High Charging Voltage Seems to have killed my Battery - PeachParts Mercedes-Benz Forum

Captain Scott · #1 12-19-2005, 08:00 PM

My battery (not sure of the vintage) stopped holding a charge the other day. After charging it up and then testing it, I found it was really toast. 2 seconds after hitting the key switch, voltage would go to almost zero and the engine would barely turn. Yep, a real dead battery.

So I bought a new battery and hooked it up. On start, the voltage climbs to the high 14s as expected. But when the RPMs increase, so does the battery voltage. It goes up to almost 16v. Now this is a brand new battery and has a full charge already so it should not be taking such a high charge for so long after starting.

So my charging system is not working properly. The alternator is definitley putting out juice because the voltage raises after start. But the regulation system is not holding the voltage back to proper levels and seems to be the culprit.

Can someone guide me here? Is the voltage regulator the only culprit based on this condition? Or is there anything else to check out before I start spending money?

Scott

Strife · #2 12-19-2005, 08:40 PM

It certainly sounds like the regulator, although it could possibly be a bad ground from the battery to the body of the car - possible to be bad in an older car. Regulators don't work very well under very light loads (like the battery not really connected) In any event, find and fix this ASAP, this is not good for any electronics in the car, even more expensive than a battery, and I don't know at exactly what voltage the OVP relay (if your car has one) goes off.

BTW, "high 14's" seems high. 13.8-14.5 would be the range that I would expect on a normal system. The regulators I've replaced in MB alternators recently were marked 14 volts.

Captain Scott · #3 12-20-2005, 09:38 PM

OK, I think the VR may be the culprit too. But I'm still questioning the high voltage. I mean if the VR is bad or going bad in the alternator, then why the heck would it be putting out 16v when taching up around 3000?????? When silicone components start to die, they usually put out low ot no voltage. Whaddauthink?

Strife · #4 12-21-2005, 02:22 AM

This is not unusual - it can go up or down depending on if the silicon or the brushes fail first (don't forget, there are diodes in the alternator also). It seems from my reading that on these alternators the brushes generally go first.

Captain Scott · #5 12-21-2005, 10:37 AM

OK, so I'll pull the old reg tonight to inspect the brushes and comutator and then get the new VR is on the way. I'll start there and assume the actual alternator is still good since it puts out voltage. It doesn't make any noise that I can tell...
S

G-Benz · #6 12-23-2005, 01:46 PM

As far as a bad VR putting out excessive voltage:

The VR does just that...regulate voltage. The alternator will dish out all the charging amps it can muster, so the VR needs to temper that when conditions don't call for such a charging load.

When the VR went out on my W124 alternator, suphuric acid vapors were venting out of the battery!! You could smell it (and see it) while driving!

When I measured voltage at the battery, it was 17.5V!!!

Captain Scott · #7 01-01-2006, 08:51 PM

I should have checked the alternator thoroughly before ordering the VR.
It has an integrated diode package tht cannot be changed...

Rats. Now I get to support Fedex by returning the un-needed VR and buying a new alt.

S