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Home >> Mercedes Benz >> 2019 >> Sprinter 3500XD 2D Cab Chassis >> Repair and Diagnosis >> TMC Recommended Practices >> Cab and Controls >> RP 418B - Heavy-Duty In-Cab R-134A & R-1234YF Air Conditioning Systems >> RP 418B - Heavy-Duty In-Cab R-134A & R-1234YF Air Conditioning Systems >> System Design And Initial Installation

System Design And Initial Installation

  1. 6.1 Compressor 
    1. 6.1.1 Compressor Mounting Brackets.  The compressor mounting bracket(s) shall be engine mounted and provide for proper belt alignment and adequate belt adjustment. Fasteners with locking features should be used for mounting the brackets and compressor. See Section 6.1.4  for drive belt alignment.
    2. 6.1.2 Compressor Mounting.  The compressor shall be mounted per the compressor manufacturer's recommendations. The compressor shall be sized to ensure adequate system performance.
    3. 6.1.3 Compressor Clutch.  The clutch shall be assembled to the compressor to allow for the proper air gap as recommended by the component manufacturers).
    4. 6.1.4 Drive Belt(s) Alignment.  Specified alignment is as follows:

      V-Belt Serpentine Belt

      1/2° = 1/10" per ft. 1/3° = 1/16" per ft.

      per span length per span length

      To measure alignment, use a straightedge from pulley to pulley. For measuring exact degrees, utilize an additional straightedge and measure the distance between the first and second edges. Excessively long belts with an unsupported length of 18 inches or greater should have idler pulley(s) installed to prevent belt whip.

    5. 6.1.5 Clutch Electrical Requirements.  The minimum voltage at the clutch coil shall be 11.0 volts with secure ground for a 12-volt system to ensure proper clutch engagement. The measurement should be taken while the clutch is engaged and blower fan on high speed. The voltage should be maintained for a minimum of five (5) minutes. For 48-volt systems, the minimum clutch voltage shall be 44.5 volts.
  2. 6.2 Condenser 
    1. 6.2.1 Condenser Requirements.  A means shall be provided to pass air through the condenser, either by the engine fan with the condenser radiator mounted, or by one or more independently operated fans. On systems where the condenser is mounted in front of the vehicle radiator or charge air cooler, it should be mounted in an unobstructed air stream and not touching the adjoining heat exchange surface. The condenser shall be mounted and sized so as to maintain operating system pressures per the vehicle manufacturer's recommendations. System pressures should not exceed 375 psi maximum normal operating pressure for the high side (discharge side) of the system.
  3. 6.3 Receiver-Drier and Accumulator 
    1. 6.3.1 General.  The receiver-drier and/or accumulator shall be mounted to an isolated component such as the cab or radiator (or isolation mounted) to minimize vibration. Do not direct mount to the engine or frame, nor shall it be mounted near a source of high ambient heat. If exposure to high ambient heat cannot be eliminated, the receiver-drier and/or accumulator shall be insulated with an appropriate material.
