Low side pressure should be the same as high side with car off.

80-90 psi at rest sounds like it's got a charge.

Where did you get the 45 psi?
If the car is off, that's not good.

Low side should pull down to 20-30 psi when compressor is running, high side should be 200 psi +.

Pressures depend upon outside temp and humidity, plus airflow thru the condenser.

A good indicator of the system working is when the return to the compressor line is cold.

Jim

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14 E250 BlueTEC black. 45k miles
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94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles