On my 85 500sel the AC system works fine except when the car
has been setting in hot sun all day. Then, nothing happens-
no ac, heat or vent. Is this a climate control problem-if
so where is the unit and is it expensive?
Thanks,
Fred

If nothing seams to be happening, try wiggling the ignition key. The ignition switch has numerous outputs and the one feeding A/C often faills.

I had an intermittent problem at one time with my AC, in that it would run fine a little while and either cut out or go into FULL HEAT mode. This sucked in August. Once it "warmed up" a circuit board component would fail. Once cool it worked again.

My educated guess is that you should swap your PBU.

A rebuilt Push Button Unit for my '87 260E was under 175 with a fair warranty.

75% of the AC electronics are in that one box. Right there with the climate control buttons. Easy to replace with only a phillips screwdriver.

Good info about the 87 300e. Only problem is that it is wrong for an 85 500SEL. Even though the pushbuttons look similar, all the units prior to 86 have separate controllers for blower control and temperature. There are no electronics in the SEL push button controller!

That doesn't mean that the PBC isn't the problem though. It is very common for internal solder contacts to break due to the flexing caused by the pushbuttons and the temp wheel. They are a very good candidate for repairs. Their circuits are easy to see and follow, unlike the later units that do have the controllers internal.