1984 300D alternator Battery puzzler - Page 2 - PeachParts Mercedes-Benz Forum

vstech · #16 07-20-2012, 02:59 PM

Quote:

Originally Posted by funola

He reported (11.8 at idle 12.7 reved). In my experience, the alternator is incapable of 12.7 v reved with glow plugs on, it may make 11 to 11.5 v, not 12.7.

don't forget the time factor... assume the plugs are staying on.

start the car, get out and measure voltage at idle. 11.7
reach over and grab the linkage, then rev the motor, and look at the meter, it's now reading 12.7...
if the plugs are on, the amps they are pulling have dropped, due to resistance increase from heat.
I think it's possible for the alt to produce 12.7 with hot plugs stuck on.

to test, OP, pull the GP harness socket off, once the car is started, and report back with voltage at idle.

layback40 · #17 07-20-2012, 09:00 PM

Quote:

Originally Posted by Brian Carlton

The justification is in the wiring diagram for the light. You have that FSM, of course?

My diagram shows no hot connection to the wire from the light to the reg.
With the globe out, there is 0 V at the end of the wire wire where it plugs onto the alt. Maybe there are more than 1 version?
Where does this extra power supply via a resistor that you speak of connect to the exciter wire? Where does it originate from?

Codifex Maximus · #18 07-21-2012, 04:03 AM

My method from a previous post:

Test the voltage across the battery with the car off and then started. You should have a little better than 12V with the car not running. With it running, depending on the state of charge on the battery and current load, you should have anywhere from 13V to ~14.5V.

If voltage across battery with engine running is not at least 13V, then check these things:
Does alternator light come on with key on but car not started? No? Check bulb and replace if necessary.
Check alternator belt. Worn? Replace. Loose? Tighten.
Are electrical connections to battery clean and tight? No? Clean and tighten them with the engine off.
Check wiring connections to alternator. Make sure they are clean and tight.
Check voltage regulator for brush wear or damage. If damaged or worn, replace.
Check engine grounds. Tighten, clean or replace as necessary.

If none of these common things work, have battery and/or alternator checked. Replace defective units.

If you have good voltage at battery with engine running, you may have a large current drain sapping your battery.

So, if you've done all the aforementioned things... do try to disconnect the glowplug unit from power and see if it's sapping your power. Also, as Layback and others have said, you must have power to the armature windings to get any power generation. Invariably this means that the circuit thru the alternator bulb on the dashboard must be good - this means the bulb must be good to complete the circuit. When the alternator is working, at least in a basic way, there will be a balance and the light will go out. This bulb MUST light when engine is off or there is a fault in the circuit. If the bulb remains lit after car start then there is a fault elsewhere in the charging circuit or alternator or belt.

layback40 · #19 07-21-2012, 04:47 AM

Quote:

Originally Posted by Codifex Maximus

My method from a previous post:

Test the voltage across the battery with the car off and then started. You should have a little better than 12V with the car not running. With it running, depending on the state of charge on the battery and current load, you should have anywhere from 13V to ~14.5V.

If voltage across battery with engine running is not at least 13V, then check these things:
Does alternator light come on with key on but car not started? No? Check bulb and replace if necessary.
Check alternator belt. Worn? Replace. Loose? Tighten.
Are electrical connections to battery clean and tight? No? Clean and tighten them with the engine off.
Check wiring connections to alternator. Make sure they are clean and tight.
Check voltage regulator for brush wear or damage. If damaged or worn, replace.
Check engine grounds. Tighten, clean or replace as necessary.

If none of these common things work, have battery and/or alternator checked. Replace defective units.

If you have good voltage at battery with engine running, you may have a large current drain sapping your battery.

So, if you've done all the aforementioned things... do try to disconnect the glowplug unit from power and see if it's sapping your power. Also, as Layback and others have said, you must have power to the armature windings to get any power generation. Invariably this means that the circuit thru the alternator bulb on the dashboard must be good - this means the bulb must be good to complete the circuit. When the alternator is working, at least in a basic way, there will be a balance and the light will go out. This bulb MUST light when engine is off or there is a fault in the circuit. If the bulb remains lit after car start then there is a fault elsewhere in the charging circuit or alternator or belt.

Very good advice !
Once this is ALL done a clear picture will be seen.

marybeth · #20 07-21-2012, 01:22 PM

Fine advise. I have done most-but will go over step by step when my son returns from school. He needs the 300D to go to from school/work. so at present he drives little and just keeps recharing the battery (via charger) every evening.

marybeth · #21 07-21-2012, 02:08 PM

Ding Ding Ding-we have a winner! .....MBDOC with glow plug relay suggestion.

With glow plug relay out voltage jumps to 13.20 at idle, 13.75 at rev.

With new (good used) glow plug relay installed and high beams on, AC on, radio on, windshield wipers on , turn signals on, the volatge is 12.89 at idle, 13.12 at rev.

