School Bus Module and Mercedes Diesel Engine Code Problem

Mechanics Hub - November 3, 2013
Voltage Leakage with Interface Modules

Another problem we are dealing with is the VIM vehicle interface modules especially on the HDX Thomas pusher buses. They are exposed to the elements and attract corrosion and since they have cooling vents to the atmosphere this causes voltage to creep over and energize other circuits.

On one of our 2007 HDX buses when you disconnect the ground from the battery (except for the ground connector to the rear control box) this causes the switches to activate on and off and also the back up alarm starts to chirp.
The interface module (controls starting, backup alarm etc) is mounted above the engine so it's going to be replaced. The interface module is in a plastic case and consists of a metal grid electrical circuit. Once corrosion creeps in and around this circuit... relays and other accessory circuits will be activated. You will know this when the key switch is off the relays will be clicking away or an accessory will be activated intermittently. (i.e.  back up alarm)

The photo is a little blurry but you can see the corrosion that has built up around the electrical grid. With the heat dissipation required there are vents around the module. On the Thomas HDX buses the VIM is located above the steering axle bolted to the frame inside a metal box. In the past we have tried to seal the box which Thomas Bus Corp recommended.

Grid Heater Codes No More

With our Mercedes MBE 900 diesel engines coming off warranty we have had to take over the troubleshooting and repairs. We got the dealer to remove the air intake grid heater system through re-programming because the codes were happening left and right on our 2007 and 2008 buses.

The grid heater was basically a toaster and helped the warm up cycle during start up. We already have block heaters and don't live in Alaska so we decided to go ahead with this step. Since then our code problems without the grid heaters has dropped significantly. In some case the engine would derate because of this code.

Please comment and share any stories you have in this area. Appreciate some feedback from you if you have experienced these problems. This way we can divert time and frustration starting from scratch when these types of glitches occur.

Diesel Engines