Just add the ground for now.
This is a step by step diagnosis, and you do not want to confuse the issue further.

.

See post #4. Could have saved you from pulling and replacing the alternator again.

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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

Glad it was the meter lying and not your regulator gone nuts. I was almost smelling the burning electronics from here....

I'm an engineer, here are my 2 cents on troubleshooting

- you must be able to trust your instruments (I think we just proved that)
- diagrams of the system are essential, don't assume
- only try 1 thing at a time
- if the thing you tried does not help, put it back the way it was before trying something else
- keep a journal of what you did and what you observed each attempt
- sometimes you got to step back and marinate on what you have seen, it's amazing how doing that will give you a fresh insight

Good luck

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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

just put the dvm acronym together... didnt know what the dvm was before... lol

I thought everyone knows what a DVM is. You should have asked if you didn't know what it stand for. I will spell it out from now on

DVM= digital volt meter

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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

DVM's are nice because they are simple, have very high ohms impedance, and have no parallax.

Analogue meters allow you to see tiny variations of readings if they are low enough frequency.

I like to have both available to me.

I, too, agree with checking your grounds at this point.

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Codifex
1981 240D ChinaBlue (Got her running with a donor engine.)
1983 300DTurbo w/sunroof.
1984 300TD manual sunroof. (Electrical Gremlins)

New update... I had the second alternator retested and failed.... two bad ones in a row... plus my first one which was bad... Also i learned a safety tip from the guy at the auto store... never grab the fan while using an impact wrench to hold it still.... He shredding his hand up.... i felt bad cause i was just about to tell him that wasnt a good idea when he hit the trigger... all in all I learned to check and then recheck your testing equipment and also test new parts before you install them.... I now have all new wires and grounds and battery lines... before long i can call my car brand new...

Go to another shop if you want to test the alternator. It is unlikely 2 new alternators were bad. The fact you measured 16.5 v and then learned that the DVM was teading 2 v high put the alternator at 14 v which is correct charging voltage. The fact that the bench test was zero v indicate it was not hooked up properly or the wrong test dongle was used.

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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

just drove car for an hour or so and came home... wife went to take car out right after... car started very sluggish which is the same problem i had with the shutoff pressure.)still haven't figured it out) Then she tried to restart it only to have the same starter solenoid just click... i had to jump the car again but didn't make a difference still wouldn't start... i was messing around with the glow plug relay and once unplugged the car started fine no problem.. plugged it back in and it wouldn't not start... im beginning to think my problem from before is the glow plug relay? does this seem to match? Also my voltmeter was wrong and i also think that my idle may be a tad low... around 650... I cannot stand electric issues this one is giving me the run around..

could be the relay staying on all the time; there is a way to add a wire at the GP relay connector that will light the GP light in the dash as long as the GP's are actually on (i have to check my notes). without this mod the GP light will go out when the relay thinks it's ok to start the engine; however the GP's actually stay on longer after the light goes out.

I checked the light and and i still could hear the clicks of it cutting out after about 30 seconds which made me think it was still good... I checked the 80 amp fuse strip inside of it and found that the two screws that hold the fuse strip down were very loose... I tightened the two screws down and now my car has started everytime... cross my fingers this is all it was...

that's very possible that the loose fuse was the problem; the relay would keep resetting itself and continue to draw current. i installed a permanent volt meter in my SD to keep an eye on the voltage; when the GP relay is on it only shows 11.2 volts while running but once it turns off it goes to upper 13V. the GP's require a very large amount of energy.

This makes no sense to me.

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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 was trying to find something that might show this is the problem and saw this small thread.....

http://www.peachparts.com/shopforum/diesel-discussion/306327-glow-plug-fuse-screw-tightness-check.html

maybe everyone with a intermittent start problem should check this... worth a shot anyway

alternator worked good for two days... tried to start again today and just click... battery was at 11 volts.... jumped it... alternator was hitting a solid 9 volts at any rpm.... would not charge battery at all. new battery cables ground and new wires and no change in problem. my thought is maybe the overcharge protection sensor....thought I would also mention the battery light that came on last several times did not illuminate this time...