the idiot light that usually indicates a bad alternator, not an over-charging one.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Just as a test I drove her around town for about 30 mins today with the AC on and the radio playing with no trouble at all. Then I got on interstate 35 and cruised for about ten minutes before the battery light came on. The AC and radio didn't kick off but the Cruise Control cut out when the light came on.

After getting home and parking the battery checked in at 12.5 volts. Does this sound like a voltage regulator problem?

It's driving me absolutely crazy.

the battery was hissing and throwing oudt fluid tells me that too much energy was being pumped into it as you drove. Go to Radio Shack and get a plug that will fit in the cigar lighter with wires hanging off of it. Get a voltmeter and connect to the wires and watch the voltage as you drive at high speed. If it goes higher than about 14 volts, your regulator is not doing its job of keeping the charging voltage under control.

You either need to get a volt meter or a mechanic who understands electrical systems. As for normal voltage, as high as 14.4V is considered normal. A lead acid battery can produce some gas during normal charging depending on how much charge it is taking.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Alright, I bought a cheap little cig lighter voltmeter from Autozone today. It's not the most accurate thing in the world, but it did show me what was happening. During regular around town driving, it read anywhere from 13-14.3 volts. Once I got her on the highway the Battery light began going on and off, and every time the Battery light was on, the meter read anywhere from 16-19 volts.

I guessing this is for sure the voltage regulator right?

Anyone know if I can get to the regulator w/o dropping the alternator? I'll look when I get home, but man I hate getting to the alternator in that car.

This is why I said to measure the voltage when the problem is occurring - anything else is just conjecture and a waste of time. Now you have established that it is definitely overcharging. Sounds like a voltage regulator issue to me. I'm not familiar with the inner workings of the W140 VR, maybe some of the experts here can speak up.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

Pulled the stupid alternator out this afternoon. Took it in to Autozone and it was in fact WAY overdoing it. The guy said it put out a peak of 21V on their machine. Luckily the freaking thing was only 5 months old and will be replaced free of charge.

I must have pulled the alternator out of that car at least 5 times in the 3 years I've owned it, and I never seem to remember that you have to pull the smog pump to get it out. As wonderful to work on as the W140s are, Mercedes sure made the alternator, and even worse the starter, a royal pain to get to.

She should be good to go by Saturday afternoon once the new alternator arrives. Don't suppose anyone's interested in purchasing a 93 300SE on the cheap......