no heat in my 500 sec
 

How the problem appears

Start the car with the heat on the climate control wheel is on red the ec or normal or up/down button. Thermostat is fine and new 80C MB original unit. when the car gets hot there is no heat at all. Restart the car the heat is instantly back. Backup in 1min heat dies and stays dead. All this is with the heat control valve on MAX or 26C. Turn the heat control valve back to white or blue the heat turns off as it should. However, turn it back to red the heat does not restart EVER. Turn the engine off and on again heat is back and stays for about 1 min. Rad flushed out. When you drive it the same symtpoms. park the car switch engine on and off heat back on. What the hell is going on?

monovalve is been fixed with repair kit.....

Could be solder joint cracking on the ACC circuit board. It is not too difficult to resolder these boards as s often done with cruise control amplifiers.

I don't know about the 500SEC, but when my 95 E420 heat was not working, I was getting a DTC 31 on socket 16 of the 38-pin connector. That indicated a bad mono valve. After replacing the valve, I had heat again. It only took 45 minutes to change the valve as it sits right behind the module blower box.

Do you have a code reader to scan for trouble codes?

Here's a good link to replacing it. http://www.*********.com/forums/showthread.php?4535-HOW-TO-Replace-Monovalve-1995-E420&highlight=valve