Where to Mount an Under-Hood Rodent Repeller (and How to Power It)

An under-hood rodent repeller is only as good as where you put it. Mounted in the wrong spot, it can sit behind a solid part and reach almost nothing, or get cooked by the exhaust. Mounted well, it faces the open space where rodents like to settle and needs little attention after that. This guide uses our own 2-pack as the example, but the same rules apply to most under-hood units.

Before you start

• Park, switch off the engine, take the key out and let the engine cool down fully. Exhaust parts stay hot long after you stop.
• Read the manual that came with your unit, especially the wiring section if you plan to use the vehicle power cable.
• On a hybrid or electric car, stay well away from orange high-voltage cables and anything marked as high voltage. If you're unsure, ask a mechanic.
• Have a flashlight, the cable ties and a pair of side cutters ready.

Step 1: Pick a safe spot

Look for a fixed bracket or flat panel at the side of the engine bay: near the battery tray, along the inner fender or close to the firewall, rather than on top of the engine. Then rule out anything on this list:

• Hot parts. Keep clear of the exhaust manifold, turbocharger and catalytic converter. Our unit is rated from -13°F to 176°F, and normal engine coolant runs hotter than that, so don't mount it on the engine itself, the radiator or the coolant hoses either.
• Moving parts. Stay away from belts, pulleys and the radiator cooling fan, and route the power cable so it can't drift into them.
• Things you must not tie to. Don't fasten the unit to fuel lines, brake lines, the steering shaft, the throttle linkage or ignition parts. Use a solid bracket that doesn't move.
• Things you need to reach. Leave the dipstick, fluid caps, hood latch and battery terminals clear so routine checks stay easy.

Each of our units is about 3 × 1.2 × 4.6 in. Before you close the hood for the first time, check the clearance, then lower the hood slowly and make sure nothing touches.

Step 2: Give it a clear line of sight

Ultrasonic sound behaves more like light than like the bass from a car stereo: it doesn't pass through solid objects. An engine cover, the battery or the air filter box will block it, and whatever is behind them gets very little. So:

• Face the speaker toward the open space you want to cover, not toward a panel a few inches away.
• Aim it toward the places where rodents get in or nest: around the battery, the air filter box and the base of the windshield, where the cabin air intake usually sits.
• Remember that coverage figures are measured in open space. Our unit is rated at 215–538 sq ft per unit in open space, and a crowded engine bay gets less.
• Have two units? In a large engine bay, put one on each side. Or use one in the engine bay and one in the trunk or garage. The sound doesn't travel from one space to another, so each space needs its own unit.

Fasten the unit with cable ties pulled snug, trim the tails, and give it a firm tug to check that it won't rattle loose on rough roads.

Step 3: Choose how to power it

Our unit has three power options. Each one suits a different situation:

Whichever you choose, route the cable away from hot and moving parts, tie it down every few inches and keep it from being pinched when the hood closes.

Step 4: Do the battery-drain math

A common worry is whether a repeller will leave you with a flat battery. Here's the arithmetic for our unit, which draws about 5 mA (0.1 W) when it's on:

Because the vibration sensor puts it on standby while you drive, the real figure is a little lower for a car you use every day. Many car batteries are rated somewhere around 40 to 70 Ah, so a week's use is a small slice. Two things are worth knowing, though: your car's own electronics already draw a little power while it's parked, and a starting battery needs plenty of charge left to crank the engine, especially in cold weather.

The practical rule: for a car you drive at least every few days, the draw is minor. If the car will sit for weeks, the battery is old or it's very cold, switch to AA batteries or keep the car battery on a maintainer.

Step 5: Switch it on and check the status light

Most adults can't hear ultrasonic sound, so the lights are your main way to tell the unit is running.

• Flip the power switch and check that the status light comes on. Check your manual for what each color of the red/green light means.
• Remember the standby feature: the unit pauses while the engine runs and switches back on about 10 seconds after you park. Wait a moment before you decide it isn't working.
• Check the light once a week for the first month, then whenever you open the hood, and always after a battery change, a jump start or work in the engine bay.
• No light? Check the switch, then the cable connections or the batteries.

Step 6: Keep it dry, and remove it before pressure-washing

Our unit has no published waterproof rating, so treat it as a dry-location device that tolerates a normal engine bay:

• Mount it high and to the side, out of the path of spray from the wheels and water draining from the base of the windshield.
• Take it out before you pressure-wash or steam-clean the engine bay. Snip the cable ties, set the unit aside and refit it with fresh ties afterwards. There are 8 in the box, so you'll have spares.
• If it does get soaked, switch it off and let it dry completely before switching it back on.

When to use AA batteries

The vehicle cable is the most convenient option for a daily driver, but AA batteries are the better choice when:

• the car is in storage for weeks or over the winter and you don't want any draw on its battery;
• the car battery is old or weak, or you live somewhere very cold;
• you'd rather not wire anything to the car, for example on a leased car;
• the vehicle has a 24V system, which the DC 5–12V cable isn't rated for;
• you're using the second unit in a shed, storage room or trunk with no power source.

Use four fresh alkaline batteries of the same type, check the status light weekly at first to learn how long a set lasts in your setup, and take the batteries out if you put the unit away for a long time.

Quick checklist

• Engine cool, key out, manual read.
• Fixed spot at the side of the bay, clear of heat, belts, the fan and fuel or brake lines.
• Speaker facing the open space where rodents get in or nest.
• Power chosen for how the car is used; cable tied clear of hot and moving parts.
• Status light checked after parking, then weekly.
• Unit removed before any pressure-washing.

A repeller is one layer of protection, not the whole plan. For the rest, see our guide on how to keep mice out of your car engine.

From our store

Under-Hood Ultrasonic Rodent Repeller (2-Pack)

Two units with ultrasonic sound (18–36 kHz) and flashing LEDs, auto standby while you drive, and three power options. Results vary with placement. Ships free from our U.S. partner warehouse, with 30-day returns from Depoodeal.

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