Sunday, February 01, 2015

Mercedes MBE 900 Diesel Engine Emission Code dpf out of range high

This code we're working on is a "DPF out of range high" on a Mercedes MBE 900 Diesel Engine. What triggers this code is a pressure differential between the inlet and outlet of the diesel particulate filter cartridge. In the video you can see the clamps on the outside of the muffler that can be removed allowing the cartridge to be removed for cleaning.

There are several shops in town that will do a cleaning of these cartridges. It's a two step process depending on how badly the blockage is. When using detroit diesel diagnostic software you can go in and see what range the filter is running at. It's 0 to 4 range with four being severe. Once the engine control module detects blockage it will run a regen while the engine is in service.

 In a perfect world that's all the emission system would need to do running regenerations to keep operating without any trouble. The DPF amber dash light illuminates when the regen process is taking place. There is no worries if it flashes on and off indicating a normal function instigated by the engine on board computer.

Mercedes calls the engine computer an MCM (motor control module). When hooked up with the laptop and software the tech can find the code and click on the "troubleshoot code" button at the bottom of the screen. This directs you to a step by step process to check one thing at a time then moving on to the next step. In this case the outlet braided pressure hose that reads outlet exhaust pressure of the filter cartridge has to be removed and tested for blockage.

It could be blocked with soot and ash build up and the best thing to do is replace it. This fault was cured replacing the hose. If it did not help the filter cartridge may be plugged up so a forced regen is a good idea. Eventually over time the filter cartridge needs to be serviced or replaced. Mercedes recommends servicing at 125,000 km or 75,000 miles.

We have 18 of these diesel engines in our fleet and have a good deal of experience which is your basic "hands on" experience that is of course the best teacher.

Saturday, January 24, 2015

Espar Diesel Heater New Impact Switch and Harness Replaces Original Push Button Version

All of our Navistar conventional school buses come with the Espar Hydronic 5 (E-Guardian) diesel coolant heater. The impact switches that came with the heaters are very sensitive and would pop when ever the bus went over rough roads.

We did a partial fix by using a hole saw and reaching in to the heater housing and resetting the switch. This was a hassle but necessary to keep them operating. Low and behold the engineers at Espar came out with a new impact switch and harness that does not cut out over rough road.

The part numbers for this unit are below. There are 3 harness options and just one impact switch. Impact switch number: 252800705050OZ . The harness that fits the Hydronic 5 we have is 25280070100200OZ. The International dealer can get you the parts or anyone who has dealer status for Espar.

A great advantage to getting Espar heaters is the software is free to download. Go to the Espar website to get the software. The heater comes on in cycles starting with the blower and fuel pump then the electrode to start the burner.

The fuel used is minimal about 1/2 a cup of diesel per hour. With anti idling laws around schools these heaters preheat the buses long enough to get the interior temperature up fast. They operate for 2 hours at a time during each cycle. The faster a diesel engine can warm up saves fuel due to less warm up time in the morning.

If you are interested in these heaters it's possible to install them after factory with a complete install kit. You just have to get a fuel feed from the tank and hook up the coolant plumbing and electrical. Our shop installed a Webasto heating system on an International conventional bus because it was parked out of town and we wanted more assurance that it was going to start in cold weather.

Thanks for viewing this page. Please comment and feel free to share this post.

Below is a video I did on setting the 7 day timer on an Espar

Friday, January 16, 2015

Allison 3000 Series Automatic Transmission Turbine Sensor Code Repair

The Allison 3000 series automatic transmission or the older label "MD3060" in my opinion is an excellent design. The modules come off one at a time and they are really quite easy to work on. You need the right tools and manuals of course but gone are the days of balls, springs and extra assembly parts that were common with older auto transmissions. 

The photo below reveals the turbine sensor which is internal. The bottom of these transmissions have an aluminum cast unit called the control module. It has all of the solenoids and valve body that controls shifting. The code we had was a turbine sensor which stopped working only at certain times during a bus run. The turbine sensor being a winding which delivers the turbine speed signal to the transmission control unit was failing when the oil temperature got to a certain level. 

Click on Each Photo to Enlarge

Using the Allison DOC software we were able to go on a road test and observe the turbine speed sensor and exactly what it was doing. The graph you see below in the photo show the activity of the output speed, engine speed and turbine sensors. These three signals are what the TCU reads to send the right information to the control module during operation for a proper shift at the right time and in the correct range.

Looking at the laptop image (click on the image to see more detail) you can see how great it is to be able to watch what the components are doing which makes it really easy to troubleshoot. What happened eventually when the transmission temperature got to around 140 degrees the turbine sensor flat lined on the graph reading causing the shifting to fail. At that time it was confirmed that the turbine speed sensor needed to be replaced and the labour to re and re the control module would not be wasted.

