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 Power Steering Pump Front Seal

This is a Haldex 2 stage Power Steering Pump removed from one of our 1998 Pusher School Buses. The problem found was the power steering level was rising on it's own. No, it's not magic but a front seal allowing engine oil to enter the pump suction side.

The power steering pump is bolted in tandem to the rear of the air compressor which has an open crankcase with lots of engine oil being splashed about. Since the suction side of the pump is facing in this direction, if the seal is faulty engine oil will enter the pump and end up in the power steering reservoir.

One quick check to confirm this is the engine oil is black as usual from hydrocarbons while P.S. oil is standard 10w30 engine oil so there is a noticeable difference from the normal appearance.
To change the seal the pump must be removed and the front housing taken off. Always mark the pump stages before dis-assembly for easy reassembly.

With the end housing taken off the seal snap ring can be removed then the front seal can be easily replaced. This pump operates not only the power steering but the hydraulic fan motor as well. There are 2 output ports on the pump which supply pressure oil to run the 2 systems. This pump is mainly found on the 3116 cat Engines in our fleet & work very well in normal day to day operations.

Diesel Engine Cranks Does Not Start

If a diesel engine cranks but does not start the first thing you want to look at is the power feed to the engine controls. In this case it's a Mercedes mbe 900 diesel engine in a 2007 conventional school bus. The first check is the fuse block since this powers up the ignition relays that operate the engine control modules.

Here is the Engine Power Distribution Module (photo below) with the relay that sends a signal to the engine control module.
The wavy line on circuit here has a ground to one side. With no ground the coil does not energize which will not allow the circuit to close.



Mercedes diesel engines have 2 modules, one on the engine and the other inside the cab. The ignition switch activates the relays that gives us the start operation. We found voltage to the cab module but when the engine was cranked over there was a voltage drop down to '0' Volts.

Tracing the wires and checking for continuity was the first step since a voltage drop means a resistance problem in one of the circuits. With the help of the Thomas Bus Tech Line we found the circuits to both relays then found the ground to be non-existent for both ignition relays.
These 2 ground wires (photo below) energize the coils within the relays. No grounds of course caused these relays to be become inactive.
After running a temporary ground the engine started. So obviously we had to check the ground connections on the firewall and the frame. After some digging there was a broken connector to a frame ground below the cab. Once repaired, the engine was back to normal once again.
The photo below shows the broken ground cable found just below the cab firewall on the outside of the frame rail.


This is Mechanical information that points out that a bad ground can cause a lot of electrical problems and should always be checked first starting at the batteries. A digital volt meter will tell you a lot with voltage and resistance readings. The higher the resistance the lower the amperage.

Allison Transmission Problems-Hard Shifting

Allison Transmission Problems vary and one typical complaint is hard shifting. I recently looked at a 1999 Thomas Saf-t-liner [pusher] with an Allison MD 3000 series transmission. It had a hard shift into 2nd gear and a hard downshift as well. No codes came up so it was time to check the mechanical side and do a clutch pressure test. The test has to be turned on "enable clutch test" with the "ProLink" so each shifting range can be turned on with the unit running in the shop.

The pressures are compared to main convertor pressure and sits around 270 psi on this test. The individual clutch pressures must be within 5 psi. If there is a big difference that tells us the clutch itself is worn or there is a hydraulic problem. This test revealed no problem in this area and one conclusion could be a clutch that has to be physically inspected for imperfections. There is always 2 clutches applied to get a forward gear in this case it's C1 and C4 for 2nd gear.

The next step is a big one, whether to disassemble the trans and inspect for problems or live with the hard shift. I tried to "fast adapt" the shift parameters with the "ProLink" which did not change anything. This is one of the allison transmission problems you might encounter, if we get into this project any deeper I'll keep you posted.

Thomas Buses - Allison Transmission Wiring Modification

Thomas Buses are driven by an Allison MD 3000 Series Automatic transmission. This 1995 Saf-t-liner model had no power to the operators trans shift pad. The shift pad is also the Trans ECU [electronic control unit] so as I have said before always check the source , in this case the batteries.

There was a power feed open between the batteries and the rear control box at the engine compartment. I was able to limp the bus home by jumping battery power to the terminal board inside the control box which goes to directly to the operators cab supplying battery power to the shift pad/ECU.

This is obviously how I determined the open from the battery to the control box/terminal board [standard wiring on all Saf-t-liners]. The code for this fault is 35-00 [push both arrows on the shift pad at the same time with key on to retrieve code {under normal circumstances}

The control box at the engine compartment houses the engine intake heater components, lights etc. as well as the main battery power to the front electrical control box below the driver's side window.





