Mechanic Repairs In The School Bus Garage

Various School Bus Repair Photos From The Bus Garage

Every day mechanic duties change and this video is a small example of what goes on in our school bus garage. The diesel mechanic jobs involve the ISC Cummins and Mercedes MBE 900 diesel engines. These 2 engines are the oldest in our fleet and have given us some trouble because of age and mileage but we always repaired them as needed with the proper tools and software.

There are also small repairs which include oil and coolant leaks, Air brake system leaks and periodic brake jobs. Electrical problems are consistent with multiplex wiring on the newer models that use modules to run lights, motors and accessories.

One important tool that every diesel mechanic must have is the appropriate software for troubleshooting engine codes and performing static and running tests to find out what diesel component is failing or has failed completely.

Many times the engine code will tell the mechanic that the EGR exhaust gas recirculation valve is not in the right position. That's one of the easiest problems to have since 9 times out of 10 the EGR is sticking because of a heavy soot buildup.

Multiplex Wiring

Multiplex wiring is a common problem in the electrical department. Using the AccessFreightliner website is essential. If you go into the "PartsPro" section and enter the last 6 digits of the vehicle ID you can find an excellent schematic of the modules and other wiring systems. Doing a "search" and entering a keyword like "ABS" for example will bring up a schematic for the antilock brake system.

If you don't have a user and password for accessfreightliner phone the dealer and ask them about getting one. I'm not sure what the rules are but it's worth a try especially if you own Freightliner trucks or buses. In our case the buses are the C2 conventional saf-t-liner. They came to our fleet in 2005. Before that year the freightliner school bus was made out of the FS65 medium duty truck chassis.

The EOS Electronic Oil Separator

The video shows an EOS (electrostatic oil separator). This piece is part of the valve cover and separates the crankcase gases from the liquid which is funneled back into the crankcase while the vapors are routed back into the intake. This component costs 800.00 if it comes time replace it. If you get an EOS engine code you're stuck with ordering a new one. That's the downfall of getting into emission controlled engines. You need a healthy budget to repair them.

The Cummins VGT

The Cummins B 6.7 liter diesel engines in our fleet are great runners but if you have to replace the turbocharger in our case there was oil leakage into the intake side (compressor wheel). It cost 2400.00 for the VGT variable geometry turbocharger reman and a fair bit of labor to do the re and re.

The VGT is a great design and not only changes it's angle to act as an exhaust brake but also helps the engine do a regen by blocking the exhaust flow to develop engine heat for a faster and more efficient regen. That's all I want to say for now ...thanks for the visit and please leave a comment below. I would like to hear some feedback from you.

Diesel Engine Emissions A New Frontier In Preventive Maintenance

When 2007 rolled around and the EPA increased their emission standards everybody was waiting on what the original equipment manufacturers were going to do.

The diesel engine has always evolved through the decades but the challenge in the new millennium was to reduce soot and ash discharge levels out the tail pipe.

The catalytic muffler was the first on the scene along with ultra low sulfur diesel and low ash engine oil. The muffler transformed into the DPF diesel particulate filter which traps soot and then through an oxidation process remove harmful particulates from entering the atmosphere.

The very difficult problem for the engineers and the battle to stay within EPA specifications is costing Cummins, Mercedes and Navistar a lot of money in research and development. The progression to cooled exhaust gas recirculation worked for many years along with the DPF technology.

However the EGR by itself wasn't enough to keep up with environmental demands. Navistar this year will no longer supply diesel engines in the medium duty market. The EGR pictured above is out of a MaxxForce DT diesel engine. We have several of these engines in out fleet and this component needs regular maintenance or replacement at least once a year.

At the vary least removal and cleaning of the soot and ash buildup is necessary or the shaft that opens and closes the valve sticks and eventually causes the EGR valve to stay open which creates problems with power and take off ability. Every one of our fleet buses are checked for stuck EGR valves especially the older units.

Mercedes EGR valves have the same problem and eventually the electric motor that opens and closes the valve mechanism does not have enough strength to over come the soot covered shaft and valve assembly. At the end of this year Navistar medium duty diesel engines like the MaxxForce DT and MaxxForce 7 will not be manufactured any more.

The Cummins ISB will be taking over the field with it's own technology including the SCR (selective catalytic reduction) system that uses DEF diesel exhaust fluid to reduce emissions. They also have the VGT variable geometry turbocharger to increased heat to assist in the regen process that cooks out the soot build up in the emission system.

