Are you familiar with the term "ambient temperature?" You should be if you do any maintenance on your truck. Defined as the air temperature outside or under the hood in which your engine and components operate, ambient temperature affects the way your truck's systems perform.

Ambient temperature varies according to location and use. For example, on a cold winter day, the engine's ambient temperature can vary greatly. After sitting at zero degrees F, the engine will be "cold soaked" to that temperature. After starting and running for a while at zero outside, under-hood ambient temperature may be anywhere on the spectrum, even 100 degrees or more, depending on the load on the engine. Ambient for the truck is still zero, but ambient for under-hood components is much higher.

Why should you care? The laws of physics often depend not on absolute temperatures, but on temperature differences. Here's a look at what ambient temperatures means to different truck components:

Tires

If temperatures rise 150 degrees over ambient, there may be no detrimental effect to your tires when ambient temperature is 80 F or cooler, but on a 90 degree day, 150 over ambient brings tire temperatures to a dangerous 240 degrees. In the south, when the sun has baked the blacktop to as much as 120, a temperature increase of 150 degrees will raise internal casing temperatures high enough to melt the bond between tire rubber and steel cord. Tires delaminate when they get much above 250 F, resulting in chunking and thrown treads. Correct tire pressure is always important, but it becomes critical when ambient temperature is above 90 degrees. Always check tire pressures "cold," when tire temperature is at ambient. Never let air out of a warm tire, even if the pressure seems high. A tire should sit for three hours before you gauge it.

Batteries

Batteries are affected several ways by low ambient temperatures. Not only does battery output drop with cold, so does a battery's ability to take a charge. By industry convention, all battery ratings except Cranking Amps (CA, at 32 degrees F) and Cold Cranking Amps (CCA at 0 degrees) are measured at 80 degrees. That works well for automobiles. Their batteries are located under the hood where it gets nice and warm. In winter, car's batteries usually stay topped off. But truck batteries are located in chassis-mounted battery boxes, away from heat. They are subject to ambient temperature that may reduce their ability to be recharged by 50 percent to 60 percent, more in extremely cold conditions.

Picture your truck on a winter night, idling with the heater fan, marker lights and creature comforts (refrigerator, microwave, TV and VCR) all draining your batteries. The next day, your alternator can put out all the power the batteries need, but if the batteries can't accept all that the alternator puts out, you lose power. You start at 100 percent charge. A day later, you're at 80 percent, then 65 percent, then 52 and so on. Pretty soon, your lights dim, you have trouble starting, and you blame the batteries. The real problem is ambient temperature.

Air Conditioners

Air conditioner standards are written for specific ambient temperatures. Recommended Practice RP 432 from the Technology and Maintenance Council calls for engine-off air conditioner systems to maintain 78 degrees or lower in a cab for a minimum of 10 hours at 100 degrees F ambient temperature, with 50 percent humidity and a 20 mph wind load. But what happens if you're in 110-degree ambient conditions, or more. Since ratings have to do with heat transfer properties relative to ambient temperature, you had better expect your cab to be closer to 88 degrees on a 110-degree day, and 93 when it's 115 outside. Of course, if the humidity is lowered enough, that 88-degree interior may seem cool compared to ambient temperature.

Brakes

The higher the ambient temperature, the more brakes are prone to fade. Brake fade is a function of lining temperature as brakes convert your truck's kinetic energy (motion) to thermal energy (heat). Eventually, brakes heat to the point where gases cooking out of the linings lubricate the friction surfaces. At lower ambient temperatures, heat transfer from hot to cold masses is sufficient that brakes don't get hot enough to fade. As the brake's environment (the air, the wheel, the tire and the brake drum) gets hotter, the temperature differential is reduced and heat cannot flow out of the brakes fast enough.

Cooling Systems