Thomas HDX Rear Emergency Window Switch Improvisation

Mechanics Hub - October 7, 2013
If you have Thomas HDX school buses in your fleet you might have been experiencing problems with the rear window alarm coming on unexpectedly. If moisture gets into the seal area the exposed ground wire in the window frame grounds out and sets off the alarm.

The main problem with the design is the ground wire is exposed to the environment. In normal conditions the activation of the alarm is made when the window is opened up which is a safety measure to alert the driver. If the driver is on the road and there is any moisture contacting the ground connection he has to stop and try and dry our the area around the wire connection.

Here is the finished product with the "vandalock" system used on older Thomas bus models. It's all mechanical with the bolt mechanism opening and closing the start interlock switch manually. When it's moved toward the window frame it not only locks the window closed but also disables starting. The second switch facing the window activates the alarm when the window is opened.

The original ground wire was riveted to the window frame and being exposed to the environment caused the problem with the alarm coming on at random especially during wet weather. Looking back at the first photo you will see that the bracket to secure the window to the latch and the switch box for the alarm switch have both been fabricated.

A closer look shows you the riveted ground wire again. The emergency window has to be removed completely and the wiring removed. It then has to be re-routed to the vandalock assy.

Here is the vandalock assembly before the installation. The switch inside the black box acts as the interlock for no start when the bolt has locked the window for security over night. The switch on the right activates the alarm when the window is opened up. Once we installed this system the problem was cured.