I've noticed that some Bosch voltage regulators have a 120 ohm resistor across the rotor winding while others do not have this resistor at all.

Does anyone know the purpose of this resistor?

__________________
Tom Evans
69 280SE 91K (wish I still had it)
73 220D 98K
78 300D 134K
78 300D 185K
81 300SD 301K (still have)
82 300SD 321K (still have)
84 300D 268K
84 6.2 Diesel GMC Suburban 225K (a whole different story!)

A photo would help, but I can venture a guess that the resistor isn't actually positioned across the rotor contacts. It's more likely positioned between a source of 12v+ and the regulator circuit, and it's purpose is to provide a drop in regulator operating voltage to extend the life of the circuitry.

Sure it's a resistor and not a diode to shunt load-dump?

__________________

Gone to the dark side

- Jeff

I stand corrected. The 120 ohm resistor is connected between the D+ connector and the alternator ground. Yes, it is a 120 ohm resistor, not a diode.

Attached are an MB schematic diagram with the resistor shown and a photo of both versions of the regulator.

One more thing: It doesn't depend on the make of voltage regulator, I've seen Bosch units with and without the resistor.

Attached Thumbnails

   

__________________
Tom Evans
69 280SE 91K (wish I still had it)
73 220D 98K
78 300D 134K
78 300D 185K
81 300SD 301K (still have)
82 300SD 321K (still have)
84 300D 268K
84 6.2 Diesel GMC Suburban 225K (a whole different story!)

Probably a damping resistor for the field coil, to help with voltage stability. The voltage applied to the field is directly proportional to the output voltage.

Big generators operate exactly like a car alternator except:
The regulator is a lot more sophisticated. It has actual electronics and uses PID control to improve regulation. Also the regulator can be externally adjusted with a control signal to alter the output voltage.
Also instead of taking whatever frequency the alternator makes to diodes to make DC, (frequency is directly proportional to rotational speed) a big generator has a governor on its engine to precisely maintain usually 1800 r/min.

The biggest part of all this is that you have to compensate when the generator load changes dramatically in a short time. Load from 0 to 100 percent is tough but not as bad as 100 to 0 (due to overspeed and overvoltage if you don't pull it back quick). 0-100 tests are awesome on a 2.5 or 3 MW, the black smoke and screaming turbos (usually 4 or 6) would make Dieselmeken giddy.

The larger systems that have multiple generators operating in parallel have complex control systems that bias the regulator and governor so that the generator output precisely matches the ac waveform of the bus you are trying to parallel with. A device called a sync check relay compares the waveforms and gives the permissive for the breaker to close to bring the generator on the bus.

Old school systems you had to do this by hand, dialing in the regulator and governor while watching meters and closing the breaker by hand at just the precise moment. New systems do all this with computer magic.

Just a little sidebar on what is going on in miniature in your car alternator.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

More than likely, it helps to establish a reference voltage. The solid state circuitry may be different in older/newer regulators where this may be built in on some versions.

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

Is it for diesels, where there is a possibility of the diesel engine running with the ignition power off, to prevent regulator damage?

__________________

Gone to the dark side

- Jeff

No, the regulator "turns on/off" electronically when the battery is charged. The alternator only looks at the battery voltage and doesn't care if gas/diesel.

The resister is used to provide 'excitation' current to the alternator winding during initial startup. Alternator is different from dynamo, it needs an initial dose of current to 'excite' the winding to give sustaining voltage. Normally, the battery light provide a path for the excitation current so the resister is not needed. However, if the light bulb burnt out and there is no resister then the alternator will not generate any voltage.

It is always better to buy one with the resister just in case the light bulb burnt out.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

If Uncle Sam drafted you in the 60's and you didn't want to go, you are called a draft resister. A resistor is an electronic part.

Edit: I suppose criminals sent to the electric chair is part of an electric circuit and being a resistor.

__________________
85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I agree with ah-Kay. Bypass resistor so if the bulb burns out you don't lose your excitation.

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

Theory above isn't exactly correct. There is an excitation current required, but it is to "boot-up" the internal voltage regulator. The Vreg has its own mini-alternator coils (shown in M-B schematic). This is particular to the "Motorola-style" design used in Bosch alternators. Several web-sites detail all you might care to know about various alternator designs.

Correct. I'll add that my 1984 & 85 300D, the "worn brake pads" and "low fuel" bulb currents add in for alternator boot-up. That does double-duty as a "bulb check" function. One correction is that in old alternators the armature has often become magnetized, which can allow them to "boot-up" at high rpm even if the dash bulbs are blown. If you need to apply a resistor in an emergency, my 300D's have a 1-wire connector strapped to the aft side of the coolant reservoir that goes to D+ on the alternator. Disconnect that and apply ~100 ohm between ground and the side going to the alt to force a boot-up. If no resistor, you could just ground the wire, but better to limit the current using a small 12 V bulb or such. If the Vreg ever fails, you can change it on the car. It unbolts from the alternator as a regulator/brushes assembly.

Most older GM cars used the same type alternator. They well know the "bad dash bulb, no charging" issue and long ago started the load resistor addition.

For humor, read your M-B Owner's Manual. It says if your BATT bulb doesn't light w/ the key before starting, drive immediately to a dealer for an emergency bulb replacement. I'll bet the dealers loved that business.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

The way you have it drawn is it a ground path for the alternator not charging light.

The current would normally flow through the field but if the brushes have failed there isn't a path. In normal operation, once the engine is running, voltage at the diode trio ( where the light is attached ) matches the light and the light goes out. ( + 12 V on both sides of the light )

On my 97 C280 / SL320 I'm pretty sure the resistor is from the regulator side of the field and ground. In that case it acts to set a minimum charging level. This should limit voltage variation and keep the car going in the event of a regulator failure.

You apparently missed my point.

In a gas car, turn off the ignition and the engine stops. In a diesel, vacuum failure can mean that the engine will continue to run, which is why the STOP lever is provided under the hood. In this case, there wouldn't be any voltage applied to the regulator by the ignition switch while the engine continues to run, and my question / comment was whether this could cause a problem (such as the regulator sensing 0V and going full output). Apparently this is not the concern nor function of this resistor from the following posts.

__________________

Gone to the dark side

- Jeff

First of all, there is quite a bit of misinformation on this thread, some of it is wrong, some is just ridiculous.

When the engine is running and the alternator is fine, the alternator supplies it's own voltage to the regulator and the rotor. It does this not through a separate coil, but from the one and only stator coil, via three diodes called a "trio". You can indeed see them on the diagram.

But when the alternator is starting up, there needs to be some external voltage supplied to get things started. This voltage comes through the indicator lamp to the D+ connection, and finds ground through the regulator. Because of the lamp being in series, this start up voltage is less than 12V, so the alternator has a "soft start", good for circuit life. Why does the bulb go off? Because the D+ connection gets 12V+ from the trio when the alternator is running, so that the bulb has 12v+ on both legs. You can place a resistor in parallel with the BULB to give you a back up in case the bulb fails. However, this component is obviously not in parallel with the bulb.

I dug out three spare regulators today. Two of them had no component in this location, but are obviously set up to accept something, as there are terminal lugs and a recess in the case. The third has a component, but it's obviously not a resistor. I'm going to detach it later on and try to determine what it is. I have a few theories:

First, many of these regulators are thermally regulated. When they become warm, the charging voltage drops in order to prevent overcharging. It's possible that the component is related to that function. Another possibility is that it's some sort of current regulator, although I don't understand how it could work, given the way it's wired. I'll try to get to the bottom of it later tonight.