Mechanic Tips-School Bus Fleet Preventive Maintenance

Every Fleet Mechanic will have there own ideas on how to set up the best PM program for their equipment which will vary for different locations. With different climates, road conditions and power trains the procedures will never be exactly the same. Our shop likes to condense the annual PM since the regular checks are carried out every 5000 Kilometers (3000 Miles). Oil changes are done every 15,000 Km with an oil sample to be analyzed. Fuel filters are replaced at every oil change.

The Allison 2000 and 3000 Transmissions  are all running on TES-295 Synthetic oil which has drastically cut down on repairs. Right now we change the oil every 3 years, the newer buses have prognostics which tells us with a dash light indicator when the filter and oil needs to be changed. Air filters are replaced every year regardless of the mileage. Steering Filters are serviced along with changing the oil every 2 years. Valve Sets are carried out according to the Manufacturer i.e Cummins every 5000 Hours or 240,000 Km (Electronic Models). With the introduction to emission controls in 2007 we now have to perform manual regens if required and replace the DPF (diesel particulate filter) every 150,000 Km (90,000 Miles)

Air Dryers every 2 years with a new unloader kit and dessicant filter. Dirt is a factor on school buses so we service interior heater filters and the entire lighting system (Non-LED lights). We check the starting and charging systems including the battery pack. If necessary we will run tests on the batteries and alternator. years ago alternators and starters were removed for rebuild but the time and parts did not add up as an economic strategy so we decided to give these components a 5 year life span before determining if they would need replacement. The cost has come down so much on electrical reman units, it's feasible to go this route with the added bonus of a warranty period.
 
Spring brake chambers are replaced every 5 years regardless of mileage and condition. The Mercedes MBE 900 requires a coolant and thermostat change every 2 years, the other diesel engines in our fleet do not have a time or mileage limit but we're working on that to prevent antifreeze deterioration and thermostat failures. Our tires tread depth is measured every Summer and pressures checked during each service.

Common sense plays a role with any preventive maintenance program and we found the best way to set one up is to sit all the Mechanics down in one room and go through the fleet  figuring out the best program to cover all the bases. Breakdowns will quickly eat up your labor time and parts budget like wildfire so keeping on top of maintenance with a feasible program will pay you back 10 times over.                                                                     

Air Brake Theory Revealed

Here is an excellent video on air brakes. This is the best presentation I've ever seen that will give you a detailed overview on exactly how air brakes work. The air brake mechanical and air systems work together. This video is referring to a school bus but is the same brake system found on most transport vehicles.



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The cutaways and detailed views of the air brake system explained here will help you understand the principles and terminology that you need to know. There are many things that happen in air brake systems like moisture that builds up from the heat that is generated from the compressor.

This is why draining air tanks daily is very important especially in cold weather conditions. This is a lot of information to absorb but the power of video allows you to watch it more than one time.

Bus & Truck Air Brakes System-Bendix AD-9 Air Dryer

The Bendix AD-9 air dryer is easily serviced by disassembling it from the frame and removing the bottom cover bolts. The unloader located on the bottom housing can be removed and a kit installed. The kit comes with new o-rings, spring, valve and seat. The main filter which is filled with dessicant absorbs the oil and moisture that accumulates in the air brake system.
Since the air compressor creates heat this causes moisture to build up, so this is why air brake systems have an air governor that unloads the compressor at 120 p.s.i. and signals the dryer to dump the build-up of water & oil in the bottom housing to atmosphere. That's the quick air release you hear when building up the air system, the air guage should be reading a minimum of 120 p.s.i.
The dessicant filter has a threaded stud attached so it can be easily removed during a service. The Bendix AD-9 is a very reliable component and is usually trouble free but if you notice a lot of oil coming from the bottom housing exhaust port then that's telling you the compressor is pumping out more oil than it should. Bendix has a splash card that you hold under the exhaust port, that gives an indication of how bad the oil loss from the compressor really is. The next step is to replace the compressor and of course, service the air dryer. Under normal conditions the air dryer can be serviced every 1-2 years depending on the condition of the air compressor and highway miles or hours of operation.

The Bendix Air Dryer also have a built-in heater wired up to sense cold weather and keep the unloader valve from freezing up. Every Truck & Bus air system has an air dryer installed from the factory, it's the most important preventive maintenance item and should not be neglected.