Charging Light at Speed & very High VAC readings... - PeachParts Mercedes-Benz Forum

atomix8 · #1 07-03-2010, 11:41 AM

My 240D has starting lighting up the charging light dimly at idle, and sometimes with bright bursts on the highway. The light primarily comes on when the headlights are run (when I'd be pulling the most charging demand.)

The battery does appear to be getting warm on long drives, and may have boiled over slightly from this (or from hard cornering?). There is a small bit of battery acid in the battery tray.

1. I've checked and tightened the alternator/fan belt, put on belt dressing and I'm pretty convinced it's not slipping.
(edit) 1.5. Checked battery-chassis and engine-chassis grounds and resistance, both good. (Mercedes loctites these! Gotlieb Daimler would be proud)
2. No dimming of the lights at idle, so I believe I have plenty of current at idle.
3. Battery measures ~14 volts when cars is off. This seems high.
4. Alternator/voltage regulator seems to be charging to 14.2V when car is at idle and reved to a steady speed.

Here's where it gets weird. Switching the voltmeter to VAC...

5. Car off, I get a reading of ~27.5 VAC. WHAT?! How's that possible?
6. Car on, VAC reading creeps up, like the alternator is charging the battery with AC voltage...to over 30 VAC.

So, where do I go from here?
--Bad voltmeter?
--Bad regulator (is 14.2 too high?)?
--$200 alternator...hopefully not this option!

I've searched, and my case seems 'special'
http://www.peachparts.com/shopforum/sitemap/t-173574.html

Biodiesel300TD · #2 07-03-2010, 11:44 AM

I had the same issue a while back on my wagon. I could barely see it in the daylight, but at night it was much easier to see. I replaced the alternator and the issue went away. It could also be the voltage regulator.

#3 07-03-2010, 11:52 AM

I would probably start with the voltage regulator; but it does sound like an alternator issue, maybe a bad diode.

leathermang · #4 07-03-2010, 12:34 PM

There is also the possibility of a decrease in the proper values in the charging light sensor circuit... so it is good to make lots of checks before throwing parts at something like this... although except for the expense... it is nice to have those new parts on there..

layback40 · #5 07-03-2010, 11:19 PM

Remove and clean the earth strap connections. You cant use a resistance reading to test how good connections are as it is done at very low currents (miliamps). Make sure all the maim connections on the back of the alt are clean and tight. likewise on terminal blocks etc all the way to the battery. I have had this sort of problem on 2 cars, the second time, I replaced the main wire from the back of the alt to the battery. It fixed the problem. As the weather warms & things get hot & dry out corrosion on connections can be a PITA.
I dont like the sound of your test meter readings, if you know some one with an old analog test meter (needle on gauge type) try it. It would be worth letting one of the stores test your alt for you for free.

Diesel911 · #6 07-04-2010, 01:06 AM

From my notes from another forum:
ALTERNATOR DIODE check with volt meter
With your meter across the battery, on Volts DC, you should read 12.5 - 14VDC

If not, set your meter to Volts AC and you should have a reading of ~100mVAC.
If the VAC is too high, then look for a rectifying diode failure. If VAC is OK but the VDC is not correct, then the problem is with the voltage regulator.
ALTERNATOR REGULATOR charging rate chart

I assume the above test is done with the Car Runing.

Remove the Regulator and take a look at the Brushes for Brusnes that are 2mm or smaller and Brushes with Oil on them. Note if the Screws that Hold it in were tight.

Take a good look at the connector to see if there is any looseness or corrosion that would cause an intermittant contact. Is the Connector Renainting Spring OK?

You could remove the Alternator and have it tested for free.

You also might wat to do an internet search for what happens when your Battery gets old or has a defect inside.

atomix8 · #7 07-04-2010, 10:10 AM

Appreciate all the good ideas...

All ground connections look visually good, although the alternator cable seems a little rough. Where does the other end go? Looks like through the firewall and then maybe into the main wiring harness somewhere?

Layback, I have a digital multimeter, but it's a cheap craftsman one. I don't trust it's readings of VAC which show ~27.5VAC even with the battery out of the car.

I think my next step is to check the VAC with a good multimeter.

Diesel911, I pulled the regulator about 1,000 miles ago and the brushes looked good (lots of material). The brushes did seem rather loose though, lots of play in all directions. Is this normal?