It's a heavy duty challenge to keep up with emission demands but when the going gets tough Cummins diesel engines get going. They have really excelled over other diesel engine manufacturers and went outside the box to over come the high demands from the EPA to reduce emissions levels to zero through out the transportation industry.  

School Bus Heaters Article

I recently wrote an article for School Transportation News Magazine on Bus Heaters. The nice thing about researching for this subject is I learned a lot of new Mechanic information on the subject. Since our fleet has over 2 dozen bus heaters it was a topic of interest. Here is the link that will take you to the online digital version.
The pre-heating capabilities from these little heaters are phenomenal. There was a lot of positive feedback from fleet operators who saved fuel reduced engine wear and harmful emissions from cold diesel engine start ups. They don't cost a lot to run and can be adapted to bio-fuel and gasoline. The heat ratings go as high as 45,000 BTUs. One thing to point out especially with school buses is the safety and comfort of the kids these heaters provide. Imagine a bus breaking down on the highway during frigid temperatures. These heaters would be a huge asset to have around for that purpose along with defrosting the windows much quicker solving another common safety issue.


Here's a great video on an Espar coolant heater installed on a boat. The install here is the same for any kind of application.

STN Going Social With Mechanic & School Bus Industry News

School Transportion News has a new social network that covers everything imaginable in the school bus transportation industry. I especially enjoy the TechNet section which covers school bus repairs and new technology. This is a discussion group that anyone can join and participate in with questions and answers as well as comments.

With changing technology it's essential to keep in contact with other professionals in the field and share ideas and answers to challenges encountered on a regular basis.

Here's the link if you want to check it out.  I highly recommend this site for up to date knowledge and a free ticket to rub elbows with an experienced group of school bus mechanics willing to share their knowledge.
http://www.stnonline.com/social

Thomas Buses-How To Check & Replace A Fuel Guage Sending Unit

A fuel guage sending unit is a ground circuit for the fuel guage. The position of the sender float changes the resistance to ground by way of a resistance winding that is built into the sending unit. High or low resistance determines a full or empty fuel tank. The complaint I'm dealing with is an erratic fuel guage reading, the guage needle is bouncing all over the place every time the driver turns or stops suddenly. This Mechanic information is for anyone who knows how to turn a wrench and you'll see how easy it is to check out the sending unit.
Firstly, every fuel guage sending unit is very similar on older vehicles. On newer automotive applications the sender is built into the fuel tank electric fuel pump assy. This is an integral unit and quite expensive.
I'm working on a 2000 Thomas Bus Saf-T-Liner which is running a Cummins ISC Diesel Engine. In this case the sender is seperate and very easy to test.
The sender is located 3 seats back under the floor. The floor plate has to be removed to gain access.
The sender can be tested using a jumper wire. Disconnect the wire connector at the middle terminal of the sender [this comes from the fuel guage]. Connect the jumper to the loose wire and hook it to ground.
[click on each picture to enlarge]
Turn on the ignition switch and the guage should go right over to the full mark. What this does is tell us the gauge is OK since the wire to the sender has been grounded directly without any resistance. The sender has been bypassed and now we're sure it has to be replaced.
Remove the retaining screws and ground return wire and replace the sender the same way it came out. It's that easy!
So now you understand the circuit through a fuel guage sending unit and how easy it is to diagnose and repair. I hope this mechanic information has helped you.

Thomas Buses - Air Throttle Repair

During most of the 1990's before the electronic age the engine throttle was air controlled on Thomas Bus Saf-t-liner models. The high idle was a welcome addition since it was operated with the flick of a switch on the dash.
In the photo the air line can be loosened while the engine is running where there should be no air with the high idle switch turned on. The throttle linkage is moved into high idle with spring pressure[no air]. The regular foot throttle air line is attached to the underside of the cylinder. {Click Photos To Enlarge}

The air is controlled with an electrically actuated air valve that is normally open with full air pressure and no power from the switch. The air pressure moves the internal piston to the idle position compressing the internal springs. When the high idle switch is turned on, battery power closes the switch and blocks air pressure to the air throttle allowing spring pressure to kick the throttle linkage up to high idle[around 1000 rpms].



