After reading your posts, (thank you!) I immediately put on shoes and shuffled out into the garage at 0130am in my pajamas to follow up. (Yeah, my neighbors would expect nothing less from me, than to be working on my car at 0130am in 30 degree weather in my pjs.)

So, I have a nice heavy duty test light probe Santa brought me so... (I extended the reach of the clip for the test light using an alligator test lead, clipped to the brass 12volt post in the engine compartment.) And...

Key off- I have light when I probe the terminal on the alternator for the blue wire, with the blue wire disconnected.

Key on- I still have light when I probe the D+ blue wire terminal on the back of the alternator.

Car started and running- I STILL have light on the test light when I probe the terminal for the blue wire on the alternator. There was no change in the engine revs, and here's something weird- the infamous "battery charge" light didn't come on at all after starting the car or when I shut it down with the blue wire disconnected from the alternator. And- the cabin lights were slightly dim after starting the car- and I revved it o 1500rpms or so, then 2000- and the lights did get brighter.

So- is there anything else to try or is my new 143A just a dud?

I checked yesterday and found out I can return the alternator for exchange or money back if the alternator is "defective." The problem is that the alternator charges fine even if it isn't "excited" when it spins at 1500 rpm or more, but the "battery charge" is lighting... Ideas?

All assuming that you have not changed your 'battery light' bulb to a higher wattage (as this will throw out field currents) the following may help.

If you disconnected D+ (the blue wire) as you mentioned in your last test, then your battery light will go out- the blue wire goes to your battery light- this is the stator output.

Your alternator should reach its full rated voltage of 14v + or - depending on battery health/resistance, temperatures and load etc. at approx 500rpm engine speed (not alternator speed). It will not be able to produce enough grunt (amps) to power all the sub circuits until spin up speed which is somewhere around 750-1000 engine rpm.

If all is working correctly then as your alternator reaches its spin up voltage (idle is sufficient for this) voltage will appear to one side of your 'battery light' B+ through a bank of diodes in the alternator, the power diodes, and on the other side of your 'battery light' D+ voltage will also appear but from the stator through another set of diodes, excitation diodes. As the voltage potential is now close at both ends of your 'battery light', it will go out.

You mentioned that you had 1.8v at D+ with the engine running - this is a fail.
You should be getting full alternator output of 14v with the engine running. If you are not, which seems the case, then one of your diodes inside the alternator have failed on the excitation windings. The only true way to test this is with an oscilloscope to check the waveform at D+

You can do the test to D+ to create a positive excitation but don't run it like this for too long as it is the same as doing a full field test where it will be running at maximum power- they only like 75% continuous duty cycle and not for very long.

There are other tests to perform but some are potentially VERY dangerous to your car.

ACHTUNG/WARNING if someone suggests to bridge a wire between battery positive of B+ to DF (usually a green wire) from the rotor field, you are highly likely to fry your cars engine computer. It will put unregulated voltage out. Be very careful. Similar warning to not remove a battery terminal whilst your engine is running/charging as you will most likely fry a power diode in the alternator.
You use to be able to do this on the older mechanical voltage regulators but not with solid state.

Can you take your alternator to another place to be tested? Many places simply don't test the thing thoroughly enough. They simply spin them up, whack voltage on DF, test output at B+ and say 'yep, all is good'.

Or maybe you have access to another known good one?

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1995 E300 Diesel

@benedict

I was very shocked to find a wire with a switch and AC current heat lamp (like the indicator on an old National clothes iron) connected to the W terminal of my cars alternator routed to ground via the switch and lamp.

I have no idea why anyone would do that, the W terminal as far as I know is tapped into the field windings and the only car I saw it being used on was my VW Golf diesel, it was used to drive the tach.

I removed it and threw it into the bin, btw everything electrical on my car is functioning correctly

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

I haven't changed anything other than the alternator, battery and cleaned ground straps. The "battery charge" display only came on when the new alternator was connected at the D+ wire- but not when disconnected so I think this is pointing more at the alternator... right? I don't know of a place near me that could put it on an oscilloscope, so I'm thinking I'm going to have to uninstall and get another one, and re-install. What a PITA...

PITA definitely but probably your best option.

Bosch alternators are sold here on ebay here for around $250-$300 as reconditioned. Reconditioned means they've been degreased and thats about it - they still have the original bearings, carbon brushes, regulators in them along with possibly 100,000’s miles to them.

I'd be more inclined to find an auto electrician shop or a Bosch shop and get one truly reconditioned with a warranty. They really do need to be pulled down properly.

I understand that you won't be able to do this because you've already purchased this one and the seller/shop will only exchange. They should at least swap your pulley clutch over no charge.

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1995 E300 Diesel

I'm not sure what a 'W' terminal represents- must be an early alternator but you could get this pulse from a stator winding inside the alternator I suppose. You would only need to tap into one of any three windings for this (most alternators are wired three phase) to produce a pulsed frequency.

But why someone has wanted to earth out a winding is anyone's guess. Wiring to one of the stator windings at that point is going to give you alternating current not direct current.

People do all kinds of weird and wonderful things to their cars electrical systems.

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1995 E300 Diesel

NSA makes excellent "NEW" alternators, and are priced just about the same as a "rebuilt". When I changed out my old alternator due to a bad diode, I installed a new NSA. No problems what-so-ever. Only thing I had to change out was the clutch pulley from my old alternator and install it on the NSA unit.

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96 E300d

That's an awful lot of Aussie dollars for what is effectively a "core" ready to be rebuilt. In any case, as usual, one needs to be cautious buying stuff on eBay. True rebuilt Bosch alternators sell on-line in the states for about $200 and come with a lot of new parts.

Question: Who does this rebuilding process? Does Bosch send them back to North Elbonia (or wherever they were originally manufactured)? I suspect they have an in-country plant wherever there's enough business. Is there an "alternator rebuilding company" that does this job for anyone (like all car batteries are made by just a few companies)?

Jeremy

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

No, I highly doubt they are sent back to Bosch for rebuilding. There are many Bosch dealers around here (many auto electricians shops) but I don't think they are Bosch trained. When I went to one recently to discuss rebuilding a Bosch injection pump, they had no way of testing or calibrating the unit as there was no bench equipment for this, so I walked out. If I ever need to get an injection pump rebuilt, I'll be building a timber crate for it and shipping it to the US or Germany.

Yes it is a lot of Aussie dollars for what it is. You definitely get better value for money in the US and not just car parts. I get many things sent from the US and even with the extravagant postage cost, I'm still far better off financially. You would fall over if you knew what it cost to register and insure a car on the road in Sydney every year!

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1995 E300 Diesel