I have an 89 300e that I converted from R12 to R134 last summer. I just replaced my condenser and am now re-charging my a/c system.
My question is what should the settings be on the gauges for 134? With R12, I was around 150 on the low side and 20-30 on the high side.
But I'm not sure where I need to be with 134 gauges

In a converted system it's anybodies guess. It is a very inexact science. You have to just put enough in to get it to cool and make sure you don't overcharge. If you overcharge, the high side pressure can easily run away. Make sure you use aux fan switch for 134. The sight glass is no longer much of an indicator either.

Your 150 high side pressure for your R12 system certainly sounds low.

The 124 is probably the worst candidate for 134 conversion of all the US MB's so best of luck,

Have a great day,

My compressor went out last summer and when I bought the new one it was for a 134 system, so I went ahead and replaced the reciever dryer and the expansion valve and recharged it and it worked great. Someone helped me recharge and I didn't think to look at the gauges for 134, and now that I'm doing it again I'm kinda just winging it.
It's not to bad fight now but for some reason my middle vents arn't working. The air is coming out the defrost vents and the side vents. Ever heard of anything like that?

Your center vent problem is either the actuators or the push button unit. Try resoldering the push button unit first.

Good luck,

You can also check the actuator. If you pull the glove box, you will see the vacuum "tree" that connects to all of the actuators. I apoloigize but I don't remember which one is for the center vents, hopefully someone can follow up with that, but just hook up a mitivac to the vacuum line for the center actuator and if it holds vacuum, you are good to go. Otherwise... replacing the center actuator requires pulling the dash from what I have read. On the plus side, it could be that the actuator for the defrost vents is broken, in which case its about $30 for a new part and you can service it through the glove compartment.