Meritor Tire Inflation System by P.S.I.

Quick Facts

Manufacturer: Pressure Systems International, commonly known as P.S.I.
Product Name: Meritor Tire Inflation System by P.S.I.
Common Abbreviation: MTIS by P.S.I.
Product Type: Automatic trailer tire-inflation system
System Category: Automatic tire inflation and pressure maintenance
Primary Position: Commercial trailer axles and wheel ends
Pressure Control: Maintains a preset target pressure by adding air when required
Air Supply: Existing trailer compressed-air reservoir
Air Routing: Control box, axle passages, rotary unions, hubcaps, and tire hoses
Driver Notification: Trailer-mounted indicator lamp
Optional Technology: ThermALERT wheel-end heat warning and telematics integration where equipped
Common Applications: Dry vans, refrigerated trailers, flatbeds, tank trailers, and specialized commercial trailers
Installation: Original-equipment or approved retrofit installation
Primary Benefit: Automatically maintains trailer tire pressure to help reduce underinflation-related tire wear, fuel use, and downtime

Overview

The Meritor Tire Inflation System by P.S.I. is an automatic tire-inflation system designed for commercial trailers. It uses compressed air from the trailer’s existing air system to maintain connected tires at a preset pressure while the vehicle is operating.

Air travels from the trailer reservoir through a pressure-protection valve and control box before entering the trailer axles. The axles act as air passages, carrying air toward rotary unions at the spindle ends. Hubcap fittings and flexible hoses then deliver air to the tires.

If a tire falls below the system setting because of normal pressure loss or a slow puncture, MTIS supplies air until the target is restored or until the system can no longer compensate for the leak. A trailer-mounted indicator lamp warns the driver when excessive airflow suggests a significant leak or system problem.

Key Characteristics

Automatic trailer tire inflation
Uses the trailer’s existing compressed-air supply
Maintains a preset tire-pressure target
Inflates tires affected by normal pressure loss or slow leaks
Uses the axle structure as part of the air-delivery path
Rotary union is the principal moving component
Isolation valves help protect unaffected tires
Trailer-mounted warning indicator
Dual filtration helps control moisture and contamination
Compatible with multiple trailer axle and wheel-end configurations
Available for original-equipment and retrofit installation
Optional ThermALERT wheel-end heat-warning technology
Telematics integration available in applicable configurations

Major Components

Trailer air reservoir connection
Pressure-protection valve
System shutoff valve
Particulate filter
Control box
Pressure regulator and adjustment mechanism
Flow-sensing switch
System pressure drain or petcock
Trailer indicator lamp
Electrical wiring harness
Axle supply lines
Axle press plugs
Stator tubes
Rotary union assemblies
Through-tees
Inflation-system hubcaps
Tire hoses
Check valves or isolation valves
Optional ThermALERT components
Identification and warning decals

System Type and Design

MTIS by P.S.I. is a pressure-maintenance system rather than a load-responsive pressure-adjustment system. The control assembly is set to a target cold inflation pressure, and the system adds air whenever connected tire pressure falls below that setting.

The trailer axle carries air internally, eliminating the need for long exposed air lines along the axle beam. At each spindle, a stator and rotary union transfer air from the stationary axle into the rotating hubcap assembly.

Flexible hoses connect the hubcap through-tees to the tire valve stems. Check valves help isolate an individual leaking tire so the remaining tires do not lose pressure through the same circuit.

How the Tire Inflation System Works

Compressed air from the trailer reservoir enters the MTIS control box through a pressure-protection valve. The protection valve helps preserve sufficient air for the trailer braking system before allowing air to flow into the tire-inflation circuit.

The controller regulates pressure according to the system’s preset target. Air travels through supply lines into the trailer axles, which function as conduits to the spindle ends.

Rotary unions transfer air from the nonrotating axle components into the rotating hubs. Air then passes through the hubcaps and flexible hoses to the tires. When tire pressure reaches the system setting, airflow stops.

If airflow continues because of a puncture, loose connection, or system leak, the flow-sensing switch activates the trailer indicator lamp. This alerts the driver that the system is working continuously or requires inspection.

Position and Function Within the Vehicle

MTIS is installed on the trailer and serves its axle and wheel-end assemblies. The control box is normally mounted beneath the trailer, while the indicator lamp is positioned where the driver can see it through the tractor’s mirror.

