A/C won't turn on when it's hot

[pryce27]

In a cruel twist of fate, my 92 500E has A/C that works great when I don't need it. It turns on reliably and is ice cold at temps under 75 or so. But park it in the sun for a while at higher temps and the A/C just blows ambient air with no cooling whatsoever. Turning off and on, restarting the car, etc, doesn't seem to make a difference, just won't turn on the cold air in hot temps. I searched the forum threads and couldn't find this problem, anyone familiar enough to suggest the problem and/or a fix?

The guy who did my inspection thinks he might be able to switch out the "A/C speed sensor" to fix the problem. But he didn't seem super confident. Any ideas? Thanks!

 

[gsxr]

Check fault codes on the Basic Module / General Module (diagnostic socket pin #8). That will tell you if it's a speed sensor, or belt slippage, issue. But if restarting the car doesn't clear it, I have a feeling that isn't the issue.

After checking for fault codes, next would be verifying the pressure safety switch isn't defective. This is at the receiver/dryer, you can bypass it for test purposes by shorting the leads together. If no change, it's something else.

You could also have hot coolant entering the heater core, either from a bad monovalve (unlikely) or a control unit that isn't regulating temperature properly. Lots of possibilities here, you'll need to start eliminating one at a time...

Links to FSM, climate control section:
https://www.500eboard.co/misc/Manuals/Servicev3.2/W124/Index/Groups/83ClimateControlSystem.htm