I had the compressor blow a front seal on my '90 300E last year. I replaced the compressor and dryer assy, purged both directions with scuba air through the evap core. Pumped system down with vac pump and replaced oil, dye and R-12. Worked great for the 3 months of remaining summer (Phoenix) last year. Gauge readings were the same after 90 days as when new.

This year I have a little cooling, but not much. Gauges show 30 psi static on both gauges while off, a vacuum on the low side and about 70 psi on high side. Compressor clutch cycles on command. I can't find any red dye at fittings.

Any advice from the gurus? You guys have made Mercedes ownership possible. So good, in fact, that I just bought a '92 400E for the wife. Now I have 2 white W124's!

Did you evacuate the system properly. There is a special evacuation pump specific for the task. If you have any moisture within the system it will freeze and present blockage.....I am told.

Last summer I pumped the system down with a new 5cfm vacuum pump for 4 hours before refilling with R-12. It ran fine for the 3-4 months of summer last year.

I have found the red dye to be pretty much useless. You need to use UV dye and then search with a black light. You can get an inexpensive UV dye kit that has an injector sort of system with oil and dye. This is usable on R12 and R134 systems.

Once you have charged and have the dye in the system, look at the condensation drain water under the car with a UV lamp, preferably in the dark. If there is UV dye in that water, the problem is your evaporator. Do this soon after you have charged with UV dye because once the refrigerant leaks out it won't cool well enough to make condensation drain.

If the condensation drain water is clean, then keep an eye on all connections with the UV lamp. Common leak points are the expansion valve and the compressor front seal. You have to pull the front nut on the compressor and pull off the front clutch plate to check the shaft seal.

Good luck,

Larry, thanks for the reply. The red dye is UV active as well, I have a UV LED flashlight coming. My question was mainly the vacuum reading on the low side. Both gauges read 30 psi without the compressor, but as soon as the compressor kicks in, the low side drops into vacuum, high side goes up to ~60 psi. I am assuming that I am low on freon, as I remember the low side was 45(?) and the high side ~ 180(?). What should they be in a properly functioning, correctly charged system? Thanks for the patience. Cheers!

It's not LOW on freon, it's empty. I would not recharge it until I found and repaired the leak. The best approach would be to pressurize it with dry nitrogen and 2 oz. of R22 then search for the leak with an electronic leak detector. Failing such facilities, charge it with R12 and dye, (since it is empty your dye is gone) find the leak with your UV lamp, recover the freon, fix the leak and recharge.

Any time a system is drawing into vacuum, see to it immediately before moisture is drawn into the system.

Your best bet would be to repair the leak, evacuate and charge by weight, not pressures. Failing equipment or means to accomplish this, low side will be around 30 and high around 250 or until sight glass is clear. This will depend on ambient temperature.

Good luck,

The low side pulling into vacuum is because it's so low on refrigerant. With 30PSI static showing, you just barely have enough pressure in the system to engage the compressor. A bit more leakage and the low pressure switch on the dryer will disable the compressor.

First task is to find out where it went - easier said than done.

- JimY