Click for Coverage
Exclusive Content
Original Shows, Motorsports and Live Events
Try it free for 14 days
Due to the EU’s Global Data Protection Regulation, our website is currently unavailable to visitors from most European countries. We apologize for this inconvenience and encourage you to visit for the latest on new cars, car reviews and news, concept cars and auto show coverage, awards and much more.MOTORTREND.COM
  • JP Magazine
  • Dirt Sports + Off-Road
  • 4-Wheel & Off-Road
  • Four Wheeler

Trail’s End: December 1991, Driving An M939A2 With CTIS

Posted in Features on November 18, 2016
Share this
Photographers: Tom Berg

In the Dec. ’91 issue of Four Wheeler, we published a story about a visit we made to Eaton Corporation’s Michigan proving grounds to drive a Cummins CTA 8.3L diesel-powered M939A2 5-ton military 6x6 equipped with the company’s Centralized Tire Inflation System (CTIS). Tire inflation/deflation systems like this had already been around for decades, but this test gave us a hands-on demo of the technology.

“CTIS ties each of the M939A2’s six 22.50R20 radial tires to the truck’s air system. The air lines are not readily visible, they run through the axle ends and seals and, through an internal valve in the wheel, to the tires. An air compressor driven by the truck’s 250hp diesel engine is the CTIS’s ‘powerplant,’ keeping pressure levels up to operate the air brakes and any other air-driven accessories, including CTIS,” we explained.

We wrote that a control console beneath the center of the instrument panel allowed the driver to tell CTIS-via pushbutton-what types of surface he or she is driving on. “There are three normal settings, ‘Highway,’ ‘Cross Country,’ and ‘Sand.’ There are also ‘Emergency’ and ‘Run Flat’ buttons for more unusual off-road circumstances,” we noted. The story told how after a driver selected a button, CTIS pumped in air as needed to maintain the setting the driver has chosen. If the driver switched from Highway to a lower pressure setting, CTIS would respond by bleeding off air. The story said that with the 22.50R20 “super single,” tires, pressure would be 80 psi Highway, 35 psi Cross Country, 25 psi Sand, and 12-15 psi Emergency. If the driver forgot to select a higher setting when returning to the road, the CTIS would use a speedo-driven sensor to detect the higher speed and automatically make the adjustment to a higher pressure.

During testing, we found that there was no audible hissing from the system during deflation. Instead, there was just an audible beep from the CTIS control box when the desired setting had been reached. “CTIS did take a while to air up the tires for resuming travel on hard-surfaced roads. We had to punch in the desired setting, and yank out the hand throttle, letting the Cummins rev at about 2,000 rpm to speed up the compressor. Anywhere from 3 to 10 minutes were required to reinflate the tires, depending on how low their pressure is and how much you want to reinflate them. Still, the truck will run at reduced pressure on hard surfaces, so the driver doesn’t have to wait. CTIS will continue airing up as the vehicle moves down the road. And the process is faster than dragging around a hose to fill up each tire individually,” we wrote.

Near the end of the story we speculated if in the “not-too-distant future” a form of CTIS would be available for common 4x4s. We still wonder that. Will a CTIS-type system ever be offered for mass-produced off-road-package rigs like the Wrangler Rubicon, Ford F-150 Raptor, or Ram Power Wagon? But more importantly, would you pay for it? Or are you happy with manually altering the pressure in your 4x4’s tires?

Email your thoughts on CTIS to We’d love to hear ’em.

Connect With Us

Newsletter Sign Up

Subscribe to the Magazine

Browse Articles By Vehicle

See Results