So the bad guy is the glow plug relay.

Gotta love this forum-thanks or all the help!

mattc · #22 07-31-2012, 08:36 PM

I am having the same issue. Unplugged the glow plug relay, and it is charging to specs now! I'll deal with the glow plug indicator. Just wanted to say "THANKS" for the advice.

TnBob · #23 11-28-2012, 05:41 PM

"The Bulb for the warning light is suppsed to be 3 watts or your Alternator will not charge. "

Making all the dash light LED apparently isnt the worlds best thing to do. New battery and alternator with an LED in the dash alternator location and my battery
still goes flat. ~1200rpm or so, reading across the battery is 13.7vdc

jay_bob · #24 11-28-2012, 09:09 PM

No it has to be incandescent. LED will not work here.

I play with big generators at work. The concept is the same only ours are several orders of magnitude bigger.

No reference voltage through the bulb means the regulator cannot produce field ecxitation, without field excitation the alternator will not produce any meaningful output.

Brian Carlton · #25 11-28-2012, 09:37 PM

Quote:

Originally Posted by layback40

My diagram shows no hot connection to the wire from the light to the reg.
With the globe out, there is 0 V at the end of the wire wire where it plugs onto the alt. Maybe there are more than 1 version?
Where does this extra power supply via a resistor that you speak of connect to the exciter wire? Where does it originate from?

This is the best I can do with 70KB:

The resistor is between the brake wear indicator and the alternator light. I can assure you that the alternator will charge without an alternator light connection provided that the rpm's get above 2000.

Note that this is on the W126.

Diesel911 · #26 11-28-2012, 10:01 PM

Quote:

Originally Posted by Brian Carlton

This is the best I can do with 70KB:

The resistor is between the brake wear indicator and the alternator light. I can assure you that the alternator will charge without an alternator light connection provided that the rpm's get above 2000.

Note that this is on the W126.

I am not so sure how that would effect the Charging Circuit because there are Diodes in the Circuit.
The only thing that is clear to Me is that the Diodes prevent the Charging Light current from getting into the other circuits to the side of it.

Brian Carlton · #27 11-28-2012, 10:23 PM

Quote:

Originally Posted by Diesel911

I am not so sure how that would effect the Charging Circuit because there are Diodes in the Circuit.
The only thing that is clear to Me is that the Diodes prevent the Charging Light current from getting into the other circuits to the side of it.

The alternator gets voltage from the brake wear indicator via the resistor.

What I'm not clear about is why the rpm's make a difference. There is insufficient voltage to excite the alternator until about 2000 rpm. Then it is excited and works properly until the vehicle is shut down.

Diesel911 · #28 11-29-2012, 12:08 AM

Quote:

Originally Posted by Brian Carlton

The alternator gets voltage from the brake wear indicator via the resistor.

What I'm not clear about is why the rpm's make a difference. There is insufficient voltage to excite the alternator until about 2000 rpm. Then it is excited and works properly until the vehicle is shut down.

The view Break wear indicator circuit shown is not enough of the to show what actually happens in that circuit. And, you would think the resistor would block the flow to the Charging circuit.

Meaing with a resistor in the circuit the current is going to flow the easiest way it can until something happens to block the easy way. When that happes the resistance of the Resistor will be overcome and then current will flow through the resistor.

There is not enough of the Break wear indicator circuit shown to show that and you would think the resistor would block the flow to the Charging circuit.

Meaning with a resistor in the circuit the current is going to flow the easiest way it can until something happens to block the easy way. When that happens the resistance of the Resistor will be overcome and then current will flow through the resistor.

Brian Carlton · #29 11-29-2012, 12:20 AM

Quote:

Originally Posted by Diesel911

The view Break wear indicator circuit shown is not enough of the to show what actually happens in that circuit. And, you would think the resistor would block the flow to the Charging circuit.

Meaing with a resistor in the circuit the current is going to flow the easiest way it can until something happens to block the easy way. When that happes the resistance of the Resistor will be overcome and then current will flow through the resistor.

There is not enough of the Break wear indicator circuit shown to show that and you would think the resistor would block the flow to the Charging circuit.

Meaning with a resistor in the circuit the current is going to flow the easiest way it can until something happens to block the easy way. When that happens the resistance of the Resistor will be overcome and then current will flow through the resistor.

There is no current flow through the brake wear indicator, under normal circumstances, because the circuit is not grounded (the brake sensor isn't touching the rotor).

So, with 12V present at the resistor, current will flow through the resistor and a lower level current will flow to the alternator. This is sufficient to excite the alternator.

vstech · #30 11-29-2012, 12:24 AM

ok, so the bulb is NOT needed for the alternator to charge? wild.
I wonder why 2Krpm is needed... perhaps the much lower voltage from the alternate path is too low until the alt reaches that speed?