My plan is to totally bypass the trans battery supply circuit from the battery to the ECU. Here you can see I've already cut the wires [larger #8 guage pos. and neg. wiring come from the battery while the smaller #10 guage pos. and neg. wires are running up front to the ECU]




This is the terminal board I mentioned earlier, factory wiring with spade connectors that connect the battery feed wiring to the trans. ECU wiring.

The first picture [top] shows the #8 pos. and neg. wires hooked up and running directly to the trans ECU. This will eliminate the extra #10 wires shown in the rear control box.


In the drivers cab the rear cover has been removed for easy access to the back of the trans. ECU. The upper plug can be removed and inspected for battery power. Both pos. and neg. #18 wires [2 each] run into this plug directly from the battery. The #8 wire is downsized under the dash to accommodate the smaller wiring into the ECU plug-in.



The 12V source from the battery is shown here with no connections in between. With a Digital Multimeter you can check for a battery voltage and a good ground.






So now you know when you have a shift pad with no reading [key on] the first thing to check is the battery voltage.

I hope this has helped you become more familiar with Thomas Buses equipped with Allison MD 3000 Series Automatic Transmissions.

Thomas Buses-How To Replace A Signal Switch

Thomas Buses have a signal switch that is very easy to replace. Most of the time the switch does not cancel properly which is a pain for drivers who have other things to look after and don't want to be constantly reaching for the the signal switch to manually turn it off after every corner.
The Thomas School Bus Saf-t-liners are similar with many similar components, the signal switch design did not change for many years.

Remove the upper and lower covers on the steering column. The upper cover is easier to remove when the steering column is tilted all the way down.






Use an allen wrench to remove the 2 signal switch screws holding it to the column.

Here is a top view of the upper screw being removed. As you can see the access to the signal switch is very easy.







Unplug the 2 wiring harnesses to the signal switch, they only fit one way.

Important Note:

One thing that you should watch for is to make sure the wiring running down the steering coluumn has enough slack. I've had problems where the signals do not work or short out on the column because of not having enough extra wiring to accommodate the tilt steering column movement .

The signal arm is held in the switch with a catch and can be removed by pulling on it aggressively until it releases from the switch.

Reverse this procedure and now you know how to replace the signal switch on Thomas Buses.

Cummins Engine Repair- ISC Engine Code 434

Cummins Engine Repair not only involves mechanical but electronic repairs as well. The ISC model has an ECM that sends and receives messages that controls every performance aspect of this engine. The main source of power is the battery that depends on wiring that feeds the Cummins ECM sufficient voltage to run the engine systems.
Pictured is the laptop shot of an active code #434 a power interruption that derated the engine to a crawl. Derating protects the engine from sustaining high speeds and rpms which would cause more severe damage. This ISC Cummins Engine is in a 1999 Thomas School Bus Saf-T-Liner. With an active code you can retrieve it with the diagnostics switch [on the dash] & ignition switch turned on. The code will beep each number with a pause, in this case 4 beeps 3 beeps 4 beeps with a pause in between each #.

A closer look [click on photo to enlarge] shows numerous fuses on the positive feed wire to the Engine ECM [electronic control module]. Our code information is telling me there is a power problem so "the first thing to check is the Source."

Using a digital ohmmeter check the fuses for continuity and clean all the connections.

Note: Before this maintenance step I could not get a reading on my laptop, there was a communication error between the engine and the "Cummins Insite" software.
After cleaning all the connections and testing the batteries the communication problem was erased and the code disappeared.

There is a variety of 7.5 and 10 amp fuses inline with battery to ECU supply power. I traced the wiring back along the frame as well to check out any other possible problems. The rule of thumb in this situation is to check connections and wiring closest to the environment and work your way up the line.

Using a Snap-On Micro Vat Battery Tester I concluded that all the batteries were in good condition. This tester does a very quick assessment of battery condition and also tests starting and charging systems quickly.

The Conclusion to this problem was simple, servicing the battery connections which had enough resistance to cause the #434 code to occur. It makes sense since the ECM reads Voltage at a very sensitive level.

After an extensive road test the code did not come back and this experience was a good one telling me [and you the reader] to always check the simple things first!

Now we're ready for the next challenge with Cummins Engine Repair.

Recommended: Diesel Engine Repair Manuals

Cat 3116 Engine-Repairing Common Engine Oil Leak

The Cat 3116 Engine has a common oil leak that is very easy to fix. The first step is to remove the steering pump which is driven off the air compressor. The next step is to remove the rear housing from the air compressor, it's held on with 4 nuts on studs, there is a square o-ring that seals the rear housing to the main compressor body. The Compressor is a "Bendix 850 Model" which is a 2 stage [2 cylinder] unit.



The Cat 3116 Engine oil pressure guage and air compressor oil feed lines run off the housing pictured here bolted to the engine block which is connected to the main oil pressure gallery. There is an o-ring seal between this housing and the engine block.





