No Air Conditioning, No Fare

Miami-Dade's preventive maintenance standards help take the heat out of county policy.

Miami-Dade Transit Agency (MDTA) of Miami, Fla., is setting new standards in preventive maintenance programs that increase the reliability of rebuilt bus air conditioning compressors and air conditioning systems. As a result, millions of dollars in reduced downtime and catastrophic failures has increased ridership revenue that would otherwise have been lost (Metropolitan Dade County's policy for its bus riders is "no air conditioning, no fare"). Air conditioning is required almost 11 months out of the year for Miami's fleet of 590 buses (490 are on the streets at one time).

MDTA's success started in 1994 – with a problem. A high failure rate of rebuilt compressors was totally inconsistent with the high quality of new equipment – rebuilt compressors were sometimes failing after only three to four weeks. So MDTA teamed up with Thermo King of South Florida to solve the problem. They identified four so-called trouble areas: parts usage, mechanic training, the shop environment and facilities, and rebuild procedures. As a result, first-hand inspections and analysis of effective air conditioning maintenance programs at other transit authorities and a tour of Thermo King's manufacturing plant in Minneapolis were completed; hands-on training sessions at MDTA and Thermo King of South Florida were conducted; and a separate, enclosed, air-conditioned room designed specifically for compressor rebuilding and repair was constructed.

The past four years have brought MDTA more success in the area of preventive maintenance. In a paper presented to the American Public Transit Association, Acting Assistant General Superintendent Harpal Kapoor describes MDTA's various maintenance programs and their primary functions. The purpose was to study the inspections and their schedules as well as the overhaul procedures developed to reduce road calls and improve the reliability of the air conditioning systems. Below are excerpts from Kapoor's paper.

Inspections

"A" Inspection (6,000 miles): This inspection is mostly a visual check at every 6,000 miles. It is a feel-and-touch check of a running A/C, and provides the condition of the whole system. The potential leak points are checked, the compressor clutch is checked for overheating, the drive belt is checked for tension, condenser and evaporator coils are cleaned, and the air temperature difference between return and ambient is noted.

"B" Inspection (18,000 miles): This inspection is more detailed and involves maintenance of the A/C system to meet the specified temperature parameters. The gauges are connected before running the bus (static condition) and checked for freon in the system. The bus is run at fast idle for 10-15 minutes, and the pressure readings are noted. The other components checked are: blower motors, hoses, drive belt, clutch gap, evaporator and condenser coils, heater coil, service valves and lines. The color of the compressor oil is checked to detect contamination. The difference of temperature between return air and ambient is monitored.

180 Day Inspection: This inspection has cut down the road calls by approximately 80 percent and is the most important inspection of A/C system maintenance. It is performed at six-month intervals on each bus, or on a bus with chronic A/C problems. The inspection is performed at a centralized location, and the buses for each division are scheduled for inspection in advance. This central location is called the "A/C Shop." All types of specialized A/C tools, including leak-detection and refrigerant recovery systems, are available to the technicians performing thorough inspections of the A/C components. MDTA has worked very closely in establishing performance criteria and standards with the Original Equipment Manufacturer (OEM).

Component Overhaul

The compressor is rebuilt in MDTA's dust-free air-conditioned room. After cleaning, the compressor is disassembled in a sequence. The dimensions of the critical parts, like crankshaft rod journals, oil pump housing bore and cylinder bores, are checked with a digital vernier and compared with the OEM specifications. Some parts are rejected. Some are designated for machining and repairs. And the only parts reused are those that meet the OEM specifications. The performance of these rebuilt compressors compare well with the new compressors and have shown tremendous improvement in reliability. With the scheduled rebuilt procedures in place and strict quality control, the compressors last up to 8,400 hours or more. The training provided by the OEM helped in producing a quality product, and the learning process proved beneficial.

Factors Affecting Rebuild

The quality of a rebuilt component depends on the parts used for its rebuild. MDTA has maintained a standard procedure to use OEM parts in rebuilding components. The aftermarket part may be cheaper, but may not last as long, and could result in progressive damage to components. The non-OEM pistons and sleeves may not have the right hardness, chemistry, structure and heat treatment. The connecting rods may not be the proper length, bores may be out-of-round, the crankshafts may have serious dimensional discrepancies, and the clutch pulleys may be dimensionally incorrect. These factors have resulted in premature failures. Due to the close tolerances of assembled components, MDTA has found that it is cost effective to use the OEM parts and kits, as opposed to using non-OEM components. When assembling hoses, it is imperative to use matching components from one manufacturer. Never assemble a hose of one manufacturer mixed with a fitting of another. There is a potential danger of leaks due to mismatching tolerances of the hoses and fittings.

Conclusion

The effectiveness of a preventive maintenance program depends on its proactive approach and the standards developed to meet the specific needs of the transit properties. The inspection and maintenance intervals, along with an effective rebuild program, have dramatically helped MDTA improve the reliability of its A/C system, while saving millions of dollars caused by the reduction of downtime and catastrophic failures.