The control module is around 50 pounds so you need a tranny jack for sure. There are 2 dowels that line up the module to the main case and it's a bit of a fight prying the module down on to the jack. With some care it can be done including the removal of the wiring harness which slips through the access hole in the transmission case.

Once the sensor was replaced along with the necessary gaskets and seals the module was reassembled with new filters and sump suction filter (internal). The synthetic oil was added then running checks were performed. The road test was a success and the bus went back into service. Synthetic ATF oil is worth every penny in my book with anti foaming and heat resistant properties far superior than standard oils.

Valve body and shifting solenoids from the control module 

Here is a link to the Mechanic's Tips Handbook in pdf form. It has very good general information with torque specs for servicing and other useful tips. 

Saturday, January 03, 2015

Diesel Engine Emissions and DPF Diesel Particulat Filter Codes

If you look at this DPF diesel particulate filter you'll notice  the clamps around the outside. Fortunately for all of us diesel techs this exhaust system is serviceable. The filter inside can be removed and serviced or if it's too far gone replaced. There are many shops now that have made the investment and have the equipment to clean the DPF filter properly.
The cost is about half compared to buying a pre-cleaned unit from the dealer. This is what I experience anyway... I'm not sure what the set up is at your location. The Detroit Diesel dealer has a cleaned DPF filter on the shelf for around $800.00 while the cleaning process is in two stages if it needs a 2 step melt down then the bill may run up to $500.00.
The good thing about this operation is it does not have to be done on a regular basis. Mercedes recommends every 125,000 km for a filter service. On the other hand if there is a over fueling or if the engine is using excessive amounts of oil due to a defect then the filter will plug up much faster.

The photo above shows both the MIL (malfunction indicator light) and the engine warning light illuminated. The MIL indicates there is a fault in the emissions system. This could be the DPF needs a regen or there is a lack of air flow in the intake system or numerous other problems. Most times we have found that the DPF regen is the most common service procedure.
With the laptop we do a forced regen and this cleans out the DPF filter using heat and diesel injection (Mercedes) or DEF (diesel exhaust fluid) for Cummins 2010 and newer engines. The DOC or diesel oxidation catalyst built into the DPF causes the chemical reaction to take place breaking down the soot and ash that builds up in the filter.
Cummins has taken it one step further with the SCR (selective catalytic reduction) process using DEF to reduce emissions even further. Low ash engine oil and low sulfur diesel fuel has also contributed to reducing emissions. If you have anything to add please do so below in the comments section. What do you think of the emissions in today's diesel engines?

Saturday, December 27, 2014

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.

Tuesday, December 23, 2014

TPS Throttle Position Sensor Code

TPS Code Thomas School Bus With an Mercedes MBE 900 Diesel Engine 

The driver called us up on the radio and said he had no throttle. The bus he was driving was 10 minutes out of town. The bus was a Thomas HDX Pusher with a Mercedes MBE 900 diesel engine. The TPS which needs to send a signal to the engine ECU or in Mercedes terms it's called an MCM (Motor Control Module). This was not happening so some diagnostics were in order.

Without a throttle position signal to the MCM shifting and engine power goes crazy. Engine RPM will decrease and the transmission (Allison 3000 series automatic) will not shift properly. The 5 volt signal to the CPC (common powertrain controller) from the throttle position sensor will not happen. The CPC is the interface between the chassis controls and the engine / transmission. In other words it communicates the signals to the MCM.

After finding a proper wiring schematic and checking the circuits we traced the problem down to the TPS itself which as you saw in the video is mounted on the right side of the throttle pedal. It was a funny occurrence because we swapped throttle position sensor units with another bus with out any change. But...the problem was not with the TPS but the wiring to the throttle position sensor connector.

It was hanging by a thread and caused an intermittent throttle signal with a big voltage drop. This causes a bigger challenge for the mechanic doing the troubleshooting. Wiring is tricky at times but the only difference is the time involved to find the fault. So guess time we have a pusher bus with no throttle that connector will be one of the first places to check.

The TPS wiring and switch is on the Thomas Bus (OEM) side so we can't blame the Mercedes diesel powering this piece of equipment. In conclusion it might have been the operator's boot that hit the connector wiring while going into full throttle. It's a hunch but it could very well happen. If you have time leave a comment or a question.

I don't get to questions right away but always check back periodically. Thanks for reading this post and I hope it helped you out. Remember that a diesel engine control module needs to be told what to do. It must have a signal from a sensor before it allows the diesel engine or transmission to operate. It's great technology if you don't get mad at it and lose patience. Work with it and get the experience..... you will not regret the knowledge that you accumulate over time.

Sunday, December 14, 2014

Mercedes MBE 900 Diesel Engine Emissions and Engine Codes

EGR - Exhaust Gas Recirculation Maintenance

I want to share some observations and solutions with emissions and engine codes we have been experiencing with the Mercedes MBE 900 diesel engines in our bus fleet. We have a dozen of these engines 2007 - 2008 emission controlled and they are all off warranty so this has allowed us to learn by "doing" with the various emission and engine codes that normally occur. The time we have spent troubleshooting these various problems has taught us a lot and we have adapted to the typical problems that happen with this model engine.

