On my 1979 240D, the alternator light comes on while idling. When I start to drive down the road it goes off, but when I slow down or stop at an intersection, it comes on very dimly. This always repeats itself. I have never had any trouble (yet) with the car starting. I imagine it could be a voltage regulator problem, but my question, is with multimeter in hand, how do I check the alternator output? bear in mind, I havent used a voltmeter very often except to test continuity in circuits, so be specific with instructions!
thanks,
Jim

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1979 Mercedes 240D (Sold to a neighbor last year,now I have room for a new one,looking for a 300D this time)
1980 Mercedes 240D(bought for a parts car, but I have fun driving it!)

I picked up a cheap voltmeter from Harbor Freight(looks nice), and thought I would check my battery and alternator output. I set the voltmeter on 20dc and place the red lead on positive post of battery and black lead on negative. Read 12. something volts. Then I started the car, let it idle for a bit, and used the voltmeter the same way, with intention of reading output of alternator. While I was holding the leads, I could feel them getting warmer and insulation on the lead wires started smoking! I took them off the battery. Never did get any readings. What did I do wrong, or what went wrong?
Jim

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1979 Mercedes 240D (Sold to a neighbor last year,now I have room for a new one,looking for a 300D this time)
1980 Mercedes 240D(bought for a parts car, but I have fun driving it!)

Don't know how to check alt with meter, some parts places will check for free. My '82 300 had a similar problem, not the heat you are experiencing tho, I replaced the voltage regulator, solved.

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83 SD

84 CD

Check the voltage across the battery. Should be 12.5 V with the engine off, 13.1 or higher running, but less that 14.1.

Get a new regulator, but you may have a bad diode or two -- needs a run on a test stand to verify.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Alternators have two sets of rectifying diodes that turn the AC current into DC. The main set rectifies juice for the battery and rest of the car's electrical system. The secondary rectifies it for the voltage regulator. The alternator light straddles the two sets. When the alternator isn't charging, the current flows from the battery, through the light and then through the voltage regulator to ground. If the light goes on when at idle, and off at higher RPMs, then you know that the alt isn't putting out enough juice at low RPMs, but you don't know why that's the case.

It could be something simple like alt belts that slip a little, or a regulator that isn't contacting so well anymore, so check them. It could also mean the diodes are weak. In all the cars I've owned I've never had to diagnose an alt to this level, and I don't have a wiring diagram for your car, but if I read mine right (and assume yours is similar), with the B+/D+ wires disconnected, and the regulator removed, you can test 6 of the 9 diodes for serious leakage with a battery-powered test light. See if you can light the test light by conneting it across B+ & D+, try both ways. If it glows, even weakly, in either direction, then the alt probably has some bad diodes (or there's so much dirt it is conducting electricty). The other three diodes could be bad, but I can't see any way to test them without opening the alternator up.

Dunno about the meter. There's no way it should have been able to draw enough current to warm the lead wires on a voltage setting.

hope that helps, and please let us know what you find out.

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'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.

the meter through me a bit, as I thought the way I did the check was appropriate, could just be a bad meter. I have another, older one,not as fancy, I might try a check with that. I'm thinking it is the voltage regulator. I ordered one from "Phil",just in case I needed it this time or in the future, plus I needed to get my order totat over $100 for the free shipping! I'll let you know what I find out, when I check the alternator with the differant meter and/or install the regulator.
thanks
Jim

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1979 Mercedes 240D (Sold to a neighbor last year,now I have room for a new one,looking for a 300D this time)
1980 Mercedes 240D(bought for a parts car, but I have fun driving it!)

It sounds like you had your meter in the amp setting, which would make your meter a short across the battery.

Ensure the setting is DC Volts and about 20VDC range. 12VDC=engine off; 13+VDC=engine running.

Then place the setting to AC Volts and check across the battery. Should be less than ! volt AC.

