R134a performs so poorly in the W124 to begin with when the temperatures heat up that not putting enough in decreases performance even more.

Pressures will never be the same as R12, and that is half the problem because the additional pressures heat up the condenser and then the system can't shed the heat regardless of the fan set up. It'll only perform half decent on the freeway, and below 90F. Anything above that, especially at idle, and expect 50-60F coming from the vents. At idle, in 110F, mine is hopeless...75F coming out the vents. On the freeway it'll keep around low 50's, but unless its an hour drive, the car never cools down. Turn the fan down low and you immediately realize it did nothing to cool the cabin.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k