By Bruce W. Smith
It’s a small device that can be held in the palm of your hand, yet it’s the most important safety item on diesel pickups working in or around petroleum production facilities.
If your truck does’t have one, you’re not getting onsite.
The device is called a positive air shutdown system, or PAS.
If you are a contractor who has spent much time around the mining and petroleum-production trades, PAS devices are old hat.
But bring up the subject to the typical construction or landscaping contractor who is trying to expand their business and you’re more than likely to get a blank stare.
Yet having a PAS on a heavy-duty pickup is one of those upgrades that provides a safety benefit like no other aftermarket product out there.
It also opens the doors to getting a contract where such a device is a mandatory component on diesel pickups working where gas vapors pose a safety hazard.
Installing a PAS also makes diesel pickups compliant with regulations mandated by many contracts including septic tank removal, forestry projects and oil/gas well service.
The operation of a PAS is very simple: A diesel engine only needs air and fuel to run. Cut off one and the engine quits.
Where an emergency shutdown system like PAS comes into play is when that fuel source comes from outside the truck in the form of hydrocarbon vapor; you can turn off the truck’s ignition or kill the diesel supply, but the engine will keep running and revving until it blows up using that volatile outside fuel source.
(Think spraying a can of starting fluid into the air intake but with an unrestricted supply.)
So to prevent a runaway diesel engine there has to be a mechanical component that closes off the air supply, ensuring that outside fuel source through the air intake stops. Hence, the PAS.
There are a number of different PAS systems available. One of the best designs for diesel pickups is the one offered by BD Diesel Performance.
The $1,135 (US) kit is compact, smartly designed and easy to install.
All the parts are aluminum and stainless, so corrosion will never be a factor, and the electrical connections are waterproof.
BD’s PAS, an in-line butterfly valve activated by a heavy-duty solenoid, is placed in the charge-air-cooler air intake line (CAC). The rpm sensor is wired into the main engine harness to pick up the crankshaft position.
The system’s computer, housed in a small control box, will trip the solenoid and close the valve automatically when the engine hits a predetermined rpm (our Duramax is set to 4,600rpm ) – or manually when the driver flips the dash-mounted switch to the “off” position.
Shutdown is almost instantaneous. To restart the engine requires opening the hood and physically resetting the trip mechanism on the PAS.
We installed a BD PAS kit on the Duramax in our 2008 GMC Sierra 3500HD, Big Red, so whomever wins it next April will be all set to use the truck for whatever work comes its way.