The system performs several functions:

Receives compressed air from the trailer reservoir
Protects the brake system through a pressure-protection valve
Regulates air to the preset tire-pressure target
Routes air through the trailer axles
Transfers air into the rotating wheel ends
Inflates tires that fall below the preset pressure
Helps compensate for normal pressure loss and slow punctures
Isolates leaking tire positions when possible
Warns the driver when excessive airflow occurs
Supports optional wheel-end heat detection and telematics

Common Specifications

System Type: Automatic trailer tire-inflation system
Pressure Function: Adds air to maintain a preset target pressure
Pressure Setting: Application-specific and selected for the installed tires and axle load
Air Supply: Trailer compressed-air reservoir
Brake-System Protection: Pressure-protection valve
Air Routing: Controller, axle passages, stators, rotary unions, hubcaps, and tire hoses
Driver Alert: Trailer-mounted indicator lamp
Tire Arrangements: Application-specific dual or wide-base single configurations
Axle Compatibility: Approved Meritor and non-Meritor trailer axles when properly configured
Wheel-End Lubrication: Oil or grease configurations with compatible hubcaps and components
Installation: Original equipment or approved retrofit
Optional Equipment: ThermALERT wheel-end heat warning and telematics integration

Pressure Settings and Operating Capacity

The target pressure is selected according to the tire manufacturer’s load-and-inflation requirements, trailer axle loading, tire size, wheel capacity, and fleet operating conditions. The setting should normally reflect the required cold tire pressure for the application.

MTIS can only add air when trailer reservoir pressure exceeds the pressure required by the tires. It cannot generate pressure independently of the tractor and trailer compressed-air system.

The system’s ability to compensate for leakage depends on the size of the leak, available reservoir pressure, compressor capacity, air-system condition, and number of affected tires. A rapid or severe leak can exceed its delivery capacity.

Pressure Maintenance and Leak Compensation

MTIS is designed to maintain the configured target pressure while the vehicle is operating. It compensates for normal diffusion, temperature-related pressure changes, small valve leaks, and certain tread punctures that release air slowly enough for the system to keep pace.

The system is not a tire-repair device. A tire receiving continuous air still requires inspection and repair. Continued operation with a damaged tire may allow the condition to worsen even when inflation pressure appears acceptable.

Isolation valves help prevent pressure loss from spreading to the remaining tires when one tire or hose develops a leak. Their effectiveness depends on correct component condition and installation.

Indicator Lamp and Driver Alerts

The trailer indicator lamp is connected to a flow-sensing switch in the control assembly. The lamp illuminates when airflow exceeds the system’s normal threshold, indicating that one or more tires may be low or that the system has a leak.

A lamp may illuminate temporarily after startup if tires require air. If it remains on, turns on frequently, or activates while driving after pressures should have stabilized, the trailer should be inspected.

The standard indicator does not identify the exact tire or component causing the airflow. The driver or technician must inspect every tire, hose, hubcap, fitting, axle connection, and control component.

Air Supply, Controls, and Electronic Integration

The trailer reservoir provides the system’s air supply. A pressure-protection valve prioritizes brake-system requirements and prevents the inflation system from using air when reservoir pressure is below the valve’s opening threshold.

The control box contains the shutoff valve, filters, regulator, flow-sensing components, drain, and related fittings. Its pressure adjustment should only be changed according to approved procedures and verified with an accurate test gauge.

Optional configurations may communicate system activity or ThermALERT wheel-end heat warnings through compatible trailer ABS power-line carrier communication, telematics devices, or P.S.I. TireView LIVE equipment.

ThermALERT Wheel-End Heat Warning

ThermALERT is an optional MTIS feature designed to warn of excessive wheel-end heat. High temperatures may result from bearing problems, loss of lubrication, dragging brakes, or other developing wheel-end failures.

When the heat-sensing components reach their activation condition, the system produces a warning intended to alert the driver. The vehicle should be stopped safely and the affected wheel end inspected according to the applicable service procedure.

ThermALERT supplements rather than replaces regular brake, bearing, seal, lubrication, and wheel-end inspections.

Installation and Pressure Adjustment

Installation involves the trailer air supply, control box, axle preparation, spindle components, rotary unions, compatible hubcaps, tire hoses, electrical wiring, indicator lamp, and system identification decals.

Axle preparation must follow the approved procedure for the specific axle and spindle. Incorrect drilling, sealing, press-plug installation, stator positioning, or rotary-union installation can create leaks or damage wheel-end components.

After installation, the system must be leak-tested and its pressure setting verified. The pressure should only be adjusted using an accurate gauge and the applicable P.S.I. or Meritor service procedure.

Trailer, Axle, and Wheel-End Compatibility

MTIS must be matched to the trailer’s complete axle, spindle, hub, lubrication, tire, and wheel configuration. Compatibility depends on:

Trailer manufacturer and model
Trailer axle manufacturer and model
Axle spindle design
Number of controlled axles
Oil- or grease-lubricated wheel ends
Hubcap design and bolt pattern
Dual or wide-base single tire arrangement
Tire size and target pressure
Wheel and valve-stem configuration
Trailer air-reservoir capacity
Brake-system air requirements
ABS and electrical configuration
ThermALERT or telematics requirements
Original-equipment or retrofit installation

Common Applications

Dry van trailers
Refrigerated trailers
Flatbed trailers
Tank trailers
Moving-floor trailers
Drop-deck trailers
Dump trailers
Automotive transport trailers
Long-haul commercial trailers
Regional delivery trailers
Dual-tire trailer axles
Approved wide-base single-tire trailer axles

Advantages

Automatically adds air to underinflated trailer tires
Uses the trailer’s existing compressed-air supply
Helps maintain a consistent preset pressure
Can compensate for normal pressure loss and certain slow leaks
Helps reduce underinflation-related tread wear
Supports improved fuel efficiency
May extend tire and casing life
Can reduce tire-related roadside service events
Indicator lamp provides a visible driver warning
Isolation valves help protect unaffected tires
Available for new trailers and approved retrofits
Optional ThermALERT warns of excessive wheel-end heat
Can integrate with compatible telematics equipment

Limitations or Considerations

Maintains a fixed target rather than automatically changing pressure with axle load
Requires a dependable trailer compressed-air supply
Cannot compensate for every rapid or severe tire leak
Does not repair punctures or structural tire damage
Indicator lamp does not identify the exact leaking tire
Air lines, filters, fittings, rotary unions, hubcaps, and hoses require inspection
Contaminated or wet air can affect system operation
Pressure settings must match the tires and maximum axle loading
Axle preparation must follow approved procedures
Wheel-end service requires attention to inflation-system components
Manual tire inspections remain necessary
The system does not increase tire, wheel, axle, or trailer ratings

Driver and Operator Experience

MTIS operates automatically while the trailer air system is charged. Drivers normally do not need to add air manually during ordinary operation, but they remain responsible for tire inspection and responding to warnings.

A brief indicator-lamp event may occur while the system restores pressure. A lamp that remains illuminated or repeatedly activates can indicate a tire puncture, loose hose, leaking fitting, low reservoir pressure, or system fault.

The system can temporarily maintain pressure during certain slow leaks, giving the driver an opportunity to reach an appropriate repair location. This capability should not be interpreted as permission to continue operating indefinitely with a damaged tire.

Sensor Programming and Tire-Position Assignment

Standard MTIS is a pneumatic inflation system and does not require individual electronic pressure sensors to be assigned to every tire position. The trailer indicator lamp reports excessive system airflow rather than displaying separate pressure readings for each tire.

If MTIS is combined with TireView LIVE or another compatible monitoring platform, sensor identification, trailer assignment, alert thresholds, and tire-position mapping must be programmed according to that system’s instructions.

Fleet records should identify the trailer, axle positions, installed MTIS configuration, target pressure, tire sizes, and any telematics hardware associated with the system.

Inspection and Wear Points

Routine inspection should include:

Control-box enclosure and mounting
Pressure-protection valve
System shutoff valve
Particulate filters
Pressure regulator
Drain petcock
Flow-sensing switch
Air-supply lines and fittings
Axle press plugs and seals
Stator tubes
Rotary unions
Hubcaps and through-tees
Tire hoses and knurled fittings
Tire valve-stem connections
Indicator lamp and wiring
ThermALERT components where equipped
Signs of air leakage, abrasion, heat damage, or contamination

Maintenance and Ownership Notes

Inspect every tire during pre-trip and scheduled maintenance
Verify tire pressure manually at scheduled intervals
Confirm the system shutoff valve is open during normal operation
Drain and service filters according to the maintenance schedule
Inspect air lines and hoses for rubbing, cracking, and kinking
Check fittings for leakage
Inspect rotary unions and hubcaps during wheel-end service
Check wheel-end lubricant level and condition
Test the indicator lamp and wiring
Verify target pressure after regulator service
Maintain the tractor and trailer air dryers and reservoirs
Investigate repeated indicator-lamp activation
Use approved replacement parts
Follow the applicable MTIS maintenance manual

Common Tire-Inflation-System Problems

Indicator lamp remains illuminated: A low tire, tire puncture, loose hose, leaking fitting, low reservoir pressure, or system fault may cause continued airflow.
System will not inflate tires: A closed shutoff valve, insufficient reservoir pressure, blocked filter, damaged regulator, restricted line, or failed rotary union may prevent air delivery.
One tire remains low: A damaged tire hose, closed or blocked isolation valve, valve-stem problem, tire puncture, or wheel-end restriction may isolate that tire.
Incorrect system pressure: Regulator adjustment, an inaccurate test gauge, restricted flow, contamination, or component wear may cause an incorrect pressure setting.
Air leakage at the hub: A worn rotary union, damaged through-tee, hubcap seal problem, loose fitting, or improperly installed stator may be responsible.
Moisture or contamination: Air-dryer problems, neglected filters, or reservoir contamination can restrict valves and damage internal components.
False warning indication: Damaged wiring, a faulty flow switch, incorrect adjustment, or an electrical connection problem may illuminate the indicator.
Wheel-end heat warning: A ThermALERT activation may indicate bearing damage, inadequate lubrication, dragging brakes, or another serious wheel-end condition.

Safety and Service Precautions

Chock the trailer wheels before beginning service
Turn off the system and drain pressure before disconnecting components
Never work beneath a trailer supported only by a jack
Wear eye protection when working with compressed air
Keep hands and tools clear of pressurized fittings
Do not exceed approved tire or wheel pressure ratings
Do not assume a tire is safe solely because the system maintains pressure
Inspect any tire that causes continuous system operation
Follow approved axle-preparation and spindle-service procedures
Protect rotary unions and stator components during wheel-end service
Respond immediately to a ThermALERT warning
Follow P.S.I., Meritor, tire-manufacturer, axle-manufacturer, and fleet procedures

Identification Information

System information may be found on the control box, trailer decals, hubcaps, axle tags, and individual components. Record the following information before ordering parts:

Trailer manufacturer, model, and VIN
Axle manufacturer and model
Spindle configuration
MTIS system model or kit number
Control-box part number
Preset tire pressure
Number of controlled axles
Dual or wide-base single tire arrangement
Hubcap manufacturer and part number
Oil- or grease-lubricated wheel end
Rotary-union and stator configuration
Tire-hose configuration
ThermALERT equipment where installed
Telematics hardware where installed
Tire size, load range, and required pressure

Buying Considerations

Identify the trailer and axle configuration
Confirm axle and spindle compatibility
Determine the number of controlled axles
Choose dual or wide-base single tire equipment
Verify tire load and pressure requirements
Confirm the desired preset pressure
Check trailer reservoir capacity and condition
Verify oil- or grease-lubricated hubcap requirements
Determine original-equipment or retrofit installation
Review axle-preparation requirements
Confirm indicator-lamp and electrical requirements
Decide whether ThermALERT is required
Review telematics integration options
Confirm replacement-parts availability
Use a qualified trailer and tire-inflation-system installer

Variants and Configurations

Standard MTIS configurations
MTIS with ThermALERT
Dual-tire systems
Wide-base single-tire systems
Single-axle installations
Tandem-axle installations
Multi-axle trailer installations
Oil-lubricated wheel-end configurations
Grease-lubricated wheel-end configurations
Different preset pressure settings
Meritor trailer-axle applications
Approved non-Meritor axle applications
Original-equipment installations
Fleet retrofit kits
Telematics-enabled configurations

System parts and installation details vary by axle, spindle, hubcap, tire arrangement, and trailer application. The complete kit and component numbers should be verified before ordering.

Comparison to Similar Equipment

Compared with the Hendrickson TIREMAAX PRO-LB Tire Inflation System, MTIS maintains a preset pressure rather than automatically changing its target according to trailer axle load.

Compared with the Hendrickson TIREMAAX PRO Tire Inflation System, standard MTIS focuses primarily on adding air to tires below the target. TIREMAAX PRO also provides active pressure relief.

Compared with the STEMCO AerisIS Tire Inflation System, MTIS uses P.S.I.’s control, axle-routing, rotary-union, and wheel-end component architecture. Compatibility and service support depend on the trailer fleet.

