What really does cause induction contamination? - TAT - The ...

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Air conditioning and serial data BUS communication by Jack Stepanian and Sam Nazarian A VX Holden Commodore presented itself to the TaT research workshop with inoperative air conditioning along with a note that all it blew was ‘hot air’. It was revealed, during history taking, that a third party had attempted to diagnose the cause and told the customer that the system had enough gas and it should be OK, even though it wasn’t. We decided on a text book approach to diagnosis. This story will reveal how the dashboard air conditioning switches alert the multifunction control unit, often referred to as body control module, and how it communicates with the power train control module via a serial data BUS line, to enable activation of the compressor clutch. First, the fuses and the high and low pressure hoses were checked and we even had a play with the dials on the dash, with no result (pic 1). Manifold gauges were connected and both high and low pressures measured. Yes, compressor on and off via a relay based on two parameters a) input from air conditioning pressure sensor and b) serial data BUS request message received from the multifunction control unit. The multifunction control unit in turn initiates a request message to the power train control module based on two parameters, the activation of air conditioning master switch and the heater blower relay. So where do we start? Perhaps communication between the two control units. Serial data BUS communication and block diagram While there are many ways of checking whether there is communication between the two control units, including using an oscilloscope, we opted to use the scan tool and check if communication was present (pic 2). Multifunction control unit and circuit diagram The circuitry of a multifunction control module is arranged such that the body control module needs to see two active inputs. One is the grounding signal of zero volts produced by the blower inhibitor relay which in turn enables the operation of the blower motor via the blower motor resistor pack and the other is a 12 volt supply signal generated by the master a/c switch. Appropriate terminals were checked and both were OK. Power train control module and circuit diagram Since the power train control module is responsible for turning the compressor on and off via a relay, the controlling relay was identified and bridged to check compressor clutch operation and all were OK (pic 3). 1. there was enough gas for the system to operate but the compressor was not engaging. A scan tool was connected to the on board diagnostic connector and we looked for any trouble codes in every control module. While there were a few unrelated codes, these were promptly noted and erased and the process repeated, again with no result. A quick look at data lists of some of the control modules proved to be indecisive since we were not sure of the process. It was time to profile the chain of command to facilitate a defined prognosis. Chain of command A quick glance at the block diagram revealed that the power train control module is responsible for turning the The Automotive Technician 24 2. Communication channel was not isolated (it was normal) and the search continued elsewhere. Power train interface module (PIM) In the block diagram it was also noted that the multifunction control unit at times communicates with the power train control module via an interface module. This interface module is only used for V8 engines where PIM translates universal asynchronies receive transmit (UART) messages into CLASS II. By the way, a V6 engine uses UART as a form of protocol to communicate with other control units and since this car was a V6 it was one less item on our agenda to check. 3. Closer examination of the circuit diagram revealed that while the compressor clutch solenoid is controlled by a relay, there seemed to be a ‘spy’ or a form of feedback wire from the compressor solenoid winding side back to the control unit (being pulled up to supply voltage). A quick look at the data list revealed no such data information available and a physical measurement of the wire to and from the power train control module proved to be OK. Prognosis Now we had a set of dashboard switches that were OK, a serial data BUS communication that was OK and the compressor relay circuit which was OK. So our attention turned to the air conditioning pressure sensor (pic 4).
4. The sensor is a pressure transducer that generates a voltage proportional to the amount of pressure in the high side. In order to simulate a similar output voltage 5. range of between zero and five volts, a linear potentiometer (pic 5) was arranged with 10 kilo ohms resistance. This is to minimise impedance loading effects on the control unit (see TaT issue 23, October 2011). The potentiometer was fitted with the wiper centre arm in the centre of the sensor’s connector terminal (pic 6) and resistance slowly varied from one The turning on and off thresholds were double checked with a scan tool data list and all were OK (pic 7). Range of hysteresis Hysteresis is where the operating parameters such as the cutting-in and cutting-out are not at the same setting. This is to minimise the hunting effect. Here are the two set of hysteresis: a) Thermo fan operation – the second fan (driver’s side) cuts in at 2033 kPa 3.46 volts and cuts out at 1552 kPa 2.83 volts. b) A/c clutch drop out – when excessive pressure is noted, perhaps due to malfunctioning cooling fan, and pressure begins to build up the a/c clutch drops out at 2928 kPa 4.6 volts and the clutch cuts back in at 2336 kPa 3.9 volts. Fan operation a) First thermo fan and clutch cuts in at 240 kPa 1.02 volts b) Second fan (driver’s side) cuts in at 2033 kPa 3.46 volts The culprit was the sensor. The system was degassed and a new sensor fitted (pic 8). It was then evacuated and the correct amount of refrigerant was introduced into the system while the output voltage was being measured from the new sensor (pic 9). 9. 10. 11. The first was to directly illustrate the intricacy of the chain of command and how to follow this chain of command using appropriate tools such as circuit diagrams. 6. 8. extreme to the other. Presto! Everything came to life.The compressor clutch did cut in, fans started to operate and as it was dialled more the compressor cut out. Pure magic. In order to measure dynamic current draw (spikes, slippage, ripples and all) a 0.1 ohm 5 watt ceramic resistor (pic 10) was positioned in place of relay terminals 30 and 87 and the current draw noted (pic 11). Current draw was 3.5 amps and there were no spikes in the pattern, which is good. In conclusion The second was to indirectly illustrate that sometimes replacing components to remedy symptoms may not be the appropriate solution without fully appreciating how the system ought to work. Armed with this information, where would your next starting diagnosis point be? Should you be confronted with the same symptom, would you simply replace the pressure sensor or relay? Where does one stop and step back to opt for an informed holistic approach. 7. You’re probably asking yourself did we really have to go through all this effort to decipher the cause of the problem. While the answer may seem simplistic, the thrust of this article was twofold. Invest in a 10-kilo ohm linear potentiometer and substitute it for the air conditioning pressure sensor and note the range of hysteresis as fans and clutch are cutting in and out – it’s fun. The Automotive Technician 25
Page 1 and 2: February 2012 Issue 25 www.tat.net.
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