The disassembled air throttle consists of springs pistons and seals and is easily serviced. The one thing that does fail is the 1/4 inch adjustment bolt on the end cap wearing through the washer that seats on the internal spring. Thomas buses have changed since this decade and are electronically controlled with an onboard computer but if you still own a saf-t-liner pusher these systems are reliable and easy to maintain.

Thomas Buses-HDX Rear Emergency Window Modification

Thomas Bus HDX Pushers have a rear emergency exit window that has 2 ground switches. One for the alarm, warning the driver that the window is open and the other is an interlock that does not allow the bus to start when the window is latched.

The problem we were having was water seeping through the seal causing a circuit to ground tripping the alarm even when the window was fully closed. The quick fix was to grab an air line and dry off around the switch and grounds. Pictured here is the alarm ground switch[click on pic to enlarge]

This is the window locking latch that must be unlatched for the bus to start. The rivet on the outside is going to ground while the handle contacts a strip riveted on the inside of the frame.






We installed 2 intermittent switches in a currently available Thomas Part that encases the 2 switches and does the same job as the factory install.






The window must be removed to rewire the new installation. The 2 original ground switch wires are attached to the new switches in the casing and grounded at the case.






Once installed the [starting interlock] bolt style latch locks the window and the other switch is depressed [just above] to shut off the window alarm.
The latch catches on a "C" shaped flat bar that was fabricated to lock the window.




The latch bolt is in the open [run] position and the bus will start, this is a safety factor that newer Thomas Buses come with from the factory.

I will be updating this post soon with part numbers.

Cummins Engine Repair - Torquing Cylinder Head Bolts

Cummins Engine Repair - Torquing Head Bolts on a Cummins 24 valve turbo diesel. The torque pattern is circular starting from the middle and working both ways to each end of the cylinder head.

Torque Specs Are As Follows:

• 59 ft. lbs.
• 77 ft. lbs.
• Check again at 77 ft. lbs.
• Finally advance each head bolt an additional 90 degrees
• Since each flat side of the "hexagonal" bolt head is 60 degrees [60 X 6 sides=360 degrees]
• One flat and 1/2 = 90 degrees.
  

This is the most important step when replacing any engine cylinder head gasket. If the proper torque specs are not followed bolt stretch [over torqued] or coolant leaks [under torqued] will occur and you'll have to start all over again with your Cummins Engine Repair.

Recommended: Diesel Engine Repair Manuals

Haldex Brakes-Self Adjusting Slack Adjusters

Haldex Brakes self adjusting slack adjusters are installed at the factory on all Thomas Buses equipped with air brake systems. This allows much better control over brake adjustment especially for a large school bus fleet operation. At the same time there are operating and maintenance procedures that must be followed to get the best performance out of Haldex Self Adjusting Slack Adjusters.

Here Are Some Important Maintenance Steps To Follow:

• Visual and operational checks must be made at every scheduled PM check
• Check adjusters and anchor brackets for visual damage
• Automatic Adjusters should not be operated as manual adjusters
• Standard chasis grease may be used to lubricate Haldex Slack adjusters
• Lubricate the Standard 409-10 model every 50,000 miles or 3 mths
• Lubricate Reduced Maintenance 409-10 once a year
• No-Lube 429-10 adjusters are sealed and do not require lubrication
• DO NOT use Moly-disulfide grease as this will effect the operation of the slack adjuster
• Replace the slack adjuster if the TORQUE is less than 13 ft lbs when backing off the adjuster. You will hear the ratcheting sound while doing this.
• Haldex Slack Adjusters should be checked for operation between 90-100 p.s.i. in the air brake system reservoir. Anything over 100 p.s.i. starts to measure deflection in the foundation brakes and does not give a true reading
• To self adjust Haldex Brakes the operator has to make several full brake applications [spring brakes released] preferably with brakes hot and over 90 p.s.i in the air brake reservoir.
  
  Here is a very good video explaining proper Brake Adjustment on an air brake system. The self adjusting slack adjuster in this video is not a Haldex but the exact same procedure applies. In my experience with a properly adjusted Haldex slack adjuster the push rod travel averages at 1.5 inches of travel. Well below the Department Of Transport maximum allowed brake adjustment specs.
  
  Self adjusting slack adjusters are now standard equipment on all Thomas Buses and are a real asset for large fleets who must keep up with Department Of Transport Standards especially with air brakes and proper adjustment. I hope this post has informed you on the proper maintenance of Haldex Brakes.