No Start After Alignment
 

I got everything adjusted, removed the spreader bar, closed the hood, then jumped in a turned the key. The glow plug light burnt dimly and the tachometer needle started a weak dance between 1 and 2K. After a second or two, the glow light went bright yellow, the tach needle red-lined, and a short buzzing sound came from somewhere. But the car didn't turn over.
I've heard stories of a neutral safety switch. I think the PO might have said something about it being disabled, but I wouldn't swear to that. Does this problem sound familiar to anybody? I did put it in neutral in order to roll it over the slip plates, if that provides any more useful information.

Weak battery maybe?
I could have sworn there is a big fuse somewhere inside of the glow plug relay.

I would check the battery voltage and then pop open the glow plug relay to take a look.

11.92V at the battery and the strip fuse is intact.
I put the trickle charger on the battery, but the symptoms were very strange.

11.92 volts is low and is probably the root of your problem. A good battery will show more like 13-14 volts.

12.6 volts

Fully charged automotive batteries should measure at 12.6 volts or above. When the engine is running, this measurement should be 13.7 to 14.7 volts. If you don't have a multimeter to tell you the voltage of your battery, you can do a test of your electrical system by starting the car and turning on the headlights.

Well, it looks like probably a new battery for me. I wonder if it is worth getting a Sam's club membership yet...

Check the engine to chassis ground strap and make sure it's tight and not corroded.

I’d check the engine to chassis ground first thing.

11.70 V is fully discharged, you are at about 25% . Charge the battery for a few hours then put a load test on the battery to determine it's condition.

To check the NSS or the shifter bushings, put it in park and bridge two screws in the plastic case on the passenger wheel well. If it will spin the starter, it is could be the NSS or bushings.

Did you do the alignment? Someone else?

I’ve observed an alignment once where the tech kept turning the vehicle on and off. Leave the key in the wrong position and glow plugs and fan might keep running. Add a few starts and a weak battery, and... voila.

On the W123 Sedans with a turbo there is a wiring junction box on the passenger Fender Well that you can use a jumper wire on.

It goes right down to the Starter

See post #8
https://www.benzworld.org/forums/w123-e-ce-d-cd-td/1626516-start-84-300d-without-key.html

I agree with Phil, it sure sounds like you lost the main ground under the chassis. It is on the left side behind the left front suspension. It is a braided strap between the engine block and the body.

You have all the classical symptoms of current either not getting where it needs to, or finding improper paths, to return back to the battery.

Check at Wal-Mart and at Home Depot. My last battery was $130 at Home Depot, no core required, so I sold the old battery for $40 to the local metal recycle place.

Interesting. There was a core at HD when I bought a group 78. Believe it was $15.

A 49 is heavy enough that not taking the core refund can be a good idea for a few bucks. But I’ve seen core fees as high as $22.

Napa sells Deka batteries and I prefer them and Exide to any JCI batteries since that all got offshores to Mexico. IMO/E the exide leaking thing was old news due to overtorquing and not applying dielectric grease. A little prevention and they’re great. And USA made. I’ve had multiple JCI batteries go south on me too fast.

I’m also a fan of the white MB Battery if they still make it...