    2. 6.3.2  The receiver-drier inlet and outlet fittings should be horizontal as mounted.
    3. 6.3.3 The fittings of in-line driers should be horizontal as mounted.
    4. 6.3.4  The canister of an accumulator should be mounted vertically.
  4. 6.4 Moisture Indicator and Sight Glass 
    1. 6.4.1 General.  If used, the moisture indicator and/or sight glass shall be located near the receiver-drier outlet.
  5. 6.5 Thermostatic Expansion Valve (TXV) and Orifice Tube 
    1. 6.5.1. General.  The TXV should be attached to the evaporator inlet. If remote-mounted, insulate the TXV and line to the evaporator. For right angle TXV, the capillary tube shall be installed and insulated per the component manufacturer's recommendations. The orifice tube shall be located at the evaporator inlet.
  6. 6.6 Evaporator 
    1. 6.6.1 Location.  The evaporator shall be located to provide for ease of maintenance and replacement.
    2. 6.6.2 Air Filtration.  A removable air filter should be incorporated to prevent air flow restriction caused by dirt and debris.
    3. 6.6.3 Moisture Accumulator and Drainage.  A moisture accumulator and drainage system shall be used to remove condensation from the cab.
  7. 6.7 Hoses and Tubes 
    1. 6.7.1. General.  Formed tube assemblies and refigerant compatible hose material will minimize moisture entering and refrigerant escaping from the air conditioning system.
    2. 6.7.2 Sizes.  The hose sizes used in a system should be chosen to ensure proper system performance. In general, the liquid lines between the condenser and evaporator shall be the smallest diameter, the compressor discharge line shall be the next largest, and the suction line between the evaporator and the compressor shall be the largest diameter. All hoses going to and from the compressor should be isolation clamped to the engine to avoid loosening the fittings and to prevent fitting breakage. In all cases, spacing between clamps shall not exceed 18 inches. Isolation clamps should be utilized whenever possible. Avoid using cable ties as the primary clamping device. Always avoid contact with hot surfaces such as the exhaust manifold as well as sharp surfaces which can cause abrasion. Care must be taken to keep A/C lines (hose or tubing) from being pinched, twisted, kinked, or rubbed. If required, use sleeves to prevent damage from abrasion contact.
    3. 6.7.3 Bend Radius.  The vehicle manufacturer is responsible for obtaining and applying information for the minimum bend radius and maximum unsupported length for each type and size hose used in the air conditioner system and ensuring these specifications are not exceeded. In general, the following minimum bend radii are recommended:
      • 3/8" tube OD, min. of 5/8" (3/4" preferred)
      • 1/2" tube OD, min. of 3/4"
      • 5/8" tube OD, min. of 1"
      • 3/4" tube OD, min. of 1-1/4" (1-1/2" preferred)
      • 1" tube OD, min. of 2"
  8. 6.8 System Protection and Control Devices 
    1. 6.8.1. General.  System protection and control devices should be incorporated to monitor and protect system components from abnormal conditions. These conditions are excessive high pressure, low pressure