This 2nd block is the air compressor rear support block that has an o-ring seal as well. This block also serves as an oil drain for the 3126 model which has a oil pump above that supplies high pressure oil to the HEUI system [Hydraulic Electronic Unit Injectors].




Pictured here is the oil pressure manifold housing removed. The heat is absorbed by these 2 o-rings and they become very brittle causing an oil leak. Diesel Engines run from the "heat of Compression" so you can bet the engine block is smokin' hot! when operating temperatures are met.





The manifold housing o-ring has been replaced and is secured with 2 M8 Metric bolts. These 2 areas are a common oil leak in both the Cat 3116 Engine & 3126 Diesel Engine .






Recommended: Diesel Engine Repair Manuals

Cummins Engine Repair-Adjusting Valves ISB

1. Valve adjustment on a Cummins Engine Repair is the final step. Engine position is very important because the intake and exhaust valves must be in the closed position.

Rotate the engine clockwise [looking from the front] until #6 cylinder rocker arms are both rocking. When you're on the correct stroke the exhaust valve will be moving into the closed position, then once it closes keep rotating the engine until the intake valve JUST starts to open. You are now on #1 Compression Stroke & ready to adjust 1/2 of the engine valves.

Note: The crossheads pictured here on top of the valve stems [#5 & #6 Cyl] are located with the elongated contact surface towards the exhaust manifold. The Rocker arms are held down on aluminum cradles with one retaining bolt and torqued to 27 ft. lbs.

Cummins 24 Valve Turbo Diesel Valve Adjustment Specs.

With the Engine on #1 Compression Stroke.

*Adjust #1 #2 #4 Intake Valves to .010 inch.

*Adjust #1 #3 #5 Exhaust Valves to .020 inch.

*Rotate Engine One Full Turn. #1 Cylinder Valves will rock like #6 did previously positioning the engine on #6 Compression Stroke.

*Adjust #3 #5 #6 Intake Valves.

*Adjust #2 #4 #6 Exhaust Valves.

It's not hard to adjust valves on a Cummins 24 Valve Turbo Diesel once you get the hang of it. All you have to know is how to position the engine correctly, the valve settings and some basic tools. That's it for now on Cummins Engine Repair.




Recommended: Diesel Engine Repair Manuals

Charging System Test - Heavy Duty 12 Volt System

A charging system test is a very basic operation with the right tools. Pictured is a Leece Neville 160 Amp Alternator which works very hard to keep the three 900 CCA [Cold Cranking Amps] bus batteries charged. Thomas School buses have a large demand with extra lights, electric motors and the engine air intake heater. The diesel engine in this case is a Cummins ISC Electronically controlled unit.



You're probably wondering what the heck is that device hooked up to the batteries? That's my tool of choice, a 'SUN' VAT 33 [Volt Amp Tester]. This model has been around for decades and still very reliable, our shop does have a digital tester but I like this one for doing a quick check on the charging system. The reason I'm testing the charging system is because of an unusually low voltmeter & ammeter reading.




The main leads are hooked up to the positive and negative battery terminals while the amps pickup is clamped around the positive cable to read alternator ouput back into the batteries. The engine must be running at high idle [approx. 1200 rpm]

The black control knob I'm turning puts a load on the batteries which should kick in the alternator to charge very close to maximum. Once I bring down the system voltage to 12V the alternator should normally put out a full charge of approx. 160 amps.

The test revealed the alternator is only putting out 80 amps maximum which will not keep up to the demands of a fully operational school bus & must be repaired or replaced.

This is a fast and easy way to do a charging system test on a 12 Volt Heavy Duty charging system.

Click here to sell your car online at eBay!

Thomas Bus Heater Blower Motor Replacement

There are 4 Front heater blower motors on the Thomas School Bus Saf-T-Liner Model. With Winter on it's way this is an important maintenance area, defrosting of the windshield has to be accomplished quickly.
The heater blower motors also serve as defrost motors with a quick change up on the controls. Each motor has 4 wires for high/low speed and 2 grounds.
The direction can be reversed using the ground wires. One is grounded directly while the other is connected internally back to the motor, reversing these 2 will change direction.

This Thomas Pusher Bus is a 1992 model, removing the middle and left side panels will allow access to all 4 heater blower motors.
The tandem motor in the middle looks after main defrost and heat while each motor on the ends provide floor heat to the driver and defrost/heat to both ends of the cab.
The main failures on these motors are dry bushings causing noise and seizure and blower cage noise.

Thomas Pusher School Bus - Removing Radiator

This Thomas School Bus had a leaking radiator that required removal and repair. All the body panels must be removed to gain access to the radiator. The air charge cooler must be removed as well, it cools the turbocharged intake air to create more density/volume.




Fortunately this radiator core was repairable being a 1999 model, This is the best way when running a fleet of 70 School buses. The Thomas Pushers have transversely mounted radiators since they have rear drive engines and transmissions.