My 89 240 has a bazillion (unkown) miles on it, so it acts a bit strangely sometimes. With some help from friends and forums I've fixed a few things and made a significant difference in the way it runs and idles. Recently I've noticed that turning OFF the headlights causes a huge drop in the idle that either right back to normal in an instant, or it dies immediately. Doesn't happen every time, but very often.

Battery voltage is 12.6 off, 13.4 running. Revs make no difference. I can hear belt chirp. Belt tension issue?

 

13.4 V is pretty low for running, at least with the headlights and other accessories off

the belts shouldn't squeak or chirp, if they do, either they are badly worn, something is too loose, or something is too stiff and hard to turn

you could have a badly worn alternator. it /may/ just need brushes, but if its not turning freely, you may need a new alternator (you can change the regulator/brushes a couple times but eventually the bearings and/or commutator wear out and its new alternator time).