As engineers, one of our biggest challenges is how to deal with underhood heat. Because of aerodynamics and other styling changes, the engine box on trucks is smaller and more densely packed than it ever has been.
Add a hotter-running diesel engine (the result of emission controls) and a smaller grille area, and the thermal environment can get downright nasty.

It's particularly true of 2007-model and later on-highway diesels, which you'll see more of as these trucks come off warranty.

While heat under the hood is an issue for the durability of hoses and other components, we're especially concerned about the effect on the cab and the operator inside.

Unless the truck manufacturer makes physical adjustments to the cab in terms of thermal insulation, heat from the engine box can be absorbed into the floor, doghouse, and other surfaces inside the vehicle. We've measured surface temps on metal seat bases, floorboards, and engine tunnels running 140 degrees.

This is an additional load for the A/C system that didn't exist before. The situation is similar with cleaner-burning diesels in off-highway equipment, which have even more steel plus the additional heat load of the hydraulics.

When you're dealing with engines built to meet tougher emissions standards, it's doubly important to check the A/C system thoroughly and to replace worn-out hoses and other components with OE-quality parts. If you're getting complaints about the A/C not working well, or you're helping a customer spec an aftermarket unit, remember that a hotter-running diesel can increase the heat load in the cab. Take that extra heat into account.