My 300E AC system is totally discharged (no pressure). Obviously I have a leak somewhere, but my question is how do you determine if the system has adequate lubercation present? The system uses R12 and has an aftermarket compressor installed. What is the correct oil for this type of system?

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Tony

1988 300E Metallic Blue Black/light Gray interior

MBCA Member

I am by no means an expert, but here is what I learned.

R12 uses mineral oil, R134 uses ester oil.

I don't think there is a place to check the oil level on a 300E. The oil mixes with the refrigerant. No refrigerant, no lubrication equals compressor seizure. You may well have a fried compressor.

If I were you, I would pay for a diagnostic test at a shop, where they charge the system, add dye and see if it works.

Probable culprits: compressor to manifold pipe joint, expansion valve, or evaporator. The first two can probably be detected with dye. The evaporator needs a "sniffer" at the vents.

Good luck - you are heading down the slippery slope. Lot of posts on this subject.

Chuck (AC but No Money)

The compressor is OK but the system has lost the R12 charge. I plan to pull a vacuum on the system to check and see how big of a leak I have. The AC system did not work when I purchased the car. If the system is fully discharged, would the oil also be low?

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Tony

1988 300E Metallic Blue Black/light Gray interior

MBCA Member

Possibly. It depends on the location and type of leak the system had.

In your vehicle, the refrigerant oil is circulated throughout the system in conjunction with the freon; catastrosphic failure of a high pressure hose when the system was operating would result in a substantial loss of the system's oil. A pinhole leak, on the other hand, will also result in freon loss but with decidely less lubricant loss. The problem that can arise, however, is when systems with chronic, unrepaired leaks are repeatedly recharged with freon only. Although the rate of oil loss is small, these systems can deplete their original lubricant charge over time, even though the freon is periodically replenished. This can be one of the aggravating problems in acquiring a used vehicle with an undocumented service history.

M-B specifies oil replenishment values for systems that have had slow refrigerant leaks vs. catastrophic failures. For systems with no detectable leaks but requiring up to 400 g of freon, M-B recommends the addition of no compressor oil. 20 cc of oil is advised when completely recharging the system due to leakage (in conjunction with replacement of the receiver/drier). In the event of a sudden freon loss (accident or broken hose), replace the defective part(s), receiver/drier and add 40 cc of oil. Verify check these values however, as I believe they are applicable for '90-'92 W124.