MaxxForce 7 Diesel Engine Air Compressor Failure

The one advantage to a belt driven air compressor is the minimal labour to change it out. In this school bus with a MaxxForce 7 diesel engine built by Navistar the Bendix air compressor failed after three years. The oil went as far as the air tanks so this was a severe level failure. The air valves throughout the entire system have oil contamination but nothing practical can be done to clean out the system.

The seals in valves are rubber so I have not heard of any kind of safe chemical solution that can be used to flush out the air valves. When you buy a new bus from Navistar the standard warranty on accessories is one year. I believe there is a rumor out there that Navistar is increasing this coverage to 2 years which still would not have paid off for us in this case but maybe in the future.

The compressor does run off of a belt and if you noticed the size of the drive pulley the RPMs would be very high compared to engine speed. Once the piston rings and cylinders in the compressor wear the blow-by develops to the point that oil is blown into the lines, air dryer and valves. One thing that I learned is don’t buy a new compressor which was all that was available from the dealer.

The retail on that was over $4,100.00 before taxes. But thankfully god had mercy on us and we found a reman for $800.00 which is perfectly fine. If the compressor life is decreased because it’s used and remanufactured so be it. I would pay the reman price any day because of the drastic difference. Our Cummins ISB diesel engines have never had oil issues. We just had one that would not build air pressure and all that was required was a reman head because the valves were sticking.

The main reason why they are so reliable is Cummins makes their own compressor called a ‘Holset’ which is a single piston model that makes a lot of noise when engaged but lasts for ages with very few problems. I’ve told the techs in the shop that as long as I’m the Foreman we are sticking with Cummins ISB diesels.

Next year Thomas buses will be selling their school buses with a new Detroit diesel. Our fleet has gone through years of trouble and expense (still ongoing) with the Mercedes diesels that came with Thomas buses starting in 2007 equipped with emissions. We will not be experimenting with a new design that has not yet been road tested. Our shop has the know how, dealer support and the tools to look after our Cummins engines which don’t give us a lot of grief.