Cadillac Repair: A/C LOW SIDE PRESSURE, r 134 freon, r 12 freon

Question
1992 CAD-CONVERTED TO 134A- AFTER 4 CANS FREON, I'M GETTING 20 LBS LOW SIDE PRESSURE AT IDLE, LESS AT 3,000 RPM.
   VENT THERMOMETER READING 36 DEGREES @ 80 DEG AMBIENT TEMP.
    WHAT SHOULD THE LOW SIDE PRESSURE BE AT THAT AMBIENT TEMP.???

Answer
Converting an R-12 Cadillac system to R-134 usually causes the CHECK ENGINE light to illuminate and the AC system to display a LOW REFRIGERANT message due to the low pressure sensor which is NOT calibrated for R-134a freon and there is NO resolution known at this time other than to go back to the R-12 freon.

There are things that you need to do to get the system working BUT after getting it working you WILL encounter the above paragraph problem.

1. Did you remove ALL the R-12 freon from the system ?.

2. Did you evacuate ALL the atmospheric air from the system using a vacuum pump ?.

3. Did you install 2 ounces of R134 freon oil (PAG) oil in the system ?.

4. Look for a sticker that tells you how much freon the system requires and ONLY install 80% of the R-134 freon in the system.

5. This will get the system operative BUT you will encounter the first paragraph problem.