Imagine this: You turn off your diesel engine, but it keeps revving louder and faster. The key is out, but the engine won’t stop. Within seconds, the vehicle shakes, and you fear a full-blown explosion.
This terrifying situation is called diesel engine runaway—and it’s more common than most people realize, especially in industrial work zones.
In Canada, diesel-powered equipment is used in nearly 80% of construction, mining, and oilfield operations. A malfunction like a runaway engine doesn’t just damage machinery—it can risk lives.
That’s where a positive air shut off system (PASS) comes in. This safety device cuts off the engine’s air supply, stopping combustion instantly and preventing a dangerous runaway.
In this blog, we’ll break down exactly what a positive air shut off system is, how it works, and why it's essential for diesel engine safety—especially in Canada’s high-risk industries. We’ll also walk you through how to install a PASS unit, test it properly, and keep it well-maintained.
Whether you're a fleet manager, equipment operator, or safety officer, understanding how PASS protects both workers and equipment can make a real difference.
Keep reading to stay ahead of hazards and ensure your team is prepared for the worst-case scenario—before it ever happens.
What Is a Positive Air Shut Off System?
A positive air shut off system (PASS) is a critical safety device used in diesel engines. It’s essentially a valve installed in the engine’s air intake line.
When activated—either manually or automatically—it closes off the air supply to the engine, choking the combustion process and bringing the engine to an immediate stop. This is the most effective way to prevent or stop a diesel engine from running away.
The concept of PASS dates back to the 1950s when heavy-duty diesel engines were increasingly used in industrial settings such as mining, oil and gas, and construction. Back then, systems were bulky and manually operated.
Today, PASS units are more compact, smarter, and often feature automatic sensors that detect abnormal engine behaviour and trigger shutdowns without human input.
So why do diesel engines run away in the first place? Unlike gasoline engines, diesel engines rely on compression ignition, not spark plugs.
This means any flammable vapour or fluid—such as engine oil, propane, or natural gas—can enter the intake and ignite without warning.
Once the engine starts drawing in external fuels, it can accelerate uncontrollably even if the key is turned off. This can lead to engine failure, fire, or even explosions.
PASS provides a fast, reliable way to shut down the engine by cutting off its oxygen supply—removing the one thing combustion cannot continue without.
Types of PASS Systems
Positive air shut off systems (PASS) come in different types to suit various engines, job sites, and safety requirements.
The right type depends on the level of risk, the nature of the work environment, and how much automation is preferred.
While all PASS units work by cutting off the air supply to stop a diesel engine, the way they are triggered can vary widely.
Let’s look at the three main types of PASS systems:
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Manual PASS Systems
Manual PASS systems are the simplest and most cost-effective option. These are controlled directly by the operator through a toggle switch, lever, or pull-cable.
When the operator notices signs of a diesel runaway—like uncontrolled revving—they can quickly activate the system to shut the engine down.
Manual systems are ideal for environments where operators are highly trained and can respond quickly to emergencies. However, they do rely entirely on human reaction time, which may not be fast enough in some high-risk situations.
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Automatic PASS Systems
Automatic PASS systems use sensors to monitor engine behaviour in real time. These sensors may track RPM, air intake flow, or pressure changes, and when readings cross a set threshold—indicating a possible runaway—the system automatically closes the air intake valve.
This type offers fast, consistent protection without needing operator input. It’s especially useful in hazardous environments like oilfields, where diesel engines may be exposed to flammable gases that could trigger a runaway suddenly.
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Dual‑Mode PASS Systems
Dual-mode systems combine both manual and automatic features to provide redundancy and flexibility.
For example, Headwind’s ShockerPASS allows operators to manually shut down the engine or rely on automatic sensors to do it if they don’t react in time.
This layered approach offers the best of both worlds—human control when needed and automated safety as a backup.
Dual-mode systems are often preferred in high-risk sectors like mining, transportation, and petrochemical operations where even a moment’s delay can have serious consequences.
How PASS Works – Step‑by‑Step
A positive air shut off system (PASS) is designed to stop a diesel engine quickly and safely during a runaway situation.
Whether it’s a manual or automatic unit, the core function is the same: cut off the engine’s air supply to halt combustion.
Understanding how the system works step by step can help operators use and maintain it correctly, especially in high-risk environments like Canadian oilfields, construction sites, and remote work camps.
Here’s a breakdown of the key stages involved in PASS operation:
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Detection
In automatic PASS systems, the detection phase involves constant monitoring of engine metrics—usually RPM or airflow sensors. If the engine begins to rev uncontrollably past its normal limit, the system detects the abnormal spike.
In manual systems, this detection depends on the operator. The driver or equipment operator must be alert to signs of diesel runaway, such as high-pitched revving, unresponsive ignition, or rising exhaust smoke.
