Replacing Fuel Gauge Sender Thomas School Bus

A faulty or inaccurate fuel gauge is classified as a minor problem but running out of fuel on a diesel engine in a school bus on a regular run is a problem for us. The rule of thumb in our operation is to keep the top half of the fuel tank full. However if the fuel gauge is reading inaccurately there could be an empty fuel tank when the gauge is reading 1/2 full. When a diesel engine runs out of fuel it’s a difficult task to get it running again due to air in the system. The air has to be bled out of the fuel injection system which takes time.

So the best solution is to fix the problem right away to avoid the headache. This Thomas bus in the video is a 2015 model but in 2006 when the C2 conventional buses came out the fuel gauge senders were not calibrated properly. A technical service bulletin came out from Thomas Buses stating there were faulty fuel gauge senders from the factory. Back then we experienced several buses that ran out of fuel because of the inaccurate gauges.

OEM – original equipment manufacturer

Manufacturers of the sender obviously had to re-engineer the design to get the calibration and accuracy back to OEM specifications. A new part number was issued so we could order the updated part. It was good news for us having the proper sender installed and less worry about break downs. OEM will put out a technical service bulletin if they have enough instances come in with the same issue. These bulletins are just for customer information and are not covered by warranty.

Our shop experienced a lot of electrical problems on the Thomas C2 conventional buses. The modules for body controls are connected by multiplex wiring much like cable TV communicating back and forth to different circuits at the same time. This system was designed to reduce wiring. Unfortunately corrosion set in after a few years effecting connections along with insulation wearing through and exposing bare wire.

The obvious sign of a wiring issue with this system was circuits staying active even after the bus was shut down. Corrosion would track across a connector or module and connect to a battery supply terminal. At first we took a lot of time to figure out these problems but eventually after many instances we knew where to look. Experience helped us to know exactly where to start and troubleshoot these challenging wiring problems.