The difference is normal, the amount of difference may not be. Early 124 cars had small compressors that were never great.

If your system is working properly refrigeration wise I would look to a recirc door sticking open. In the AC position the recirc door is always closed on the first stage to 70% closed. If it doesn't it will mean a 10-15 deg duct temp change.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician