A/C overhaul on a 300E

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This article is a reference for overhauling the A/C system on a 300E with conversion to R134a. Sections include reinstallation for the parts, but do not follow this reference linearly. It makes sense to leave all of the components uninstalled until the very end in order to flush the A/C lines without removing them from the car.


Time 5 hours
Cost $$
Difficulty Medium
Parts required Refrigerant, compressor oil, expansion valve, receiver-drier (with temperature and pressure switches), R134a o-rings
Applicable models All 300Es and 300CEs

Special Tools

A/C Flush Kit


FJC 2710 A/C flush kit - Used for flushing the A/C components

A/C Manifold Gauges


A/C R134A Manifold Gauge Set - Diagnostics, vacuum, and A/C recharging

Vacuum Pump


Vacuum Pump - Used to draw vacuum on the A/C system

R134a Can Tap

R134a Can Tap - Connects a refrigerant charge can to manifold gauges

Refrigerant / Oil


Factory specifications for refrigerant, oil, and their capacities are as follows:

Refrigerant Oil
Type Capacity (oz) Type Capacity (oz)
R12 35.00 Mineral 7.75
R134a 33.50 PAG 46 8.50

Note: R134a was only used in 1993 models for the 300E. If a conversion is being performed from R12 to R134a on older models, then it is suggested to use Ester oil rather than PAG oil to avoid incompatible contamination


  • Mineral Oil - Used for R12 systems
  • PAG Oil (Polyalkylene Glycol Oil) - Used for R134a systems
  • POE Oil (Polyolester Oil) - Used for either R12 or R134a systems and is compatible with either oil types. Commonly used on R12 -> R134a conversion systems.

R134a High and Low Side Fittings

  • High side service port is located underneath the air scoop
  • Low side service port is located behind the ABS relay
  • Install new R134a fittings, making sure o-rings are installed as well

Evacuating existing refrigerant

Before opening any of the sealed A/C parts, the existing refrigerant in the A/C system needs to be removed. Be responsible and properly evacuate, recover, or dispose of the current refrigerant in the system by using an A/C recovery system. If an A/C recovery system is not available, you can visit a local mechanic shop to have this performed. Often times, the shop will do it for free if they can keep the refrigerant.



  1. Remove radiator
  2. Remove upper hoseline connector to condenser from the front; 27mm nut with 23mm counterhold
  3. Remove lower pipeline connector; 19mm nut with 17mm counterhold
  4. Remove 10mm bolt holding upper hose line to condenser
  5. Remove top two clips
  6. Unplug auxiliary fan
  7. Remove condenser with auxiliary fan. Rotate the top of the condenser towards the engine while pushing down in order to gain enough clearance to remove entirely.



Fill the A/C flush kit canister with 6oz - 16oz of acetone and connect your air compressor to the inlet for flushing. Press the rubber tip of the flush gun against the hose connector of the condenser and gently squeeze the trigger, gradually increasing the pressure as you go. Be wary of stuff that will be flying out the other hole. Flush both directions, keeping can upright. Wear eye protection and flush in a well ventilated area.

At the end, blow out the rest of the acetone using compressed air with an air gun


  1. Install 2 o-rings; one on the upper hoseline and one on the lower pipeline connecting to the condenser
  2. Install in reverse order
  3. Check to make sure condenser lines up on the bottom correctly, or else the radiator will not install properly



  1. Remove poly v-belt
  2. Disconnect 3 pole connector
  3. Remove screw for hose from underneath vehicle; 6mm hex bit
  4. Remove 4 mounting bolts. The two top bolts may not be fully removable since the engine wall is in the way; 13mm
  5. Remove compressor by pulling down


Flush the compressor with whatever oil you will be using, whether it be mineral, ester, or PAG. Do not use A/C flush, solvent, acetone, or anything other than oil. Turn the compressor over and let the old oil drain out of the inlet holes for the hoses. Turn the compressor clutch by hand to cycle and move oil around. Fill with clean oil and repeat until the oil that drains out is clean.


  1. Fill compressor with 4 oz of oil
  2. Replace the 2 o-rings on the compressor
  3. Reinstall in reverse order

A/C Hose Line Assembly


The main A/C hose line assembly can be flushed while installed in the car as long as the compressor and condenser are removed.

  1. Disconnect large AC hose where the low side service port connects to the firewall; 31 mm with 27 mm counter-hold
  2. With AC compressor removed, place a zip lock bag around the hose block flange to catch any flush fluid that will come out
  3. Flush through the large hose where the 31 mm nut was disconnected and fluid should come out where the compressor connects
  4. Flush through the small hose where the condenser connects and fluid should come out of the same place through a different path
  5. After flushing with fluid, blow compressed air through the lines until there isn't any remaining A/C flush residue left



  1. Unplug temperature sensor (red) pigtail wires
  2. Unplug pressure switch
  3. Remove hose to expansion valve; 19mm nut
  4. Remove small screws holding brackets on left and right side to engine bay and low speed fan resistor; 8mm
  5. Remove receiver-drier with old temperature sensor and pressure switch


  1. Install a new pressure switch and a temperature sensor on the receiver-drier
  2. Replace o-rings
  3. Fill with 0.33 oz of oil
  4. Connect expansion valve hose and tighten 19mm nut
  5. Screw in 8mm screws

Expansion Valve Replacement and Evaporator Flush

Expansion Valve Removal

  1. Remove mount for check valves behind brake booster; single screw
  2. Separate AC hose bracket on firewall; 10mm bolt and 10mm nut
  3. Remove 10mm nut on expansion valve mounting plate; 10mm, use a 1/4 swivel socket
  4. Separate pipes from expansion valve by pulling on them from engine bay side of firewall
  5. Unscrew and remove the 2x 3mm hex screws on expansion valve; warning: easy to strip, be careful
  6. Pull expansion valve out to remove

Evaporator Flush

In order to flush the evaporator while it is in the car, modifications to the old expansion valve need to be made so don't throw it away just yet!

  1. Drill out the lower hole on the expansion valve completely so both openings are open at all times.
  2. Reinstall this modified expansion valve
  3. Reinstall the two hoses that run to the expansion valve
  4. Cover the lower hose opening near the driver headlight with a zip lock bag to catch the flush
  5. Use the AC flush kit at the larger upper hose to flush the evaporator and both AC lines
  6. After flushing with AC solvent, flush out the remaining solvent in the lines with compressed air until clean
  7. Remove the modified expansion valve


  1. Installation is reverse of removal
  2. It is easier to install the expansion valve when the o-rings are installed on the connecting pipes instead of the expansion valve itself.

Refilling the refrigerant

Connecting the gauges

The basic manifold gauge set usually has three hoses. Two hose will be attached to the service ports on the vehicle during service. Each hose has its own identifying color. In most cases, the hose intended for the low pressure service port is blue, and the hose intended for the high pressure service port is red. The middle hose should be yellow. That yellow hose will be attached to the refrigerant cylinder while charging or the vacuum pump when the system is being evacuated of air and moisture. Your manifold gauge set should have a corresponding gauge and control knob for each of the two service hoses. Like the color of the hose, the gauges and control knobs will usually be colored to indicate high or low pressure. [1]

A typical R134a manifold gauge set has the following colors:

  • Red - High side
  • Blue - Low side
  • Yellow - Vacuum/refrigerant source

The low side pressure gauge is called a compound gauge. That means it can be used to measure pressure or vacuum. The numbers around the outside of this gauge indicate pressure in pounds per square inch (PSIG), and the numbers near the bottom indicate vacuum in inches of mercury. The smaller scales near the middle of the gauge list the temperature relationship of different refrigerants. The gauges pictured here lists the temperature of R12, R22, and R502. Regardless of which refrigerant is being used, the scale designated as PSI is the one used to read system pressures when charging and diagnosing an a/c system. The working pressure of this gauge is from 0 to 120 PSI. The red, high side gauge is used to measure the high pressure side of the a/c system. This gauge has no markings that indicate vacuum. It reads positive pressure only. The working pressure of this gauge is also much higher than the low side gauge. Notice the scale on this high side gauge reads from 0 to 500 PSI. [2]

Connect the blue line to the low side service port, red to the high side service port, and yellow to the vacuum pump. Start with all of the valves open for vacuum procedures.

Drawing a vacuum

Drawing a vacuum on the A/C system is necessary for removing moisture and air from the system and serves as a quick and dirty test for system leaks. A proper pressure test with R22 and nitrogen should be used if leaks are suspected.

With all of the valves open and the vacuum pump connected to an air source, vacuum will begin to draw. Using the highest pressure the air source can supply, attempt to draw up to 30 inHg of vacuum as indicated on the low pressure manifold gauge. Shut off the valves on the gauge for low and high side to hold the vacuum in the system without drawing pressure from an air source. Check for vacuum leaks after letting the system stand still for 30 minutes to an hour.

Charging refrigerant

While maintaining the vacuum drawn in the last step, attach the R134a can tap to the refrigerant charge can and connect the yellow hose to the can tap. Make sure that the can tap valve is closed. Charge a small amount of refrigerant into the low side by opening the low side valve on the manifold gauge as well as the can tap valve. Keep the high side valve closed at all times.

Start the engine and run the A/C and blower motor on full blast. The A/C compressor clutch should engage at this point. If not, try jumping the pins on the A/C pressure switch. Continue charging the system by weight until the system is full. Once the system is charged, cold air should be coming out of the vents.

When charging is complete, low side pressure should be between 30 psi and 50 psi. If the pressure is too high, check that the auxiliary fan is operating at low speed to cool off the A/C condensor.


Compressor won't engage

Auxiliary fan won't activate on low speed