I'll pull the regulator again and do a visual check, it was a PITA to get back in last time, so I'm wondering if there could be a problem I created...

Diesel911 · #8 07-04-2010, 02:50 PM

The 2 Fat Red Wires on the Alternator Connector in the end are actually connected to (+) terminal of the the Battery. Both should have Battery Voltage all of the time.

The Blue Wire goes to the No Charging Light and will have slightly less than the Batter Voltage only when the key is on. Key of no Voltage at all.

Concerning what was checked 1,000 miles ago. When troubleshooting you need to methodically check everthing as long as the problem presists.
The only relief you get from that is that there are sometimes things more likely than others that can cause the problem.

I have posted the below pic several times but no one has seemed interested.

This is an Alternator Test Cord I made using a Alternator Connector I got from the Junk Yard and you will need a 3 watt Light Bulb to duplicat the No Charg Light.
If your Alternator does not charge correctly with the Test Cord you know your Alternator or the Ground is at fault.
I also think it could be used in an emergency to keep you charging as long as your remember to un-clip it from the (+) Battery Terminal.

In the first pic the Red Arrow points to the Light Bulb.

micalk · #9 07-04-2010, 04:13 PM

Quote:

Originally Posted by atomix8

All ground connections look visually good, although the alternator cable seems a little rough. Where does the other end go? Looks like through the firewall and then maybe into the main wiring harness somewhere?

Visually good doesn't cut it. You need to do voltage drop checks. With the engine running, a/c full blast (or heater if a/c is a no-go), lights on, rear defrost on, etc., check dc voltage from negative battery post (not the terminal of the cable, but the battery post itself) to a good ground point on the fender somewhere - not the engine, has to be a clean bolt or something on the chassis. Should be zero volts, but if you see anything above 0.15 VDC or so, check the voltage drop across the terminals of the battery cable, and by that I mean the terminal on the battery post and the terminal where it connects to ground. You want to check the integrity of the cable vice the connections to the battery and chassis. If the cable checks good then clean both ends of the negative battery cable and recheck. Do the same checks for the positive post to the starter. If you suspect the alternator, you might try performing the same check for the grounds through the engine: voltage drops from alternator housing to engine block, and engine block to chassis.

Quote:

Originally Posted by atomix8

Layback, I have a digital multimeter, but it's a cheap craftsman one. I don't trust it's readings of VAC which show ~27.5VAC even with the battery out of the car.

I think my next step is to check the VAC with a good multimeter.

Diesel911, I pulled the regulator about 1,000 miles ago and the brushes looked good (lots of material). The brushes did seem rather loose though, lots of play in all directions. Is this normal?

Perfectly normal. New brushes extend about 3/8" from the body of the regulator, anything under 3/16" is suspect, I'd say. If you are concerned, check the rotors of the alternator where the brushes sit.

Quote:

Originally Posted by atomix8

I'll pull the regulator again and do a visual check, it was a PITA to get back in last time, so I'm wondering if there could be a problem I created...

From the schematic it looks like the alternator connects back to the starter along with the positive side of the battery, the fuse box, and the glow plugs. If that's the case you may want to clean all those connections just for good measure.

barry123400 · #10 07-04-2010, 05:55 PM

I do not think you should have 27.5 volts ac at the battery or anywhere outside the internals of the alternator. I suspect it is time for a new alternator or diode pack in there.

I was also wondering even if this might be heating the battery as well as you mentioned it has released some acid externally. If you read 14 volts still on the battery after the car has sat for half an hour I still would change the alternator.

I can see no way the regulator is capable of producing the ac component. It has to be getting past the diode pack and might even be upsetting the voltage regulator. Under normal conditions when you shut your car down for half an hour about 12.75 volts should be the maximum retained normal battery voltage.

atomix8 · #11 07-23-2010, 07:42 AM

"Solution Post"

Well, for better or worse, even with a good multimeter that read 0 VAC at the battery and alternator I was still getting the charging light.

I replaced the voltage regulator with a new one from Meyle (made in China, whoops). Still had the light.

So I went junkyard diving, local yard had SIX(!!!) W123s to choose from, so I pulled a rebuilt Bosch alternator. It still wasn't a great one, bearings are noisy, but for $22, I'll live. Installed my new voltage regulator, and light is gone.

Couldn't diagnose problem using recommended methods...but when you replace all the parts, it eventually fixes the problem...