Truck/Trailer Refrigeration Unit - Sunbelt Transport Refrigeration

4.21 MICROPROCESSORNOTEThe erasable, programmable, read only memory(EPROM) chip (component U3 on themicroprocessor logic board) has a window on itwhich is covered with a label listing the revisionlevel of the software. The window is used to erasethe chip’s memory with the use of ultravioletlight. The label prevents light from entering thechip and erasing the memory. Under NOcircumstances should this label be removed.CAUTIONUnder no circumstances should atechnician electrically probe the processorat any point, other than the connectorterminals where the harness attaches.Microprocessor components operate atdifferent voltage levels and at extremely lowcurrent levels. Improper use of voltmeters,jumper wires, continuity testers, etc. couldpermanently damage the processor.As mentioned above, some microprocessor inputsoperate at voltage levels other than the conventional 12vdc. Connector points and the associated approximatevoltage levels are listed below for reference only. Underno circumstances should 12 vdc be applied at theseconnection points.Grounded wrist cuffs are available at most radio,computer and electronic supply stores. It isrecommended that these be worn whenever handling amicroprocessor.Table 4-3. Connection Point VoltageConnection Point Approximate VoltageCDT, RAS, WTS 2.5 vdc (Variable)MPF15.0 vdcCAUTIONMost electronic components aresusceptible to damage caused by electricalstatic discharge (ESD). In certain cases, thehuman body can have enough staticelectricity to cause resultant damage to thecomponents by touch. This is especiallytrue of the integrated circuits found on thetruck/trailer microprocessor.Although there is less danger of electrical staticdischarge ESD damage in the outdoor environmentwhere the processor is likely to be handled, proper boardhandling techniques should always be stressed. Boardsshould always be handled by their edges, in much thesame way one would handle a photograph. This not onlyprecludes the possibility of ESD damage, but also lowersthe possibility of physical damage to the electroniccomponents. Although the microprocessor boards arefairly rugged when assembled, they are more fragile whenseparated and should always be handled carefully.During emergency situations, the test board may beused to keep a unit running and prevent a critical loadfrom spoiling. Since the microprocessor is totallydisconnected from the unit, it cannot monitor theengine’s safety switches for oil pressure and coolanttemperature. Since the engine is running unprotected whenthe test board is used, it is imperative that should aproblem develop with the microprocessor, it be replacedimmediately. The test board is intended to be atrouble-shooting tool only.When using the test board to troubleshoot, the unitshould be started in low speed, unloaded cool in the sameway as when the processor starts the unit. Good judgmentshould also be used when cycling any unit with the testboard. Rapid cycling should be avoided.When welding is required on the unit frame, or on thefront area of the trailer, ALL wiring to themicroprocessor MUST be disconnected. When welding isperformed on other areas of the trailer, the welderground connection MUST be in close proximity to thearea being welded. It is also good practice to remove bothbattery cables before welding on either the unit frame orthe truck to prevent possible damage to othercomponents such as the alternator and voltage regulator.a. Replacing Key Board (Trailer Unit)Should damage to the Key Board of themicroprocessor occur, it is possible to replace only theKey Board.It is absolutely imperative that whenever amicroprocessor is removed from a unit for any reason,that the old Key Board gasket be removed, and a new onebe installed. All replacement microprocessors and KeyBoards are packaged with replacement gaskets for thispurpose.The tightening sequence shown below should beadhered to when tightening the retaining bolts or nuts.The recommended torque for securing nuts or bolts is 15inch-lbs. Over tightening the nuts or bolts, or using atightening sequence other than the one shown, may causewarping of the Key Board.513 482Figure 4-20. Tightening Sequence forMicroprocessor (Trailer Unit)b. Hour MeterThe hour meter can be set to any value via the serialport, if the meter has less then five hours on it. This allowsa replacement microprocessor to be set to the same hoursas the microprocessor it is replacing.The microprocessor has two programmable registerswhich are set via the serial port. These registers arecompared to one of the hour meters (diesel, standby, orswitch on). If the hour meter is greater than the registerthen the proper alarm is set.764-19 62-02884