The wire that runs from the bulb to alternator exists only to wake up the alternator and provide visual indication that it isn't charging.

Dropping power here when the engine is running won't cause a problem as the regulator is already awake. Battery charging voltage is sensed at the alternator output wire. Some alternators have a separate sending wire that runs near the battery connection. This is done to minimize low system voltage due to voltage drop at high currents.

Leaving power applied with a stopped engine will only serve to drain the battery.

This trigger wire isn't always needed, many alternators will begin to charge if engine speed is raised to say 2000 to get things going. If the engine is brought back to idle, the alternator will continue to charge.

Do you mean in parallel? In series would be open circuit if either one bust.

__________________
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.

The part in question is definitely a resistor. Bosch even refers to it as a resistor in their literature. The color bands say it is a 120 ohm resistor which is what it reads.

Here's something more that is puzzling. It appears that only the 55 amp/28 MM slip ring model has the resistor. The 65 amp/32 MM slip ring model does not have the 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!)

Yes, I fixed my post.

Here are the three regulators. From left to right, a KAE and two from factory rebuilds, probably WAI's. The one on the right has the mystery component. The other two seem to have provision for the component, but nothing is installed.

I desoldered the mystery part and cleaned it up. It's marked 68 ohms L3, 5%, it's also marked RB59B. Finally, there is a manufacturer's mark, but it's illegible. Resistance measures 68 ohms on the nose. It may be a wirewound, so maybe it's used as a choke. Don't know, and can't find a reference for it.

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AAAARRRRGGGGH! After giving it much thought, I think I've figured it out. But first, discarded ideas:

- back up path for bootstrap current: it's not in the right place for this

- radio suppression: it's really an odd component in an odd place if this is the intent

- surge suppression for the rotor coil: aside from being in the wrong place, why not use a reverse bias diode and do it right?

- current limiter or voltage divider. Just not in the right place for either function.

- Heat compensation. Aside from being in the wrong place, it's not a thermistor. If anything, it makes the alternator run hotter.

So here it is...wait for it...






In the end, the simplest explanation works best. If the rotor or regulator fails, goes completely open for any reason, the resistor provides an alternate path to ground for the dash light. Otherwise, there would be no indication that the alternator has failed. I don't know why it's in some regulators and not others. It might be that some regulators accomplish this with internal circuitry. It's a pretty inefficient way to do it. When the alternator is operating normally, the resistor is a useless load that generates lots of internal heat. No wonder they call them idiot lights.

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This statement holds true, very true.

__________________
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.

I'm just surprised that someone didn't post that the resistor was to provide a gravity path so the electrons can fall to ground. Given many German wiring diagrams have positive at the top and ground at the bottom it has to be true.

See also post 13.

Also, has anyone physically verified that the resistor is on the + side of the field? I'm going to have to dig up my old regulator and trace the circuit down. I thought mine was to establish a minimum charging level / backup in case the regulator fails open.

Yes, post 13 is also completely off the track. This resistor has no bearing on charge level at all. It's located just as shown in the OP's diagram, providing a path between D+ and ground. The normal ground path for the light is through the voltage regulator/field circuit via D+. In the event this path is open for some reason, this resistor allows the indicator to find ground and light up. That's all it does.


How about that...I've even found an official reference for it!

http://apps.bosch.com.au/AAExtranet_TechSearch/docs/repairinstruction/regsB9-0.pdf

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I just got out from under the car after trying a couple of different VRs. I took a VR with a resistor removed and tried it. No problem with operation whatsoever.

I checked six VRs, four with resistors and two without. The internal circuitry is different between the two versions. Measuring between the two terminals where the resistor is connected yields different results. The resistor version indicates that there is a semiconductor junction (diode, transistor) between the resistor pins. The non-resistor version indicates that there is a fixed resistance (~ 4K ohm) between the pins.

Bottom line, it looks like either version will work.

Thanks to MxFrank for the Bosch article.

__________________
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!)

How?

Well, for one thing it IS a ground for the "charging light", which it why it terminates on D+. The reason it's a resistor and not a length of wire is specifically because you don't want to dead ground the alternator. But let's complete the quote:

This just isn't the way it works. If you look carefully at the diagram, you can see that this is a "A" circuit regulator, meaning that it controls field ground. In order to serve as a back up mechanism, you would need to locate the resistor between the negative side of the field coil and D-. The resistor would need to be inside the can, bridging the power transistor. The resistor would also need to have a much larger value, because whatever voltage is applied across the field has to always be low enough to keep the alternator from running away. I'm not sure it could be made to work safely.

So if I'm reading this correctly Frank, this resistor is to provide an idiot light in the case where (common failure) the brushes lose contact, and you are not charging, ... where previous designs did not illuminate the light? BTDT, IMO it was a poor design and I had that failure when my car was only 3y/o, with no ammeter and no voltmeter the only indication that I had that my car was not charging (on Christmas Eve, at night, in a blizzard, in the middle of nowhere, with my family in the car, ...), was that the ABS/SRS lights came on!

__________________

Gone to the dark side

- Jeff

The resistor would offer enhanced indicator functionality in the case of a brush failure. (We're not allowed to say that anything OEM was a poor design.)

... you weren't out with your family in you fairly-new $74,000 best in the world wagon in a blizzard, stranded without warning, ... or you might bend that rule.

But I can agree that this made it a more robust design, ... than the original design. Better?

__________________

Gone to the dark side

- Jeff