Voltage regulator screws stuck! 1978 300D
 

I've tried pb blaster. Don't really know what else to do to get this voltage regulator off. I tried a vise clamp, a few different screwdrivers but it won't budge. Any recommendations?

Try getting the an appropriate sized bit placing it in a 1/4 size socket and using a socket wrench. That might give you some additional torque you might not have with a screwdriver.

The bit in a socket is a great idea. You should also place the bit by itself on the screw head, and if you can get a hammer in there, tap on the bit to see if it will loosen it up any. It might also help to better seat the bit if the screw head has rounded any.

These are great ideas but there's so little space in there! I think I need a different size screw bit. Mine aren't fitting in there just right.

Answer
 

Remove the alternator from the car.

Use one of these with the correct bit.
https://www.google.com/search?q=manual+impact+driver&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

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oh, man, easier said than done. i've been reading thread on how to remove the alternator and maybe mine has a different design? it's a 78. or maybe i'm just confused. i removed the lower bolt and removed the nut from the upper bolt. can't seem to figure out what to do past that to get this thing out!

here's a pic of the other side of the bolt - the upper one that i removed the nut from. can i attempt to hammer (if i can get in there) the other side of the bolt to get it out?

that pic should be turned to the left

Once the main bolts are loose, adjust the tensioner to slacken the belts then remove them. Pull the main bolts and the alternator and tension bracket comes out as a combined unit. Then the tension bracket can be removed from the alternator.

finally got it off! not easy. i had to remove all three bolts. impact driver worked for the voltage regulator. i hope that's the problem and not the alternator because i do not want to have to take this thing out again! I've attached a pic of the old and new regulators.

Your 1978 300D has that kind of bracket? That is odd... I didn't even know that type would fit.

Well, I hooked it all back up and turned on the car and the brake light and that light with the circle and 6 triangles (what is that light?) stayed on, even when the brakes were off. That's what it was doing before. I touched a voltmeter to the battery and it read 12.3.

Does this mean that I need a new alternator? Should I drive it around first or will that not do anything?

12.3v is NOT charging enough...what happens when you increase rpm with the voltmeter attached. Maybe turn the headlights on. Bring it up to 1700 rpms. I think with some regulators/alts. the rpm has to be over a certain amount/range before it kicks in and charges. I think that circle with triangles is low brake pad warning light.( front brakes) The light that says brakes could be the e- brake on or low brake fluid in reservoir indication.

wait for another member more experienced in electrical to weigh in before giving up on that alternator. There could be more you could do to fix this. I am also not positive about 12.3 being too low, but pretty sure it is suppossed to be at least above 12.8 to be o.k. I wonder if your battery was fully charged before you hooked it back up to the new regulator?

There is a metal springy strap inside the cavity that pushes on part of the regulator - make sure its not rusty and is actually contacting the regulator - also make sure the mounting screw boss where the regulator is grounded on is good and clean and smooth.

12.8 is too low - ideally you should see 13.8 or something.