The EGR Cooler above had to be replaced due to a leakage problem. With a coolant leak they will eventually plug up with a sludge build up if it does not get attended to. If the EGR cooler is not leaking and has a soot build up it will require removal and a hot tank cleaning at your local radiator repair shop.

They are not cheap to replace however if they leak you'll experience coolant loss and engine code problems because of the soot wet mess that will develop inside the cooler exhaust core. This will cause poor exhaust gas flow and reduce engine power. The EGR cooler exhaust gases flow to the EGR valve which mixes the cooled gases with the charge air intake reducing NOx (oxides of nitrogen) creating reduced emissions out of the tail pipe.

Charge Air Cooler Failure Mercedes Code 2631

The charge air cooler to the left split open right at the core on the engine side. This problem was hard to detect because of the noise going on when the engine is running.

The reason we found it was because of a pressure check on the cooler itself. There was a 2631 Mercedes engine code which is a "turbo boost performance" issue.

Mercedes has a procedure for troubleshooting this code and it's quite involved. You have to check the boost pressure sensor for soot build up then using the software check the EGR operation along with the intake throttle valve etc. Any part of the engine that isn't allowing full flow including the air intake and boost side will produce the 2631 code.

In this case we have blown charge air cooler causing a big drop in boost pressure. The good part is we found a problem the down side is it's located 10 days away in Memphis. This bus is down until then. NOTE: We used a regulator to pressure test this cooler at 30 psi and made plugs that can be clamped at both ends.

Mercedes Diesel Engine "low doser fuel pressure" Code

The doser code activated because the fuel pressure supply in the system was too low and this caused the regeneration process to fail.

The "doser" system shoots a mist of diesel fuel down stream to cause the regeneration operation to activate at the DPF (diesel particulate filter).

No regen operation meant the DPF was going to build up soot and ash to the point where the engine would eventually shut down. This is serious since the engine control unit is programmed to protect the engine and will not allow it to run when the emission parameters are out of the ball park.

The Mercedes power service literature is very good for troubleshooting information. This test shows how to measure the return fuel going back to the tank. At idle the lines are hooked directly to the secondary fuel filter return and a tee fitting is hooked into the overflow valve.

There are 2 fuel regulators one for the doser fuel side and one for the high pressure injection side (unit pumps). If there is too much fuel returning back to the tank the fuel pressure reading drops.

The fuel pump is a positive flow gear pump that is engine driven. Fuel pressure must get up to at least 65 psi to supply the doser system (does the exhaust regen operation) and the unit pumps which provides the high pressure fuel to the injectors.

Once we changed the 2 valves in the fuel return lines "overflow valve" and "doser fuel pressure regulator valve" the exhaust regen started working again and the engine power improved drastically. So it was a win - win in this case.

If you're interested in viewing more blog posts photos and videos check out and look for "blogs". I'm posting under the name "schoolbusmechanic"

Friday, December 05, 2014

Cummins Diesel Engine Training

Training is everything especially if you own the newest diesel engines in the market. I recently went down to Vancouver (BC) to get some training at the Cummins training centre and it was well worth it. The ISB 6.7 liter diesel engine is by far (in my opinion) the best medium duty engine available.

They have designed their own emission system including selective catalytic reduction and DPF (diesel particulate filter). Getting the emission levels down to EPA levels is a huge challenge. Cummins is reducing 90% of harmful exhaust out of the pipe. We own 9 of the ISBs and getting 10 more next year.

The support from our local dealer is very good which was one of the factors we looked at but I know even if the support wasn't there this would have been the engine of choice regardless. Having 'Insite Lite' software is a "must have" and without it you're sunk. Throwing parts at a problem and guessing what's wrong will cost you a ton of cash.

Another part of the ISB is the VGT (variable geometry turbocharger). Providing boost pressure for engine power is only one of the uses for this component. It also serves as the exhaust brake and  helps heat up the engine during a parked regeneration. The angle of the turbo changes to cause the restriction needed.

Not only did we have classroom training but also went out and did a lot of hands on training using the Insite Lite software. The tests you can run with this software is endless. Gone are the days when you had to crack an injection line to look for a miss. Now it's all on the software with the click of a button.

For technical information there is Quickserve Online where you can find parts and manuals to help out with repairs. A manual is essential and without it you are taking a chance of screwing up the job. Information is like gold to a mechanic having the proper schematics, torque specs and step by step instructions.

I recommend anyone working steady on the ISB 6.7 or any Cummins engine needs to go get a Cummins training refresher. There you can ask questions and get the right training so you can dive into any repair with confidence. These days without a laptop on your toolbox with the proper software is like living in the stone age.