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Hotline Saves Day for Tech Working on Mercedes-Benz A/CPosted 1/17/2006By Craig Zuidema
An "attitude" could well be expected, given the following scenario: With temperatures in the triple digits, and the customer calling every hour to inquire about his car, the technician was understandably frustrated. "I've tried everything under this broiling hot sun to make the air conditioning work on this '98 Mercedes-Benz E320," he explained to me. "But the compressor shuts off after about three minutes, and it won't come back on until you restart the car." Without trying the tech's patience, I asked about the car's condition to gain an understanding of what actually happened to create this symptom. The A/C was nonfunctional when the car was brought in, with a low refrigerant charge in the system. A leak from the compressor's front seal was resolved by installing a remanufactured compressor. After being evacuated and recharged with the correct amount of R134A, the compressor started and ran for about one minute before switching off. The tech noticed the economy (EC) button was illuminated on the A/C push-button control unit and it didn't go out when pushed. After talking to some fellow technicians, he determined that this indicated the possible existence of stored diagnostic trouble codes (DTCs) in the control unit memory. Surfing through his shop information system, he uncovered a procedure for reading DTCs by pressing the buttons on the control panel. Good, he thought: it doesn't require some fancy, overpriced scan tool. But the procedure flat out didn't work. After a few more calls to friends, he found the right procedure. The codes were what he had expected: B1419 (electromagnetic clutch) and B1241 (refrigerant fill). He cleared the DTCs and rechecked; none reset. The A/C compressor engaged immediately after restarting the engine and cool air washed out over the tech's face from the center vent - what relief! But after a few minutes, the cool air changed to warm and the compressor shut off. The EC button light was off but the light would operate when pushed. Then the tech called Identifix. "How many things can possibly go wrong with a basic A/C system?" This article just isn't long enough to list all the tests the technician covered to verify the integrity of the refrigerant system, the power supply, ground to the control unit and whatever else he could think of. We can thank German technology for adding a few more. As it ran, the refrigerant system worked perfectly. With the compressor on, the system pressures on the recharge/recycling machine were about 37psi/2.5bar on the low side, and the high side was about 230psi/16bar. The condenser fan cycled slow and fast as the pressures fluctuated with engine speed. The blower fan and modes all worked as controlled. When DTCs tell you nothing and no mechanical problem can be discerned to explain the symptoms, what do you do? You look at data and try to understand, with the resources you have, how the system operates; not only what enables it, but what could disable it. When you run out of resources, you call for help, right? So this is why the technician called our hotline. Though a simple answer was available, the path leading to it was complex. I gave the technician a procedure for checking the actual values on the A/C control panel for the A/C system inputs. All the values displayed seemed reasonable. None of the sensors displayed the "E" that indicated an open or shorted sensor. But upon closer scrutiny, the evaporator temperature sensor read 145 degrees Fahrenheit. This was strange, since the left heater core temperature sensor read 107 degrees Fahrenheit and the right heater core temperature sensor read 106 degrees Fahrenheit. Under what conditions would the evaporator temperature exceed that of the heater core? This is just the question the A/C control panel asked itself with its embryonic electronic brain. After about three minutes operating in distress, the computer was tired, timed itself out and switched off the circuit from the A/C control panel to the compressor clutch coil. As far as it could tell, the sensor still functioned, within the broadly defined parameters set in its memory. Its programming logic told it that the evaporator should not be hotter than the heater core. If the sensor was reading incorrectly, it had no way of knowing if the evaporator was freezing up so it disabled the compressor. You might think this should cause it to set a DTC, but this is a case where the parameters for default are set differently than DTC parameters. This symptom neatly fits in between. The only saving grace - and this is important - is that the onboard diagnostics allow for checking the system inputs while the system is operating to allow the tech to diagnose the problem. After some rest and relaxation of the evaporator temperature sensor, the A/C system functioned perfectly. Everything - and everyone - was cool.
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