I'm at work and don't have the 126 CD-Rom with me, so I can't comment about low/high side pressure.
The system is designed to shut off the A/C compressor if the serpentine belt is too loose. Check out your serpentine belt tension.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.