Viair 93940 Relay Review: Power Your Compressor

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A 12V air compressor can draw far more current than a dashboard switch or pressure switch should carry on its own. The Viair 93940 40-Amp Relay solves that problem by using a low-current control circuit to switch the compressor’s heavier battery feed. This review explains its verified 12V and 40A limits, what is included, how the four terminals are used, and where a more protective relay or complete harness may be the better choice.

What Is the Viair 93940 Relay?

The VIAIR 93940 is a standard 12V, 40A relay made for compressor and other hard-wired automotive circuits. A relay contains a low-current control side and a higher-current switched side. When your pressure switch or dashboard switch energizes the coil, the relay closes the power circuit between the battery and compressor.

This arrangement keeps the compressor’s main current from flowing through the smaller control switch. VIAIR’s wiring diagram shows the relay between a fused battery feed, a pressure switch, ground, and the compressor positive lead. The molded tab gives you a fixed mounting point instead of leaving the relay hanging from its wires.

Who Should Buy It?

Key Features and Specifications Breakdown

40-Amp Maximum Rating

The 93940 is rated for a maximum of 40 amps at 12V. That makes it suitable for many single-compressor installations, but the rating should not be treated as permission to connect any compressor. Check the compressor label or manual for normal and maximum current draw, then size the relay, wire, and fuse as one complete circuit.

A relay does not reduce the current required by the compressor. Its benefit is that the heavy current travels through the relay’s switched contacts instead of through the pressure switch or dashboard control.

Four-Prong Switching Layout

The relay uses the common 30, 85, 86, and 87 terminal layout. In VIAIR’s compressor wiring diagram, terminal 30 receives fused battery positive, terminal 87 sends power to the compressor positive lead, terminal 86 receives the pressure-switch signal, and terminal 85 connects to ground. Check the markings on the relay and the supplied diagram before making connections.

Molded Case and Mounting Tab

The molded case includes an integrated tab that lets you fasten the relay to a stable surface. This reduces wire strain and keeps the relay from moving around under vibration. The standard 93940 should still be installed in a reasonably sheltered location because VIAIR sells separate sealed relay models for higher moisture and dust protection.

Relay-Only Package

The package contains the relay rather than a complete installation kit. You must supply compatible female spade terminals or a relay socket, primary wire, trigger wire, a fuse holder, a fuse, and mounting hardware. This is useful for a custom build, but it adds planning and assembly time compared with a pre-wired harness.

How It Performs in Real Use

In an Onboard Air Tank System

In a tank-based onboard air setup, a pressure switch can control the relay coil while the relay handles the compressor feed. This lets the compressor cycle as tank pressure rises and falls without forcing full compressor current through the pressure switch. The result depends on correct wire size, sound grounds, and a fuse matched to the compressor.

For Tire Inflation After a Trip

A permanently mounted compressor may run for several minutes while you reinflate multiple tires. The 93940 gives the compressor a direct switched path from the battery, which helps avoid using a small dashboard switch as the main current path. It does not change the compressor’s duty cycle, airflow, or inflation speed, so those limits still come from the compressor itself.

For Air Horns and Other 12V Accessories

The relay can also switch a compatible 12V accessory when the total load stays within 40 amps. It is useful when the control switch is not designed to carry the accessory’s full current. For any non-compressor application, confirm that the accessory, fuse, relay, and wiring are electrically compatible.

As a Replacement in an Existing System

The standardized four-prong layout makes the 93940 a possible replacement for some existing relays, but matching the terminal arrangement is not enough by itself. Verify the coil voltage, contact rating, normally open operation, terminal size, and wiring diagram before swapping parts.

Installation and Wiring Best Practices

Relay installation is simple in concept, but poor crimps, undersized wire, unfused battery feeds, and weak grounds can create heat or voltage drop. Follow the compressor manufacturer’s diagram whenever it differs from general guidance.

• Disconnect power first: Remove the battery’s negative connection before routing or terminating the new circuit.
• Fuse the battery feed: Install the main fuse close to the power source and choose its rating from the compressor specification and wire capacity, not from the relay rating alone.
• Use suitable copper wire: Select automotive-grade stranded copper wire based on current draw, total cable length, acceptable voltage drop, and local installation conditions.
• Protect every terminal: Use properly sized insulated or heat-shrink female spade terminals, make firm crimps, and support the wiring so vibration does not pull on the blades.
• Choose a sheltered mounting point: Keep the relay away from direct exhaust heat, standing water, and constant spray. Mount it with the terminals protected from moisture and debris.
• Test before final wrapping: Confirm that the pressure switch or control switch activates the relay and that voltage at the compressor stays within the compressor manufacturer’s acceptable range.

Compatibility and Electrical Safety

The 93940 is intended for 12V DC systems and should not be used as a 24V relay. A 24V supply can damage a 12V coil, while a compressor drawing more than 40 amps can overload the relay contacts. Dual-compressor systems should use a separate correctly rated relay and fuse for each compressor unless the system manufacturer provides a different approved design.

Do not assume the standard housing is fully waterproof. VIAIR lists separate sealed relay products with IP66 protection, while the 93940 is presented as the standard relay. For exposed underbody or open-bed mounting, a sealed relay, weatherproof enclosure, or manufacturer-approved harness is a safer choice.

VIAIR also lists a California Proposition 65 warning for this product. Wash your hands after handling electrical components when appropriate, keep small parts away from children, and follow the manufacturer’s safety information.

Pros and Cons

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Is It Worth the Price?

The Viair 93940 sits in the budget accessory category, but it performs an important job in a properly designed onboard air circuit. It is worth considering when you already plan to choose your own wire, fuse, terminals, and mounting location and need a branded 12V relay for a load no higher than 40 amps.

A complete pre-wired kit may offer better value when you do not already own crimping tools or electrical supplies. A sealed relay is also the better purchase for locations exposed to heavy spray or dust. The 93940 makes the most sense for a protected custom installation where simplicity and standard terminal layout matter.

How It Compares to Alternatives

A generic Bosch-style 30/40A relay may cost about the same and can be easier to replace locally, but quality, terminal construction, and published application details vary by seller. A complete fused relay harness is easier for a first-time installer because the socket and leads are already assembled, though you still need to confirm wire gauge and fuse rating.

For harsher mounting locations, VIAIR’s sealed relay range is the more suitable comparison because those models are specifically described as IP66 protected against moisture and dust. Higher-draw compressors may require VIAIR’s heavy-duty or 80A relay options instead of the standard 93940.

Frequently Asked Questions

The Bottom Line

The Viair 93940 is a strong choice for a protected, custom 12V compressor circuit that stays within its 40A maximum rating. Buy it when you need a compact four-prong relay and are comfortable supplying the rest of the wiring components; skip it for 24V systems, higher-current compressors, or exposed locations that call for an IP-rated sealed relay.