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Activation
Once a runaway is detected:
- In automatic systems, the valve is activated when RPM exceeds a preset redline (e.g., 3,000 RPM or higher), indicating uncontrolled engine acceleration.
- In manual systems, the operator pulls a cable or flicks an in-cab switch to trigger the air shut off valve.
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Air Supply Shutdown
After activation, the PASS system quickly closes the air intake valve, stopping the flow of oxygen into the engine. Since diesel engines rely on air for combustion, this action suffocates the engine, causing it to stop immediately.
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Engine Stops
With the air supply cut off, combustion can no longer continue—even if the ignition is still on or the engine is exposed to external fuel sources. The engine loses power and comes to a full stop, preventing damage, fire, or explosion.
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Reset Procedure
After shutdown, the PASS must be manually or automatically reset:
- Manual systems require the operator to twist a reset knob or pull the handle back into the open position to restart the engine.
- Automatic systems usually reset themselves once the engine RPM falls to zero and remains there for a preset delay (often 5–10 seconds), ensuring the issue has been resolved before resuming normal function.
Understanding each of these steps is key to ensuring the PASS system works correctly when it matters most.
Why It Matters
A positive air shut off system isn’t just an optional upgrade—it’s a life-saving safety measure. Diesel engine runaway may sound rare, but in Canada’s high-risk industries like oil and gas, mining, and heavy construction, it's a serious threat.
A runaway engine can explode if it continues to draw in flammable gases like propane, methane, or hydrogen sulphide (H₂S). In these moments, a PASS can mean the difference between a safe shutdown and a catastrophic blast.
Beyond protecting lives, PASS also helps prevent costly engine damage. When an engine over-revs, internal parts can fail, leading to expensive repairs or complete replacement. A properly working PASS system cuts off the air before the engine self-destructs.
Installing PASS also helps companies comply with Canadian safety regulations. For example, CSA B621 and B622 set out transport requirements for vehicles carrying dangerous goods, including emergency shutoff devices.
In Alberta, Directive 037 outlines safety standards for sour gas operations, where PASS may be mandatory. The National Energy Board (NEB) also requires engine safety systems in regulated zones.
For workers in H₂S-prone environments, understanding the full emergency procedure is essential. That’s why combining PASS installation with H₂S Alive or H₂S training is strongly recommended.
It ensures workers know not only how to react during a gas leak, but also how PASS contributes to shutdown protocols in toxic gas zones.
Industries & Applications
Positive air shut off systems are used wherever diesel engines operate in potentially hazardous environments.
These systems are not limited to one sector—they’re found across multiple industries where the risk of diesel runaway is high due to the presence of flammable vapours or gases.
You’ll commonly find PASS units installed on:
- Diesel trucks and vacuum trucks
- Generators and mobile power units
- Cranes and lifting equipment
- Mining haul trucks and loaders
- Marine diesel engines
In real-world applications, PASS plays a critical role in field operations:
- Oilfield trucks often operate in areas where natural gas and hydrogen sulphide (H₂S) can leak into the air. If this gas gets into the engine intake, it can cause a sudden runaway—PASS systems shut it down instantly.
- Remote generators, especially in northern Canada, are used for powering isolated camps and drilling rigs. A runaway here can lead to fire, property loss, or delayed operations in places where help is hours away.
- Mobile service vehicles that refuel, repair, or support heavy equipment also benefit from PASS. These units may park near active worksites with flammable materials, and engine safety is non-negotiable.
Whether on land or sea, PASS protects engines, crews, and worksites—especially in industries where one spark or over-revved engine could lead to disaster.
Detailed Installation Guide
Installing a positive air shut off system (PASS) is a crucial safety upgrade for any diesel engine operating in hazardous environments.
Whether you're outfitting an oilfield truck, generator, or mobile equipment, proper installation ensures the system will function reliably in a diesel runaway event.
Most PASS kits come with detailed instructions, but the general process follows key principles to guarantee effectiveness and compliance.
Here’s a step-by-step installation guide to help you get it right:
1. Select Proper Kit
Start by selecting a PASS kit that matches your engine’s specifications. The valve or flange size must match the intake diameter for proper sealing and airflow. Using the wrong size can lead to poor engine performance or ineffective shutdown.
2. Placement
Install the shut off valve as close to the intake manifold as possible. This placement ensures minimal delay between activation and engine shutdown, especially critical during fast-developing runaway scenarios.
3. Mount Valves and Switches
For manual PASS systems, install the in-cab toggle switch or pull cable within easy reach of the operator. This allows for quick action without fumbling during emergencies. Make sure it’s clearly labelled and visible.
4. Wiring Tips
For automatic systems, connect the solenoid or actuator to a reliable ignition power source or RPM sensor. Always follow the manufacturer’s wiring diagrams to avoid misfires or malfunctions. Secure wiring away from heat or vibration zones.
