Charging Light Theory and Operation - PeachParts Mercedes-Benz Forum

ChinaClipper · #1 06-30-2010, 02:13 AM

I have looked a a number of electrical threads but haven't found what I am looking for. Anyone know how the Charging light operates on my 86 1090D??? I have been chasing a slow discharge problem and have found a 2V reading from the back of the alternator (small Blue wire after removing from the Alternator) Can anyone explain 1) where the blue wire continues to before reaching the Ign. Sw.? 2) HOW does the charging light work? BTW Any thoughts on where the voltage is coming from would be great! Thanks! Clip...

compress ignite · #2 06-30-2010, 02:19 AM

BUT,
The Small BLUE wire is the "Signal" wire to the Alternator to Produce Power.
Without BATTERY VOLTAGE on the Small blue Wire the Alternator will not work.

If there's anything wrong with that circuit:
Bulb on the I.C. for the Alternator is Bad...NO Charge.
Interrupt in the wires from Ign.Sw. to I.C. Bulb to Alt. ... No Charge.

"Phantom Discharge":
Hook up the old DMM and start pulling fuses. (Hell,replace all the fuses)
There are ways to test UNFUSED appliances LIKE the Alternator:
A Bad(S) Diode on the ALT. CAN drain power whilst you're enjoying life elsewhere.

SUPPOSEDLY,an Alternator light on the I.C. (or a "Flickering" light) bodes
Charging Difficulties...
BUT,Lately I've seen Many instances where the Alt. light never ALERTED
and somebody was left on the side of the road with a dead battery.

on An '86...How well do you maintain your "Electrics" ?
(AKA "Clean Bright and Tight" on all connections)
(Checking OLD cables for Continuity and Resistance[Based on diameter and length] )

Jeremy5848 · #3 06-30-2010, 02:28 AM

The charging system on your 190D is almost identical to the one on my '87 300D Turbo (W124, OM603). When you turn on the ignition switch, current from the battery flows through the "Batt" or "Charge" light to the alternator, where it energizes the rotor through the slip rings, creating a magnetic field. When the engine starts, the rotating magnetic "lines of force" are "cut" by the wires in the stator, which surrounds the rotor. This generates an AC voltage that is rectified into DC by a set of diodes in the alternator. The DC goes to recharge the battery and run the various electrical loads. Another set of diodes sends a DC voltage back up the wire to the "Charge" light; "trapped" between two positive voltages, no current can pass through the Charge light so it goes out.

Of the 12 Volts available from the battery at start, about 10 Volts is dropped across the Charge light and the remaining 2 Volts is dropped across the rotor in the alternator. I just measured the voltage in my car as part of another project and it measured almost exactly 2-1/4 Volts on a reliable digital voltmeter. So I would say that your alternator is working correctly.

As for your slow discharge problem, many forumites have chased down such things. Best way seems to be to disconnect the negative battery cable and put an ammeter between the cable and ground. You will see some current even with the engine off and the key in your pocket. The current should be only a few milliamps, due to the clock, radio memory, etc. If you see a tenth of an amp or more you have a problem. If so, start pulling fuses until one or more make the current go away. The problem is in that fuses's circuit. Check all of the things that are powered from that fuse.

Typical problems are related to things that are stuck "ON," like the power antenna or other items that get power even with the key "OFF."

Jeremy

Crackstar · #4 06-30-2010, 12:20 PM

Quote:

Originally Posted by Jeremy5848

Best way seems to be to disconnect the negative battery cable and put an ammeter between the cable and ground.
Jeremy

Could some one clear this up a little bit for me? Electrical has never been my thing and I'm trying to figure out if I have a charging problem or a drain somewhere.

thanks

chuck

Jeremy5848 · #5 06-30-2010, 07:55 PM

Quote:

Originally Posted by Crackstar

Could some one clear this up a little bit for me? Electrical has never been my thing and I'm trying to figure out if I have a charging problem or a drain somewhere.

thanks

chuck

Turn off the car. Make sure nothing is left on, like headlights, interior lights, trunk light, etc. Close all the doors. Raise the hood, disconnect the negative cable from the battery (13 mm wrench). When you take the cable off, do you get a spark? Try gently touching the negative cable to the battery post -- do you get sparks? If you get more than a tiny spark, something is on that shouldn't be on and that is causing your battery to drain.

To measure the drain, you need a digital voltmeter (DVM). If you don't have a digital voltmeter (DVM) with an amps scale, go buy one. Radio Shack and most hardware and auto parts stores have them. Read the instructions, learn how to plug in the leads and set the meter to read current (measured in Amperes or milliamperes). One Ampere = 1000 milliamperes.

Put the red lead from the DVM on the negative cable. Tie it there with wire if you have to. Set it on a rag so it can't touch any metal on the car. Now touch the black lead from the DVM to the negative post of the battery. The DVM will tell you how much current is flowing from the battery to the "phantom load." If it's more than a few milliamperes (normally no more than 25, possibly as much as 100) then you have a problem.

Watching the DVM, start removing fuses from the fusebox. When removing a particular fuse causes the DVM to read lower, you have found a problem. The fuse card in the lid of your fusebox will tell you what things run off of that fuse. If you have lost the fuse card, most of them are posted on this forum somewhere.

Once you have figured out which things are causing the problem, disconnect them one by one until you figure out which one is causing the problem, then repair or replace it.

Good luck.
Jeremy

crhenkel · #6 09-05-2010, 12:53 AM

thanks for the good explaination!