If DC volts are low......regulator.
If AC volts are high........diodes

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Sam

84 300SD 350K+ miles ( Blue Belle )

I figured that I had a bad voltage regulator so I ordered one from Phil. He had two choices, either a Bosch or a German, so I chose the German.
Received my order today, and promptly went to install the voltage regulator. Anyway upon removing the old one, did notice that the carbon stacks were worn, with one especially cupped out and worse than the other. I compared the one I removed(which I think was a Bosch) to the one I was going to install(the German), and I noticed the new one had a long metal flexible tab on it, which the old one did not. I just assumed this was for contact, but I couldnt see inside of the alternator to see what it was supposed to press against. Anyway, I could not get the regulator to fit all the way into the alternator. I did try and bend the little tab upwards, and then I could get it to go in, with just a little "spring type" resistance, until I could get both screws started. I put a volt meter on my battery while the car was idleing, and it was putting out 13.8 volts, when I revved the motor up, it went up to 14.4 volts. However, when idleing, I still have the alternator light come on real faint, until I increase the throttle. Is this typical, or did I do something wrong with the little metal tab on the regulator? Does the bosch have the same metal tab, or is it more like my original I removed? I turned my lights on for about 15 minutes while the car was at idle, then checked with my voltmeter again. This time, with car at idle, it was only showing 12.85 volts. When I increased the throttle, it raised to 13.5 then after a short bit, on up to 14.4. Just curious if this is performing correctly, can you advise?
thanks,
Jim

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1979 Mercedes 240D (Sold to a neighbor last year,now I have room for a new one,looking for a 300D this time)
1980 Mercedes 240D(bought for a parts car, but I have fun driving it!)

This is simple.... a dim light means a diode is bad.

I'm an ex-battery company guy, but... I don't know the specifics of MB alternators, but generally speaking, there are 3 positive diodes, and 3 negative diodes. If one is bad, the the alternator light will 'glow'. What it means is you are 'down' about 1/3rd your alternator power. You can drive it this way, and if you're carefull, it can even last a long way. Personally, I'd recommend removing the alternator, and having it 'tested' by a good alternator shop. They should find the bad diode, and charge you for just that. $30, and you should be back on the road. (now... can you find a good alternator shop)

This sounds almost exactly like the problem I'm battling. Especially the part about driving it for a long way before it dies. I bought a new battery two weeks ago and it worked fine for a couple of weeks and then died. I've already replaced the belt and the voltage regulator which cured the glowing battery light problem I was having, just like Jim (coadman) but obviously not the undercharging.

I'm about to spring for a new alternator. JNC. Are you saying that I would only have to replace a diode in the alternator? How does one go about doing that?

- Peter.

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2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S

My real recommendation is to take the alternator to a local rebuilder. Let them tackle the repair. Tell them what it's doing, tell them how much money you have to spend.

Most alternators have 6 diodes. Individually, they are not that expensive. Most rebuilders like to replace all the positives (3) or all the negatives at a time (so they match), plus replace the bearings, turn the stator, replace the brushes, and springs.

So, it's about $25 of parts, and 2 hours labor. So, bearings, brushes, and a complete checkout out should be about $100. (unless I'm in the dark ages)

If you only want the bad diode replaced, then ask them about just that repair. (that was the $30 I was talking about, and that is taking the uninstalled alternator to a shop and working out exactly what you want for a repair)

I've got a good shop in the small town I live near. We always take our old starters, generators, alternators to this person. I'm sure he can fix me up, but it's just the fact I have to take the dang thing off, since I just put all the new belts on! Plus, the car will have to sit for 3-4 days by the time he gets the repair done(normal turnaround time).
Jim

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1979 Mercedes 240D (Sold to a neighbor last year,now I have room for a new one,looking for a 300D this time)
1980 Mercedes 240D(bought for a parts car, but I have fun driving it!)

I hope you have success with this Jim. I'll be observing the outcome with interest. Please update us when it's done.

- Peter.

__________________
2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S