      and complete refrigerant loss. Independent high-side high-pressure switches, low-side low-pressure, and high-side low-pressure switches should be included in the system, with locations and specific pressure settings per the component manufacturer's recommendations. Refer to Section 5.9 thru 5.12  for specific switch details.

  9. 6.9 System Assembly 
    1. 6.9.1 General.  Vehicle manufacturers should make every attempt to position A/C components so as to allow minimum inconvenience during engine service.
    2. 6.9.2 Preparation.  The main components of the air conditioning system are connected via hose assemblies and/or formed tubes. All hose assemblies, tube assemblies, and air conditioner system components shall be contaminant-free, dehydrated and capped as received from the supplier. The caps should have a snug fit to ensure the components stay clean, dry, and that the caps remain installed during shipment and handling. Caps should not be removed until the assembly process occurs. At installation the O-rings shall be lightly lubricated or pre-lubricated with mineral oil. This oil shall be dispensed from a closed container which minimizes exposure to air, moisture, or other contaminants. The mineral oil container should be capped when not in use. To prevent corrosion of fitting threads caused by electrolysis between dissimilar material of the fittings, apply a corrosion inhibiting compound on the fitting threads only. Caution should be used to prevent contamination of the refrigerant system when using any product other than mineral oil. Caution should be used to prevent the loss of refrigerant oil in system components during assembly. All fittings shall be torqued to appropriate manufacturer's recommendations.
  10. 6.10 System Evacuation By OEM 
    1. 6.10.1 System Leak Check.  The original equipment manufacturer (OEM) shall perform a leak check after system assembly and prior to system evacuation and charging. The leak check can be done by two methods: pressurizing with dry nitrogen, or pulling a vacuum on the air conditioner system. With either method, a short period of time should be allowed to detect system pressure change. Leaks shall be corrected prior to proceeding with evacuation and charge. After system is charged with refrigerant, it again has to be checked for leaks. See SAE J2297, "Ultraviolet Leak Detection: Stability and Compatibility Criteria of Fluorescent Refrigerant Leak Detection Dyes for Mobile R-134a and R-1234yf (HFO-1234yf) Air-Conditioning Systems," [13]; SAE J2298, "Ultraviolet Leak Detection: Procedure for Use of Refrigerant Leak Detection Dyes for Service of Mobile Air-Conditioning Systems," [14]; SAE J2299, "Ultraviolet Leak Detection: Performance Requirements for Fluorescent Refrigerant Leak Detection Dye Injection Equipment for Aftermarket Service of Mobile Air-Conditioning Systems," [15].
    2. 6.10.2 System Evacuation.  The final system evacuation shall reach a level of vacuum and be held for an adequate period of time-ideally a minimum of 1500 microns for five (5) minutes-to remove moisture trapped in the system. A micron gauge (an expanded scale vacuum gauge) must be used to verify that the required 29.90 in.Hg is pulled. A perfect vacuum is 29.921. The evacuation equipment shall be capable of evacuating both the high and low pressure sides of the system simultaneously. The recommended minimum vacuuming time should not be less than 30 minutes. However, it can be less, depending on vacuum pump capacity.
  11. 6.11 System Charging 
    1. 6.11.1 System Charging By OEM.  The correct refrigerant charge and lubrication amounts shall be determined by the vehicle manufacturer and permanently displayed on a label in the engine compartment. This label shall identify the amount of R-134a or R-1234yf refrigerant and the type and amount of lubricant oil used in the system. The label color, style, and size shall be per SAE J639 (see prior Reference 8).  The vehicle manufacturer should charge with R-134a or R-1234yf in the high pressure side of the system while the engine is NOT  running.
      CAUTION: This procedure is only for original equipment manufacturers (OEMs). This is not  a service procedure. For field service, refer to Section 7. 
  12. 6.12 Electrical System Requirements 
    1. 6.12.1 Electrical System Requirements For Air Conditioning System.  Weatherproof electrical connectors shall be used on all external harness/component connections.
  13. 6.13 IHX Design and Installation 
    1. 6.13.1 Design.  In general, two types of IHX are used for mobile HVAC systems: Coaxial (mostly usable) and Non-coaxial design heat exchangers (see SAE J3094, "Test Procedure for Internal Heat Exchangers of Mobile Air Conditioning Systems with R134a or R1234yf Refrigerant") [16].

      A typical coaxial tube IHX consists of an inner tube referred to as the low pressure suction line and outer tubes referred to as part of the high pressure liquid line. Refrigerant vapor flows through the inner tube in a counterflow direction to the liquid refrigerant flowing in the annulus between the inner and outer tube. This counterflow path provides the greatest temperature difference between the two refrigerant streams to yield optimum heat transfer. To further maximize heat exchanger performance with minimum size, the inner tube is convoluted to impart turbulence to both refrigerant flows while the straight-through design helps maintain low refrigerant pressure drops. The design of a non-coaxial tube IHX has a heat transfer section that is not based on a coaxial tube but provides for heat transfer in some other configuration.

    2. 6.13.2 Installation.  As decribed in Section 5.16.1, 

      IHX is installed at the evaporator suction line and condenser inlet line, preferably as close as possible to the condenser inlet and the evaporator outlet lines. However, typically the condenser is located some distance from evaporator (unless it's self-contained system). In this case, keep in mind that pressure drop in the liquid line is insignificant compared to the suction line. As such, never compromise the suction side pressure drop for the liquid side. For coaxial IHX designs, check braze leaks (internal or external). In addition, bends in lines should be minimized because of the pressure drop and density reduction reducing mass flow. Long sections of increasing (upward) slope are less desirable than downward slope or even vertical upward slope (vertical sections allow the oil to pool, plug and then push the oil up the pipe).

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