MERCEDES-BENZ: A/C DIAGNOSTICS & EVAPORATOR REPLACEMENTSteve Brotherton
Soon it will be summer and we are already doing air conditioning work in northern Florida. We did our first 124 chassis evaporator of the season this week. By the end of this month, our gross will increase by 30 percent, mostly on the seasonal increase in air conditioning work.
I must have worked on Mercedes-Benz automobiles for 15 years before I ever replaced an evaporator. Through the 1970s, we pulled many an evaporator out of 107 and 116 chassis models to repair a poorly designed interior drainage system. The early models had little flappers on two interior tubes that carried the condensation to a lower tray that had the external drains. Originally, we were to open the case and snip the flapper from the end of the rubber tube. Later, Mercedes-Benz came out with instructions and parts for the addition of two more external drains. The 114/115 chassis cars were equipped with heater motors that couldn’t be removed without the removal of the evaporator case. The need for a heater blower now "totals" those cars.
Metallurgical changes were made in the mid-’80s, probably to save weight. Copper/brass were replaced with aluminum. Now 10-15 years later, evaporators are probably the #2 refrigeration problem, behind compressors. Number two in occurrence, but a big #1 in cost. Many manufacturers made provisions for easy removal of their evaporators, foreseeing replacement during the car’s life. My experience with Mercedes-Benz says that they buried it because it would never need to be changed and then something went wrong. Two of its finest lines of the last 20 years, the 124 and 140 chassis, now come with footnotes about their cost of ownership due to air conditioning costs. We will probably do 10-15 of the 12-hour 124 model evaporator replacements this summer, and two or three of the 22-hour 140 chassis evaporator jobs.
There really isn’t any theory to be discussed about evaporators, and "R & R" instructions are contained in all the data systems. However, there are a few pointers worth noting. I found one recently from the iATN archives. According to James Houwen, it is possible to crack the windshield if the car is lifted with the dash brace removed. I’m glad that I didn’t figure out that one.
Dennis Warden (from the technician site www.lmvind.com) also points this out in his tips that are included in the sidebar. Also reference the photo on that shows the inside of a 140 chassis with the evaporator assembly removed. It is clear from the photo that my technician didn’t remove the dash brace. It is still a good warning, as its removal could be necessary in some instances.
I can tell from experience both how easy and expensive it is to chip the center console wood on a 140 chassis model ($675). I have also noticed that the $1,140 dash of the early models has been replaced by a $3,820 version on ‘95 and up models. Be careful and remember the risk factor when estimating.
When doing the evaporator on 124 models, we always include all the vacuum mode actuators. They have a 10-15 year life and some are almost as hard to replace as the evaporator. The part numbers are: 124 800 00 75, 124 800 02 75, 124 800 03 75, 124 800 04 75 and 124 800 11 75. When replacing the unit on the early 140 models, there is an additional spacer/baffle, p/n 140 831 08 36, that must be added as the later-model evaporator has been reduced in size.
Warden points out that before removing the evaporator (see accompanying sidebar, point #3), the system should be diagnosed to make sure all problems are solved during this labor-intensive job.
Since we are inside the car, I would like to address the great diagnostic capabilities available with the modern pushbutton controller. Three different forms of diagnostic information can be gathered here. I find the "actual value" function most helpful. Diagnostic trouble codes are also available, along with the ability to activate the individual door positions (individual flap tests) within the dash. All of these functions, actual values, fault codes and mode activation, can be done from the various buttons on the pushbutton assembly.
I really like the actual values, as one can drive the car and watch the activity of important functions that include evaporator temperature, engine temperature, blower control voltage, etc. See Chart 1 for the list of actual values for the 210 chassis E320. Other chassis are similar, but this chart is given as an example.
The procedures for reading Actual Values go like this: Turn on the ignition, press the "AUTO" button, set the temperature on each side to 72°F (this can be done quickly by pressing both the red and blue arrows at the same time), then press the "REST" button for five seconds or until the left-side display says "1."
The right-side display will then display the in-car temperature. Pressing the "AUTO" on one side makes the positions change up or down (for example, 1, 2, 3, etc). Pressing the opposite "AUTO" button runs the functions in the other direction (for example, 3, 2, 1). The test can be ended at any time by tapping the "REST" button.
Figure 1 shows the Actual Value screen. It is value #5, which is evaporator temperature. The right-side value of 06 is the temperature in Celsius.
I first ran across these tests while trying to solve a C230 problem whereby its compressor would shut off within one minute of starting. There were some fault codes involved with communication between the fan control module and the pushbutton module. I came across the actual value test while trying to make some sense of the code. I tried the actual values without the engine running. The evaporator temperature showed 155 degrees. When I discovered that the dealer had two of the sensors in stock, I had a good idea that the problem had been found.
The new sensor gave me realistic values and the problem was fixed. That model has a variable displacement compressor and it was interesting to watch the evaporator temperature while on a road test. It dropped rapidly, then slowed and finally just stopped at 42 degrees. (It was probably 95 degrees outside at the time). This feature has many values, as the list of data is comprehensive — engine temperature, engine speed, vehicle speed, battery voltage — to name a few.
The procedures for pulling Fault Codes go like this (See Chart 3): Turn on the ignition, and set the left temperature to "HI" and the right temperature to "LO." Then, simultaneously press buttons "Rest" and "EC" for five or more seconds. All of this must be done within 20 seconds. The "Recirculate" button will flash its LED. The screen will then go blank, and the first code can be brought up by pressing the right "AUTO" button.
Subsequent codes are retrieved by additional application of the right "AUTO" button. Figure 2 shows code B1234. Note that the code starts with "E" for error. Also note that the B1234 code appears as Eb1 234 on the screen.
The Individual Flap Tests are run by idling the engine, pressing the left and right "AUTO" buttons, setting temperatures to 72°, manually opening the fascia vents, and simultaneously pressing the "Rest" and "Recirculate" buttons for more than five seconds. The first step: Left display "0" and right display "LO" should appear. Pressing the left "AUTO" button changes the steps. Pressing the right "AUTO" button varies the two modes "HI" and "LO" for each step.
The functions that are checked are described in Chart 2 and can be verified by the actual air flow changes enacted. Figure 3 shows the third step (DTC) with "2" indicated on the left side and the "HI" mode indicated on the right.
All of these tests are so easy to run that they should be done frequently to gain familiarity with them. They definitely should be run before and after major dash surgery.