Battery venting excessively in 300E.
 

Got into car at work this afternoon and started it. Idle was suddenly down to 500 rpm but didn't stall. Radio was on but audio signal to speakers would shut down, then reactivate moments later. Pulled car over and shut down. Restarted and idle was back to normal, though radio was still doing its thing.

Engine and radio operated normally on the highway and the rest of the trip, but when I got back to city driving, I could smell what seemed to be battery acid. Pulled over and found acid vapors venting out of a vent hole on the side of the battery case. Continued home.

Got home, waited for car to cool down. Pulled vent caps off and noticed one cell was almost dry. Refilled with distilled water and retested.

Engine and radio were fine, but battery continued venting. Could hear what sounded like electrolyte boiling inside the case. Checked OVP fuse and fuses in box and none were blown.

My sudden fear is that the alternator is overcharging. What's going on??? Any ideas?

Either your voltage regulator has died or your battery is shot. I'd vote for the battery being overage -- they tend to lose their capacity and take too much current without actually charging. The low voltage radio dropout (lack of sound) and low rpm sounds a lot like a dead battery to me. When I bought my 300TE I had to jump it three times on the way home, wouldn't idle, the SRS and ABS lights stayed on, etc. New battery fixed it.

Were you headlights too bright? Overcharging actually means too much voltage -- you can check that with a standard cheap voltmeter -- just check the voltage across the battery with the engine on. Normal is 12.5 to 15 volts, any more and the regulator is shot.

These cars have a complicated voltage and current control system, not just a simple regulator like the old ones, and I've not played with that yet.

Any reputable mechanic and many autoparts stores can test this for you.

Don't buy a cheap battery.....!

Peter

Sounds like a battery problem to me.

It is a high resistance internal circuit, cracked , corroded , whatever. As it gives or receives current it is heated and boils the electrolyte.

An interesting test can locate/prove the actual cell. If you take two six inch lengths of welding rod (Bare coathanger wire will do - remember bare; coathangers are heavily coated with paint) and clip your volt meter laeds to them. Then place both wires into ajacent cells (into the electrolyte but not touching the plates). The voltage should be over 2 volts. The voltages should be the same for all cells.

The last 2.5 volts will be shared by the two end readings (from cell to battery terminal. One side will read something like 1.5v and the other .75 (haven't done this in a long time. You will eventually come up with seven readings and combine the two end readings to get six cell readings of over 2v.

Now the interesting part. Turn the head lights on and if the one cell gasses test the readings of it. You will probably see some real goofy readings with the polarity reversing at the cell with the high resistance. All the other cells should look the same, only slightly reduced in voltage if the lights work and current passes.

Buy a reverse terminal Optima battery. #R34 These batteries can not leak,even when shot with a gun. This will eliminate corrosion problems under the battery plate.
I put one in my 560 SEC and it works great.

John

Tested voltage at the battery with the engine running this morning and it reads 17.5 volts!!!

So I had to leave the 300E at home and take the ML to work.

Unless some elves replaced all of my courtesy lights with halogen bulbs, it seems like a overvoltage problem.

Engine eletricals seems to be doing some sort of cyclical switching as the idle revs up, a click is heard and then the idle goes down, another click, idle goes back up, etc. etc...

I turned on the headlights which were extremely bright (again with the elves), then back to normal as the engine "click" was heard, matching the cycles occurring with the engine.

I will have to test the cells this evening, but I sure hope it's just the battery. It is a good quality unit and was purchased in the fall of 1999.

Sorry, the battery can't cause overvoltage problems.

I refered to the battery thinking that only one cell was gassing. With that voltage they all should be gassing.

BTW after repairing the alternator or regulator, you should be sure the fuse in the overvoltage protection relay isn't blown. Its job is to protect the control units from these kinds of voltages.

The OVP relay must have been the "click" I was hearing this morning as the idle would surge, then die down. Fuse in OVP not blown yet.

Guess I'm doing an alternator swap this weekend. FastLane price is very reasonable, and Phil was very helpful in pointing me in the right direction.

Get the alternator checked before you just replace it -- it may just need a new voltage regulator/bush holder, which is MUCH cheaper. You definitely have an overvoltage problem! Don't drive it til you fix it, or you will fry the electronics and lamps!

Peter

I guessed that all I needed too, but here's the deal:

I have replaced just in the year 2001, water pump, radiator, radiator cap, thermostat, aux fan sensor, headlight switch, OVP, 2 sets of brake pads, one tire, a taillight housing, assorted dash bulbs, a rear window regulator assembly, and of course labor charges for some, as well as routine oil changes, and other fluid replenishments.

Replacement parts on my other two MBs: A rear window and a plastic console latch for the SL, and some 3M double stick tape for the mud flaps on the ML. No mechanical or electrical fixes whatsoever! Granted, they are newer and under warranty, but as much as I like my W124, I am starting to wonder why I bought it at all?

I figure the alternator will probably go south anyway given its age, so why not just bite the bullet and get it done now? FastLane has a rebuild for under $140, so I bought that, and a new serpentine belt, since the current one is showing signs of cracking.

After that, I guess I'll just sit and wait for the head gasket to blow next...

Sounds like my Volvo!

Check for a voltage control relay -- I know this is not in the alternator, so I suspect that there is more to voltage control on some of these cars than just the internal regulator. I will check next time I get to stop in and yak with my parts supplier, but that may be a day or two.

Unless the alternator is also making noise, I'd go for a regulator, but $140 is decent for a rebuilt one -- I spent $110 to have used parts put in the one in my 220D this year -- solved my hard start problem.

Sounds like someone didn't use MB coolant if the rad was gone -- get some in there, or at least phosphate free orange stuff. The phosphate type corrosion inhibitors eat aluminum (heads and rads!). The GM stuff appears to make something very similar to sand from the silicates they used, avoid it. See the various threads on coolant, very informative!

Peter

Haynes wiring diagram doesn't show any exotic relays or control units managing the charging system. So I am pretty sure the regulator is the culprit.

As far as the radiator, it had MB coolant. Broke at the plastic hose neck, like they all do.

Starting the dismantling process in anticipation of the alternator showing up at the end of the week. I hope to be up and running by Monday.

Yeah, the plastic neck is one of the Volvo things, too -- mine broke in the driveway while I was messing about with the injection pump, fortunately. My sister's let go on the interstate -- mine was $350 after screwing around for a week trying to get a tank on it -- the rad was shot, too, after the new tank sealed. Sis's was $750 -- had to get a rebuilt head, hers was rather badly warped.

I suspect that is what happened to my 87 300D, too -- brand new rad and what appears to be a cracked head...!

I recommend anyone with an orignal plastic rad to try shaking the upper hose connection hard periodically, in the hopes it will break while cold rather than on the highway -- no way to shut the engine off fast enough to save the head!

Peter

Gbenz, Take pics and Document it.

Alon

G- Benz:

I second Alon's suggestion - it would make a nice addition to the site DIY section at the top of the page!

Gotta borrow a digi-cam this weekend...

If I screw this one up REALLY BAD, then you will have to wait for pics taken while a REAL mechanic does it...it will be titled:

"How to do a complete electrical system overhaul and belt replacement when the idiot owner thought he could do this one himself HA HA HA"!

Footnote: This is the same G-Benz that disassembled his rear calipers in an effort to replace the pads!!! :rolleyes: