Does anyone know which side is low presure side on a 454 chevy air conditioning compressor? The right (passenger) side is marked with a S. The Left (driver) side is not marked at all. Both fittings are the same size.

The old R-12 systems used the same size fittings on both...when you convert to R-134a (which is far less expensive than staying with R-12), the HP fitting will be larger, and neither is compatible with the R-134a manifolds/gages.

Most likely, the passenger side fitting is low - or "Suction", usually taken after the expansion valve, which is on that side.

thanks, I couldn't figure out what the "S" stood for. we are changing the system over to the 134, we just purchased the conversion kit. with the kit, you still use the old compressor. thanks for the help, will feel much better while doing the recharge. (not pretty if you hook the high preasure side.)

Despite what may be said, it's best to flush out the system before charging with R-134a, mainly to get rid of the old oil. R-134a requires a different oil.

Note that if there's any R-12 in the system is should be recovered, not vented.

However, doing a "dirty conversion" shouldn't cause any issues, but if it does, it can be flushed later.

If you check the lines, the bigger is the low pressure and the smaller the high.

If you are conveting to R134A make sure you replace the reciever/drier. Unless the unit on your Barth has been replaced in the last ten years the desiccant in it is not be compatible with R134A. It is the filter of the A/C system and should be replaced anyway.
Tell us what came with the "kit", One does not fit all.

Neil.