Compared with the Aperia HALO Tire Inflator, MTIS uses the trailer’s centralized compressed-air supply and axle plumbing. HALO uses wheel rotation to generate inflation pressure at individual wheel positions.

Best Fit For

Long-haul trailer fleets
Dry van and refrigerated fleets
Trailers experiencing recurring tire-pressure loss
Fleets seeking reduced roadside tire downtime
Applications using a consistent cold tire-pressure target
Dual-tire trailer configurations
Approved wide-base single-tire configurations
New trailers specified with automatic tire inflation
Existing trailers suitable for retrofit installation
Fleets wanting optional wheel-end heat warnings
Operators integrating tire data with telematics platforms

Related Vehicle and Trailer Types

Related Truck and Trailer Models

Related Tire and Wheel Types

Related Tire and Wheel Products

Related Equipment Systems

Related Guides

Regulatory and Compliance Notes

MTIS does not change the tire, wheel, axle, suspension, or trailer load ratings. The preset pressure must remain within the approved limits of every component and satisfy the tire manufacturer’s requirements for the trailer’s maximum axle loading.

Commercial tires must still comply with applicable tread-depth, condition, load, and inflation requirements. An automatic inflation system does not make a damaged, overloaded, mismatched, or worn tire compliant.

Installation and service work must preserve trailer brake operation, air-system integrity, wheel-end sealing, ABS communication, electrical function, and required safety systems.

Real World Operating Notes

MTIS can keep a slowly leaking tire inflated long enough for the driver to reach a suitable service location, but the lamp warning should never be ignored. Continuous airflow means the system is compensating for a condition that requires inspection.

The system target should be selected for the trailer’s maximum expected axle load. Because standard MTIS does not automatically lower pressure when the trailer is empty, fleets operating many lightly loaded miles should consider how fixed pressure affects tread wear.

Wheel-end service can disturb stators, rotary unions, hubcaps, and hoses. The inflation system should be leak-tested after hub, bearing, seal, brake, or axle work.

MTIS reduces pressure-related maintenance demands but does not eliminate manual inspections. Tires can retain pressure while still having cuts, exposed cords, tread separation, impact damage, or irregular wear.

Frequently Asked Questions

What is the Meritor Tire Inflation System by P.S.I.?

It is an automatic trailer tire-inflation system that uses compressed air from the trailer reservoir to maintain connected tires at a preset pressure.

What does MTIS stand for?

MTIS stands for Meritor Tire Inflation System.

Does MTIS monitor each tire electronically?

Standard MTIS is primarily pneumatic. Its warning lamp indicates excessive system airflow but does not normally display an individual electronic pressure reading for every tire.

Can MTIS inflate a tire with a puncture?

It may compensate for certain slow punctures if the leak does not exceed system capacity. The damaged tire still requires prompt inspection and repair.

Can MTIS remove excess tire pressure?

Standard MTIS primarily adds air to maintain a fixed target. It should not be confused with systems specifically designed to provide active pressure relief or load-based adjustment.

What does the trailer indicator lamp mean?

It indicates excessive airflow through the system. A tire may be low, a hose or fitting may be leaking, or another component may require service.

What is ThermALERT?

ThermALERT is an optional wheel-end heat-warning feature intended to alert the driver to excessive temperature associated with a developing wheel-end problem.

Can MTIS be installed on an existing trailer?

Many trailers can be retrofitted when their axles, spindles, hubs, wheel ends, tires, and air systems are compatible with an approved kit.

Does MTIS replace manual tire inspections?

No. Drivers and technicians must continue inspecting pressure, tread, sidewalls, valves, wheels, hoses, and wheel-end condition.

How are the correct replacement parts identified?

Use the trailer VIN, axle and spindle information, MTIS kit number, control-box number, hubcap type, lubrication system, tire arrangement, preset pressure, and individual component numbers.

Availability and Where to Find

The Meritor Tire Inflation System by P.S.I. may be available as factory-installed equipment on new trailers or as an approved retrofit for compatible in-service trailers.

Complete systems and replacement components may be available through P.S.I. distributors, trailer manufacturers, commercial-trailer dealers, Cummins-Meritor parts channels, tire-inflation specialists, heavy-duty parts suppliers, and qualified trailer repair facilities.

Current product resources, installation information, service literature, training videos, and replacement components are available through the P.S.I. resources website.

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