5. DIY vs. Professional Installation
Many PASS kits are designed for self-installation, especially for those with mechanical or electrical experience. However, professional installers are available across Canada and may be preferred for fleet installations or high-risk job sites.
6. Time Estimate
Most PASS systems take about 90 minutes to install on a standard diesel truck or generator. Larger or more complex equipment may require more time or additional mounting brackets.
Tip: For anyone involved in installing or maintaining PASS systems, taking Hazard Recognition Training is a smart move. It helps identify environmental and mechanical risks during setup, leading to safer and more compliant installations—especially on industrial sites governed by strict safety rules.
Testing & Maintenance
A positive air shut off system (PASS) is only as reliable as its upkeep. Like any critical safety device, it must be tested and maintained regularly to ensure it performs when needed.
Neglecting routine checks can lead to failures during emergencies—especially dangerous in high-risk Canadian industries like oil, gas, and mining, where compliance and operational safety are top priorities.
Here’s how to properly test and maintain a PASS system:
1. Initial Test
- After installation, perform a complete initial test to verify functionality.
- Bench test the solenoid by applying power to ensure it closes the valve properly.
- In-cab, use the manual switch or pull cable to confirm the valve activates from inside the vehicle.
- For automatic systems, simulate or raise engine RPM to trigger the shutoff at the preset threshold and confirm the valve engages.
2. Weekly Check
Every week, conduct a manual activation test—even if the system is mostly automatic. This ensures the valve moves freely and hasn’t seized due to dirt, corrosion, or mechanical wear.
3. Visual Inspection
Inspect the unit for dust, dirt, or debris that could block valve operation. Check wiring, connections, and hoses for wear, cracks, or dislodgement. Clean around the valve and intake regularly to avoid buildup.
4. Troubleshooting
If the valve fails to activate:
- Check solenoid coil resistance using a multimeter—it should read around 1 ohm.
- Remove any debris that may block valve movement.
- Confirm proper power supply and replace any faulty fuses or relays.
5. Documentation
Maintain inspection and test logs for safety audits and regulatory compliance. Logs should note the date, technician name, test type, findings, and corrective actions taken.
Regular testing and maintenance not only extend the life of your PASS but also ensure you meet provincial and federal safety standards—keeping both your crew and equipment protected.
Limitations & Safety Considerations
While a positive air shut off system (PASS) is a vital tool in preventing diesel engine runaway, it's important to understand its limitations and use it alongside other safety protocols.
Relying solely on PASS can give a false sense of security if other ignition risks aren’t addressed.
First, PASS systems only stop combustion by cutting off airflow, but they don’t eliminate all ignition sources.
For instance, engine backfires, static discharge, or nearby sparks can still ignite flammable vapours around a vehicle or worksite. PASS protects the engine but doesn’t replace the need for proper ventilation, gas monitoring, or flame arrestors in hazardous zones.
Second, PASS systems—especially automatic ones—can be accidentally triggered. If an engine experiences a turbo overboost, steep downhill braking load, or sudden RPM spike, the system may misinterpret this as a runaway event.
This could result in an unwanted shutdown, which is not only inconvenient but potentially dangerous if the vehicle is in motion.
That’s why it's critical to calibrate PASS systems correctly and ensure operators know how to respond to false activations.
Finally, never try to replace or supplement a PASS system with DIY methods like blocking the intake with a rag, using a fire extinguisher, or fabricating homemade covers.
These methods are unreliable and dangerous—they can cause engine damage, create backpressure explosions, or even harm the operator.
In short, PASS is an essential safeguard—but it must be installed, maintained, and used correctly, as part of a larger engine safety and emergency response plan.
Conclusion
A positive air shut off system is one of the most effective defences against diesel engine runaway—a threat that can lead to fires, explosions, and serious injuries.
By shutting off the engine’s air intake, PASS cuts combustion instantly, preventing catastrophic outcomes in high-risk environments like oilfields, mining sites, and industrial yards.
We’ve explored how PASS works step by step—from detection to shutdown and reset—and the different types available, including manual, automatic, and dual-mode systems.
We’ve also outlined proper installation, routine maintenance, and key safety considerations to ensure your system performs reliably when it matters most.
Now is a good time to take action. If you operate diesel-powered equipment, audit your entire fleet to check whether PASS is installed, functioning, and compliant with relevant safety regulations.
For units already equipped, implement a regular testing and inspection schedule to keep the system in working order.
If you're running without PASS, consider it an urgent upgrade—especially if your crew works near flammable gases or in remote locations.
PASS isn’t just a technical feature—it’s a life-saving investment. In hazardous work zones, the cost of not having a working shut off system can be far greater than the price of installing one. Don’t wait for a close call to make the change.