School Bus Mechanic Garage Talk

imported_John Whelan - September 11, 2014
Allison Transmission Clutch Pressure Test

This is an old photo testing clutch pressures on an Allison 3000 series automatic transmission. The pressures would tell the story on clutch slippage and pressure retention when the transmission was at different shifting points. The tool of the day back then was a “ProLink” which was a hand held device that would read codes and allowed us to run the transmission shifting while stationary.
The C5 clutch assembly was one component that failed on us a few times. The pressure test on the C5 pressure port revealed clutch slippage because of a pressure drop compared to the main pressure reading. The clutches were slipping from wear and this transmission was in a bus assigned to a run up a steady grade.

This test was a common procedure starting in the mid nineties into the millennium. The oil we used back then was Dexron III and I'm convinced our transmission troubles have been greatly reduced using Synthetic transmission oil. With advances in technology Allison transmissions in our fleet have been flawless with nothing more than regular maintenance. The high price of synthetic is well worth it in this case.

CAT 3116 Injector Seating Tool

The 3116 Cat diesel came out in the late eighties while I was working at a GM dealership as the Medium Duty Tech. The experiences I had with these engines was interesting and having the correct tools to work on them was essential if you wanted to do the job right.

The injector seating tool was one essential tool that made sure the injector seated correctly into the cup integral with the cylinder head. Installing the injectors with the hold down was not efficient enough and sometimes this would cause a compression leak into the fuel system.

Of course the engine would not run worth beans because of the pressurizing going on in the fuel tank. The other option was to replace the injector seats which required the dealer from CAT to do the job. There's just so much money available for tools and this procedure was once in a blue moon so paying shop rate to get it done was more feasible.


GM Pickup Fuel Pump Replacement Shortcut

I don't know if you've done this before with a GM truck but removing the box mounts and lifting the box out of the way to change a fuel pump is much easier than dropping the fuel tank. It's a shortcut we use in our shop all of the time and makes for a faster and cleaner job. For some reason GM fuel pumps seem to fail on a steady basis.


Cummins B Diesel Engine Head Gasket

During the middle of the Millennium around 2003 (I love the word millennium) Cummins B engines were famous for leaking head gaskets. The leak would come from the right front corner. I'm not bashing them but it was a common repair...just saying. Other than that they were a great diesel to have in the fleet. I've known of a lot worse diesel engine models with more serious complaints.

Today's Cummins ISB is well ahead of all other medium duty engines. They have an excellent emission control system that almost totally eliminates all of the harmful NOx emissions. The SCR, DPF, VGT and Cooled EGR does a bang up job and the EPA is very happy there's a competitive diesel manufacturer that is leading the way in emission technology.

That's all I have to say about these subjects and hope you have been enlightened in some way. I would like to invite you to join us at and say what you think, say what you know and help others get through the myriad of mechanical problems that occur every day in a techies life.
Mechanic Life