grounds
 

Check grounds by going from centre of negative terminal on battery to clean area on block with your meter.I would prefer right to alternator case but too dangerous on this design with engine running. You should read virtually no voltage drop. Have friend crank engine and do same test again while cranking. Normally you may see a small voltage drop but I would start to check out cables and grounds if above about 1/2 a volt. The cables and grounds etc are quite old on most of our cars. Guess our babys are senior citizens of the car world now. Also would not hurt to check for any voltage drop between engine block and clean area of body shell. again with the engine running. Quick, fast, painless checks as you already own a meter. :sun_smile Yes i have seen just the alternators inability to charge being the only noticeable symptom of a marginal ground.

Did some more searching and learned how the charge light works
 

I crossed a line while tracing the wiring diagram in the above quote. As I mentioned in an earlier post(this thread), one side of the light is tied to battery + 12v thru the key switch via the fuse block(on my '77). the other side goes to the D- at the alternator/regulator and supplies external exciitation current while the llight is iluminated. When the alternator starts charging, 12v+ is applied to this same, point causing the same potential on both sides of the bulb so that no more current flows thru the filament and the light goes out.

Someone let me know if I finally got it. :D

Did some more searching and learned how the charge light works
 

That's pretty slick. Except that doesn't explain why the light doesn't work when the alternator isn't charging. My light works (the light comes on when starting) but failed everytime to tell me there was a problem with the alternator.

Now that I think of it though - if I had a bad ground, that would mean the alternator couldn't put it's current back out to the battery. That would mean that one side of the indicator light wouldn't have a ground reference (or the same ground referernce) as the other. Then the light wouldn't light. Maybe that's a "feature"! If your charging system dies but the light doesn't come on you know you may or may not need a new alternator/regulator, but you definitly have a bad ground between the engine and the battery.

Kip Amore

Which brings up something I forgot to include,
 

on my OM616, if the bulbs filament is open, the alternator will not charge. I suppose I could put a resistor in parallel with it so there would still be a path for the excitation current for the alternator?? :confused:

Copy/paste to address bar. Block diagram of charge circuitry
 

file:///C:/Documents%20and%20Settings/JIm/My%20Documents/alternator.jpg

:rolleyes:

Someone let me know if I did something wrong with my
 

last post as I do not understand the reply. Just trying to learn this computer/Mercedes stuff....

Aha!
 

Finally got some time and some daylight to examine the Benz this morning. Sure a ****, the is NO GROUND strap anywhere from the block to the body, much less the block to the battery. Aha! Eureka! Will the engine still be grounded to the batt? Maybe yes, but probably not, and almost certainly not in the way that the alternator needs to "see" it. Could be the car never had one, could be that it rotted away during my car's 12 year storage slumber. At any rate, good diagnostic advice on this SIG, and thanks to all who helped.

And since it's been on my mind these last couple of weeks, I think I'm going to install a traditional amp gauge in the car. Maybe in one of the heater vents, or maybe in the console somewhere where it can be easily removed if I want to keep the car stock. This problem (and expense) could have been greatly simplified with a regular old gauge. They don't call the idiot lights for nothin'.

Kip Amore

Without a ground strap the speedo cable housing carries all the current. I have personaly seen one smokin when the key was turned on and the glow plugs kicked in, never mind the current from the starter.

You pasted a file that you have on your computer. Nobody can access this file because it is not a web page. Unless you have the diagram posted on the web, nobody can see it.

You could "attach" the file to a post on Mercedes-Shop. However, it is normally difficult to read attached diagrams.

Ground strap will run from one of the starter bolts (61x engines) or one of the bell housing bolts (60x engines) to the frame on the passenger side. Without a ground strap they usually don't start (no starter action) unless both bushings are out of the shift linkage, in which case the starter grounds via the shift linkage. This can cause smoke from the shifter inside the car, along with erratic starting, welded shift linkage, etc.

Peter

Thanks, Brian. Another lesson learned. Appreciate everyones patience.
 

... :o