Rotten Egg Smell - PeachParts Mercedes-Benz Forum

racebannon · #1 03-16-2009, 02:51 PM

I have an '83 300D. Replaced the voltage regulator in Nov. and the battery in Jan. I took a longish drive yesterday and the battery started emiting steam and a sulfer smell. I checked voltage across the battery terminals about 45 minutes after getting home. The reading was 13.8 with the engine off and it never got above 14V with the engine running at high RPMs - all normal as far as I know.

The steam and smell are still happening today. After about 5 miles drive, I checked the voltage without stopping the engine. The reading was 14.8 - 14.9V with the engine at idle. It climbed over 18V at high RPMs.

I do notice the headlights dim when I let off the pedal at night.

What is the highest acceptable voltage reading when the engine is reving?
Does this sound like the voltage regulator has already gone bad?
Are there any other recommended tests?

kerry · #2 03-16-2009, 02:54 PM

18V is too high. Shouldn't me more than 14.x volts. Sounds like the voltage regulator has gone south.

racebannon · #3 03-16-2009, 02:56 PM

Is there anything that would cause the regulator to go bad in about 6 months?

bd1308 · #4 03-16-2009, 04:58 PM

In the VW aircooled world (where I came from) the new or rebuilt units Bosch puts out are junk....like i had to buy 6 or 7 of them to get one good working one!

rrgrassi · #5 03-16-2009, 05:43 PM

The rotten egg smell is the battery being overcharged. You will boil it dry if not corrected, as well as shorten the expexted life of the battery. You can also damage other electrical equipment as well, like the CCU from over voltage.

JonL · #6 03-16-2009, 06:31 PM

Years ago I saw a 280E with a bad regulator fry the car's radio. One of the electrolytic capacitors blew it's can off forcing the top cover of the radio to pop up. It was very difficult to get the radio out of the console.

Aside from the obviously bad regulator, you should check the battery and ground connections. If you've got an intermittent connection in the charging circuit, the alternator will try to put out full voltage, and the surges from the intermittent circuit will be tough on the regulator. Could also conceivably be a bad battery, maybe a loose plate or some junk inside causing a partial/intermittent short or open in one or more cells.

racebannon · #7 03-17-2009, 09:55 AM

OK. So it is overcharging. I replaced the voltage regulator last fall.
Is there anything I should look for that would have caused the voltage regulator to go bad so quickly?

SD Blue · #8 03-17-2009, 10:12 AM

Poor grounding might be causing voltage regulator problems. Check Batt.-chassis and chassis-engine grounding but even more importantly, check the grounding of the regulator-to-alternator. Mine developed some corrosion on the regulator grounding tab and caused charging problems. After removing the corrosion with a brass brush and applying a very small dab of conductive grease, where they touch, I haven't seen any further problems. (Caution: excessive conductive grease could liquify and short things out so use very sparingly.)

racebannon · #9 03-20-2009, 03:47 PM

It's real muddy in my driveway, so I have not inspected the voltage regulator yet, but I did a little more poking around:
I get about 14V from the positive to negative battery terminals.
I get the same from the battery positive to the alternator body, engine block etc.
When I go from the positive to the bolt for the battery ground, I get nothing - is that normal?

I also checked resistance from the negative battery terminal to the engine block - it jumped around a bit, but settled down to less than an ohm (seemed to depend on where I put the probe). Also had about .5 ohms to the alternator body, but with less jumping around.

I suppose all this is pointing to the regulator still - thoughts?

JonL · #10 03-20-2009, 05:51 PM

Not sure what you mean by the "bolt for the battery ground," but if the bolt is in contact with the ground strap or negative battery post, or engine block, or body... you should see 14V. Actually, 14V is very high for a battery if the engine is not running... it has a high residual charge from the overcharging or your voltmeter isn't calibrated properly.

There should be no measurable resistance from the negative terminal to the block or alternator body. .5 to 1.0 ohms is too high and might indicate corroded battery terminals or bad cables/ground straps. If they are highly corroded, they'll get hot when charging and the resistance may go up even more. This could be your problem, although I suspect there is something else wrong also. You should start by correcting these high resistances, because troubleshooting will be futile until you do.

SD Blue · #11 03-21-2009, 09:56 AM

With the resistance from chassis to engine, I would be suspicious of the ground strap. For example, mine was attached to an exhaust clamp, which was loose enough to cause very intermittent starting and charging problems.

Also, a loose/corroded ground can cause the regulator or alternator to fail. You mentioned a reading of 14Vdc, try your VOM on Vac and if significant, may show a diode failure in the alternator.

racebannon · #12 03-23-2009, 09:14 AM

Thanks for all the advice...
I cleaned the battery ground strap and while the bolt was out, tested continutiy - OL - so the bolt does not conduct electricity. The sort of high ohm readings I was getting before are in part due to my meter - probe to probe I get about .3 ohms.
I have been monitoring the voltage in the cabin via my meter hooked up to the cigarette lighter - when the car has been sitting overnight, the voltage starts a little oger 12, goes up to about 14 - 15 for a couple miles, then jumps to 18 or just over 18 (does not continue to climb). If I'm idling at a stop the voltage comes back to a reasonable 14 ish volts.

I did look at the regulator and found that it had cracks on both tabs - I did not have a new one to replace it with, so I'll leave it in there for now - the new one should arrive today.
So obviously, I screwed up the install of the voltage regulator...
Has this happened to any one else out there?
Any advice on how to not screw it up this time?

SD Blue · #13 03-23-2009, 09:54 AM

Of course, the first thing is to disconnect the battery. Hey, I know it sounds obvious but we all run the risk of being too eager and forgetting to do something simple.

When installing, insert the brushes first and slightly pull down on the body of the regulator, causing the brush springs to slightly compress. Then insert the body of the regulator into the housing. Start the screws and before tightening them, loosen the belt tension with the adjustment bolt and turn the alternator by hand to ensure the brushes aren't binding. Then tighten the regulator screws, tighten the belt adjustment and reconnecting your battery to wrap things up.

racebannon · #14 03-26-2009, 08:39 PM

Thank you to those that gave advice.

After the local parts store screwed up and did not actually order the voltage reg a week ago last monday, I ordered a new one online (so much for supporting local business). I just got it in (in the rain (fun)) and I am no longer overcharging the battery - never went above 14V. The regulator I pulled was less than 6 months old and I still don't quite understand what I did wrong last fall, but the left side tab (engine side) was cracked between the screw and the round middle part.

I also got a voltage gauge so I can monitor the charging system from the cockpit.

I must say - I'm really liking this forum - there is always something interesting to read about and or research.

Thanks again.

Skippy · #15 03-26-2009, 10:05 PM

I had the same problem on my 240D recently. I managed to break one of the tabs off the replacement voltage regulator while trying to install it. I then superglued it back on (no, seriously I did) and used some zip ties to hold the brushes down while I reinstalled. Then I just cut the zip ties and pulled them out. So far, so good.