Alternator bolts on 1984 300D stuck!!
 

It was tight on mine.I heard sometimes they break.You could find another one at hardware store.

The very bottom bolt may be very rusty and seized to the aluminum
mounting bracket.
It is the lowest bolt and subjected to all the water, salt etc... from the road.

I had this problem with the 85 300D several years ago. The bolt would turn, but not come out.
I had the engine out and was swapping parts over to the new rebuilt engine. I had thrown the timing chain.
The was bolt completely unscrewed from the threads but would not come out of the aluminum bracket.

I finally removed the mounting bracket from the engine and set it with the Alternator up right
and soaked it with Kroil for a week.
Even used an impact gun spinning it to no avail.

Finally used a smaller diameter bolt and beat the snot out of it, and drove it out.
When I got it out, the bolt was so swollen from the rust, it was almost part of the bracket.

Now I slather the bolt with Anti-Seiz before putting one back in.

Is one of the bolts held on with a nut, instead of going into the bracket? Maybe you are just spinning the nut but not actually relieving any tension?

My technique is to always go very slow and take breaks. Try to get penetrating oil in the threads overnight at least, then soak it again and start loosening it. Once you rotate it a bit, soak it again and let it cool for a couple minutes. I dont have much solid evidence, but I think most of the time if a bolt breaks after you have started loosening, its actually caused by overheating it and weakening the steel.

partial success!!!
 

Have you tried running it back in now that it's loose? Keep soaking it with penetrating oil too.

Keep in mind the alternator CANNOT BE PROPERLY ADJUSTED WITHOUT LOOSENING ALL 3 REAR 17 MM BOLTS!!!

MANY THINK INCORRECTLY that only the top outer and bottom bolts need be loosened...
This is wrong and will result in damaging the expensive adjuster bolt and bracket...

Yes, once you unscrew it slightly, keep going back and forth, keeping the threads wet w/ penetrating oil. That will help flush out the rust that is jamming the threads. Rapping on the end of the bolt can help too, by producing a hydro-shock to flush rust, once it is filled w/ oil. I use a 17 mm box wrench on that bottom bolt, and rap the wrench with a hammer to get it started. If really stuck, you can buy a stubby "knock wrench" that has a place for hammering. An air impact wrench is the same idea (anyone try HF's 700 ft-lbf "Earthquake" model?). Coat the threads w/ anti-sieze when re-installing, as I do anywhere steel and aluminum contact.

At least you don't have a 1985 CA 300D. First time I replaced the alternator was frustrating, trying to work wrenches around the rear heat shield, EGR, and AC tube. Now that the EGR valve is off and the AC high-pressure re-routed straight to the condenser, it is easy to get at the bolts. For others, note that the front nut is welded to the alternator bracket, so you might get frustrated when your wrench can't turn it.

I fixed it for you!...

It should be free... take out the top two alt bolts and it should come out.

Soaking in advance with PB Blaster is helpful.

Removing the pulley can be done with simple hand tools.
I used a combo wrench and one of the old alternator bolts, see post # 2 on my alternator update thread.
http://www.peachparts.com/shopforum/diesel-discussion/364297-diy-al129x-alternator-1978-w116-300sd.html

Easiest way to remove pulley bolt is with an impact wrench. Most auto parts stores will swap over for free.