Today I Learned That Some Jeep Grand Cherokees Can Read Body Temperature Like The Freakin’ Predator

If you’ve seen Predator (1987), you’re familiar with the alien being that was capable of destroying Schwarzenegger’s entire military rescue team, and very nearly took out the Austrian bodybuilder himself. Meanwhile, if you know the WJ Grand Cherokee, you’re aware that it’s a popular SUV that served in the Jeep product line from 1999 to 2004. But what do these two have in common?

Would you believe—it’s the ability to sense heat via infrared! The Predator was able to see in the thermal range of the electromagnetic spectrum. It picked up thermal-range infrared radiation which allowed it to track its human prey through the dense jungle. The WJ Grand Cherokee, on the other hand, uses its thermal IR sense for altogether more peaceful means, as do some following models.

Yes, when equipped with the Automatic Zone Control HVAC system, the Grand Cherokee came equipped with two infrared thermal sensors, or thermopiles. One is aimed at the driver, the other, the passenger. It was a rather unique way of determining just how hard to run the heating or cooling to serve the occupants of the vehicle.

Most automatic climate control systems use a little fan to suck the interior air over a small temperature sensor, usually a thermistor. This allows the HVAC system to know the temperature of the air inside the vehicle, and adjust its output to help reach the desired set point. But Jeep engineers realized this was missing the point. We don’t so much care about how hot or cool the air in the car is. We care about how hot or cool we are.

To find this out, the Jeep’s climate control needed a way to sense the temperature of the occupants at a distance. Nobody’s getting in their car and putting on a temperature sensor probe, after all, so it had to be done wirelessly. Enter the infrared thermopile. This is a small device that can pick up infrared radiation in the thermal part of the electromagnetic spectrum. See, objects emit infrared radiation depending on their temperature. The hotter something is, the more IR it puts out. Capture this radiation, which is essentially just light that’s too long in waveform for us to see with our eyes, and you can determine an object’s temperature to a fairly good degree of accuracy, subject to some complicated assumptions we won’t get into here.

A single thermopile can capture infrared radiation at a distance and spit out an electrical signal that correlates to temperature. You’re probably more familiar with them then you think. A thermopile is the sensor used in non-contact thermometer guns that you might have used to check the temperature of engine components or a catalytic converter. If you go ahead and take a bunch of thermopiles and put them into a rectangular array, you can actually create a crude thermal camera. Proper thermal cameras, such as those used in the filming of Predator, essentially have an array of hundreds or thousands of individual sensors that pick up IR to create a higher-resolution image. But at its heart, it’s the same idea—using infrared radiation to measure the heat of an object at range.

By my research, Jeep used this system in the WJ model from 1999 to 2004, in vehicles equipped with “Infrared Dual-Zone Climate Control.” The thermopiles were mounted behind a small plastic lens between the two temperature control knobs, which is largely transparent to infrared waves. It’s an interesting fact that materials that block visible light may not block IR, and vice versa. For example, we consider glass transparent when it comes to visible light, but it’s excellent at blocking IR radiation in the thermal spectrum.

Infrared thermopile sensors also appeared in the climate control for the WK model, built from 2006 to 2010. However, the controls had a different physical layout. It has single round sensor window in the center, and the manual specifically notes that this area should be kept free of debris to avoid impeding its operation.

On the WK design, the small round hole on the climate controls makes it look like the sensor is pointing straight out of the dash. Whereas, if there were sensors aimed at the driver and passenger, you’d expect there to be obviously two of them, and that they’d be pointing at a clear angle towards each seating position. I could be wrong, and the sensor we see on the WK could have two separate elements each with a certain viewing angle, but I’d have to tear one down to be sure. Sadly, I couldn’t get my hands on a WK climate control unit prior to publication.

It seems that the system fell out of favor with Jeep in time, as strange features often do. When the WK2 model arrived in 2011,  the manual failed to mention anything about infrared sensors, unlike previous generations. However, there’s a chance it does still have a thermopile sensor—the little round nubbin in the middle of the fan speed control looks like it could maybe be serving that purpose.

So why would Jeep abandon this innovative feature? The most likely reason is simplicity and cost. Buying an off-the-shelf interior temperature sensor and doing what every other automaker was doing was probably cheaper. Directly sampling the occupant’s surface temperature does sound nifty, it’s true, but it probably didn’t make much difference to how the climate control really worked in practice. Regular systems from other automakers seemed to satisfy customers just fine, and I’ve never heard anyone crowing that their Jeep understood them on a deeper, more personal level.

I’m always a big fan of weirdness in automobiles. I love that someone at Jeep had this totally bonkers idea of using thermopiles to sense people’s heat instead of just measuring the internal air temperature with a thermistor. Even more so, I love that the rest of the company ran with it. It sounds like something straight out of a 1980s Mercedes S-Class, but nope—it was good old Jeep doing it in the late ’90s.

Here’s to the rich world of automobiles, and everything they teach us about science and engineering. I can’t wait to find the next bit of weird vehicular ephemera, and you bet I’ll find a way to tie it to a 1980s action movie, too.

Image credits: Jeep, eBay, S-Twins, Predator (1987) via screenshot, NASA, Reduction Revolution, element 14