'85 W123 300D no-crank
 

Hi all,
I've been having some very intermittent issues with my 300D not cranking. Once in a while it will fail to crank entirely when I turn the key, and sometimes there will be a delay while the key is turned to the 'start' location. It might take 1/2 - 1 second before the starter will engage. I've replaced the ignition switch with an OEM unit a couple years ago, before this problem started. I've also put in a remanufactured starter and OEM neutral safety switch, and rebuilt the shift linkage (automatic transmission). I also cleaned the contacts at C105 (terminal block on the right front fender). Those changes seemed to help with the problem, but not completely prevented it from happening again.I've cleaned the ground connections everywhere in the car, though I would like to replace the main ground cable to the battery, and from the chassis to body.

What might I be missing? According to the wiring diagrams I've found, which I'm not really an expert at reading, it seems like power to the ignition switch is supplied either through the headlight switch or pre-glow system? Could one of those be the problem? Is this a grounding problem?

Also, it doesn't look like anyone online sells new OEM ground cables. What would be a good high-quality alternative? Should I make my own from scratch? I'd like to keep the stock look, especially since the terminals seem to be pretty high quality.

I've had a similar problem with an '89 Celica, as well as a '65 AMC Rambler American, and in both cases it turned out to be a bad ignition switch. On the Rambler, I was able to take the ignition switch apart (universal $10 autozone switch) and could see where the terminal going to the starter was bad and had been making intermittent contact.

I would test the ignition switch or take it apart and see if there's any corrosion if possible.