Dim Glow from Charging 'Idiot' Light - PeachParts Mercedes-Benz Forum

jcantor · #1 12-04-2002, 12:39 PM

I've just recently noticed that at night, when the car is cold and lots of accesories (lights, blower, seat heaters) are on, the charging system light will glow very faintly. I'm not sure if this is something new or just something I haven't noticed before. It is so faint that I can only see it when there is very little outside lighting (no streetlights nor cars behind me). It seems to go away once the car is fully warmed up and it also doesn't seem to get any brighter (or dimmer) if I turn on or off additional electrical items.

Once things warm up a bit here in NJ (currently 19F at night and 28F during the day) I'll put a meter to the electrical system to check things out. I know that the battery should be 12V or more with the car off, but I don't recal what the readings should be with it idleing with no electrical load and with full electrical load.

Anyone have these numbers or other suggestions? Or is this perhaps somewhat normal and I've just recently noticed it?

jlc

MikeTangas · #2 12-04-2002, 02:29 PM

~13.8-14vDC

Dim light in cold temps could be early warning of a worn brush pack (voltage regulator).

drbrandini · #3 12-04-2002, 08:34 PM

I am with Mike,

Most likely the voltage regulator if not your altenator is on the brink! Hopefully the first of the two $20 and 10 mins.

#4 12-05-2002, 11:45 AM

which can be the result of bad jump-starting techniques, such as jump-starting the car and immediately disconnecting the jumpers so that the newly started car's alternator is overloaded by trying to recharge a dead battery while at the same time providing power for all the car's usual accessories. The diodes are part of the brush-holder/voltage regulator, though, so that's most likely what you need to replace in any case.

jcantor · #5 12-06-2002, 10:33 AM

Thanks folks.

Now all I need to hope for is good weather Sunday.

jlc

jcantor · #6 12-19-2002, 03:55 PM

Well it took me until today to finally get around to putting a multi-meter to my car. Here’s what I got:
Battery Voltage (car off) = 12.25
Charging Voltage (car on, no accessories) = 12.55
Charging Voltage (car on, lots of accessories) = 12.25 or lower depending on # of accessories

Obviously these aren’t the numbers you had provided earlier so my question is which of the three main components is bad? Do I have a voltage regulator that can no longer control the alternator output properly; an alternator that can’t provide enough juice; or a battery that can’t take a charge very well?

If I understand the PO’s repair records, the battery is about 15 months old (25K miles) but I don’t see any indication of repair or replacement of either of the other items.

Okay, moving on to the next obvious question: I understand that there are some reasonably economical replacement voltage regulators which are just a snap in part so that repair would be very straight forward. On the alternator, however, I understand that a new one (remanufactured I presume) is quite pricey. I’ve also read here that the alternator can be repaired very easily with an inexpensive brush kit. Are both parts of this statement true (price and ease of repair)?

Thanks
jlc

#7 12-19-2002, 04:41 PM

Jeff - 12.25 V is a little low for a battery at rest. Normal should be 12.56-12.95V. Put a charger on the battery and charge it up and let it rest for a couple of hours. THEN check the "at rest" voltage again. If it goes up to the range I said above - the battery is good. If it doesn't - have your battery checked - it could be on it's last legs (weak cell). If the "at rest" voltage goes up to the range I said - then it is only an alternator problem and 95% possible it is just the brush pak. With a new brush pak expect 13.4-13.8 V at idle and over 14V at revs. Brush pak is available here at Fastlane and is two screws to replace - 10 minute job - very easy.

tryan · #8 12-19-2002, 05:15 PM

how is your electrolyte level and specific gravity?

moakesr · #9 12-20-2002, 04:26 AM

It's always worth checking the voltage drop between the negative pole of the battery and the case of the alternator.

A difference of 0.5 of a volt can make the difference between a well charged battery or one always on the brink.

I have found on older cars (8yrs+), there is often a degree of resistance in the earth strap from the engine and consequently the alternator, and also in the strap from the negative pole of the battery to the chassis.

I have corrected this on numerous cars using a 4 or 8 gauge wire from the alternator casing to the battery negative pole.

You have to be careful with this, as should the main earth strap ever fail, then you will draw full starting current through this additional cable, and it may melt !

Cheers,

Richard

stevebfl · #10 12-20-2002, 08:13 AM

Proper alternator testing really requires an ammeter, preferably inductive. Testing should not be done at idle; it should be atleast 2000rpms. The previously mentioned voltages should be achieved. As pointed out, a weak battery will absorb output (give a lower reading) but this is why the ammeter is important. The ammeter should show the rated output at test speed after correcting for load. Load is determined by turning key on engine off and viewing the reading. This reading must be added to the viewed output under load to get the actual output.

Your test do indicate that the battery is low and your charging is inadaquate at idle. Depending on the car this may not directly indicate anything. Early models charge poorly at idle.

BobK · #11 12-20-2002, 08:35 AM

Charge up the battery like suggested then test your output voltage (you may not get reliable readings with undercharged battery). Hook up your meter to the positive post on the alt and the frame of the alt. Engine running, accessories off. If you don't get at least 13.5 volts when running, your alt is in trouble. Oh, with the engine stopped, can you turn the fan on the alt by pushing on it and making the belt slip? If so, just tighten the belt. Have seen a slightly loose belt cause reduced output . If low voltage is not caused by loose belt, try a new regulator like others have said. You are only gambling the price of the regulator that way and you might luck out.

#12 12-20-2002, 09:39 AM

What I do to check for alternator belt looseness is to grab the cooling fins on the alternator pulley and try to turn the pulley in both directions. If you can turn the pulley - the belt is too loose and will slip under load. Been doing this for ten years and it hasn't failed me yet.

jcantor · #13 12-20-2002, 12:19 PM

Wow guys, lots of great suggestions here! Thanks.

If the rain lets up, I'll look into this this afternoon. One quick question, any ideas on how to safely get at the positive pole on the alternator with the engine running? It seems rather tight in there.

jlc

Ron Johnstone · #14 12-22-2002, 12:57 PM

"how to safely get at the positive pole on the alternator with the engine running? It seems rather tight in there. "

You should install the wire on the alternator with the engine shut off, making VERY sure that it will stay in place and not short out when the engine is started. Don't even think about trying to run any wires or any diagnostic tools into a running engine if there is ANY possibility that they or you will contact anything that will hurt you or the Mercedes. Keep in mind that if you get a shock, you will naturally try to get out of the area and perhaps will contact something that is turning and really hurt yourself.
Perhaps a primer on general safety procedures might be a worthwhile thread to start so we can perhaps save someone more than a skinned knuckle or two. Frankly, such a list would be a good thing for us all to read over occasionally as we frequently take chances that we really shouldn't.

#15 12-23-2002, 12:51 PM

Hello,

I agree with the voltage charge of about 12.9 volts at 2000RPM without everything on full blast. That seems about normal for my 190e and most 12V cars.

Its pretty easy to take out the alternator and check the brushes. My 190e alternator died completely as a result of worn out brushes...I had about 1/4 inch on one side and 1/2 inch on the other. I picked up brushes at a nearby retail parts store that oddly enough had the exact item that cost $40 plus tax. This was competitive with Fast Lane, and a little quicker, so I did that about a year ago with no problems since.

Joseph
1985 - 190e 2.3L 168,000 miles
1986 - Pontiac Firebird, 111,000 miles, 305 4bbl