Origin of a Fire in a Truck Sleeper Air Conditioner, Texas (2020)

An aftermarket air conditioner mounted behind the sleeper cab of a highway tractor caught fire, and in 2020 the owner of the truck asked us to find where and how it started. We examined the truck in Texas. The owner and the truck are not identified.

A unit with a history

The owner gave us the background. The split system had been added to the truck at a truck stop well over a year before the fire. It worked for one day and stopped. After a minor repair it gave off smoke later that same day. On later visits by the installer the battery charger was disconnected and then reconnected, and in the weeks before the fire the unit kept flashing an over voltage error code. Then it caught fire. It was put out with a dry chemical extinguisher, disconnected from the truck batteries and wrapped in plastic. A history like this tells an investigator where to look and what to test. It is not evidence of cause by itself.

What the unit showed

The condenser section was coated in extinguisher powder inside and out. Heat had oxidized the front of the housing in a pattern that was heaviest at the bottom right. With the top cover off, the burn patterns ran across the front of the unit and up to the top of the control relay. On the tab of another relay we found arcing, where it had arced to the bottom of the electrical compartment to the right of the control relay. The right side, where the compressor sits, was more damaged than the left side, where the fan motor is.

Origin, and the limits of a first examination

Using the method of NFPA 921, we placed the area of origin inside the unit at the bottom right, consistent with a fire starting at the compressor. Several causes were still possible there: an electrical failure, a mechanical failure or a refrigerant leak. Telling them apart would take a closer, planned examination of the area of origin, and our report said exactly that rather than name a cause the evidence could not yet carry.

Add-on equipment on trucks

Aftermarket equipment brings its own wiring and its own failure modes onto a vehicle, and it often draws power straight from the truck batteries. When a fire starts away from the engine, add-on equipment is one of the first places we look. Our vehicle fire investigation page explains how we work these losses and why origin comes before cause.