Thomas HDX School Bus Road Test

imported_John Whelan - March 27, 2019
I would like to follow up on this video. We changed the surge tank low coolant sensor and that fixed the problem with alarms and dash lights coming on. This problem did not happen all of the time. It's a phrase used a lot in the mechanic world known as "an intermittent problem". These are the hardest faults to put a finger on due to the once in a while attitude of that bratty diesel engine. The engine is a Mercedes MBE 900 model in a 2008 Thomas pusher bus known as the "HDX".

The main reason I was doing the road test was to attempt to make this issue occur before my eyes giving me more insight on when and where and under what conditions happened when everything goes sideways. Of course there were no incidents and after returning to the shop I decided to put a new sensor in the surge tank. The driver came back with good news....there were no mishaps on the run. Days after the same results. I could strike this job off of the to do list.

A minor problem considering the parts it took to fix it. Considering the possibilities if there was a wiring problem the troubleshooting time would have been much higher. The wiring to the low coolant sensor comes from the engine motor control module. You need an eyeball on a stick to see the module bolted to the side of the block. It's accessible but not smack dab in front of your face for easy access. So I'm happy it turned out the way it did.

Let's talk about the importance of road testing. I can't think of a time when I could honestly troubleshoot a driveability problem without a road test. The operator provides a story which gets the ball rolling. Then the road test will re-enact the scenario when the problem happens and hopefully give me a good feeling about where to look first. Take a driveline vibration for instance. Vibration or noise under load or coasting can indicate a differential mechanical problem or a drive line balance issue. Driveline imbalance emits a harmonic vibration with a higher frequency compared to a bad drive gear.

Sometimes I get lucky as many other Mechanics do taking a vehicle out for a test drive and as soon as you hear a particular noise you know exactly what's going on. That's the beauty of experience.


thomas school bus
road test