GMC Sierra Cooling Fan Relay Location by Model Year

Where the relay sits on 2014 and 2015 Sierras

When a cooling fan relay is fitted, it is in the underhood electrical center. This is the black fuse and relay box on the driver's side of the engine compartment, beside the battery. Release the latches and lift the cover. The underside of the lid has a printed map that identifies every fuse and relay by number and function, and the fuse section of the owner's manual shows the same layout. Look for an entry naming the cooling fan or fan control and note its position. Then match it to the cavity in the box before pulling anything. Relays in this block look alike, so counting rows against the map is more reliable than guessing by appearance.

If neither the lid nor the manual lists a cooling fan relay, the label is probably not missing. The truck most likely does not use a relay for the main fan. The 2014 Sierra 1500 introduced a new body generation, while the 2014 Sierra 2500HD and 3500HD kept the previous design until the heavy-duty models were redesigned for 2015. Fuse block layouts differ between those generations and between engine and option combinations. The only map that applies is the one printed for your truck.

How the cooling fan is switched

On trucks with an engine-driven fan, the fan bolts to a viscous clutch on the fan drive. Silicone fluid inside the clutch couples the fan to the pulley more firmly as the air behind the radiator gets hotter, and lets it freewheel when the engine is cool. Some versions are electronically controlled: the engine control module commands a solenoid in the clutch and typically monitors fan speed through a sensor. In either case there is no fan relay, because the module or the clutch itself decides how hard the fan engages.

An electric fan works differently. The engine control module watches coolant temperature and air conditioning demand. When cooling is needed, it energizes the relay coil with a small control current. The relay contacts then close and send full battery current through a fused circuit to the fan motor. The relay exists so the module never has to carry the motor's heavy load. It also means a fan that will not run could have a problem in the fuse, the relay, the wiring, the motor or the control command, not only the relay.

Configurations that change the answer

Engine choice, chassis, model year and option packages all change what you find under the hood. Light-duty and heavy-duty trucks, gas and diesel engines, and trucks equipped for towing do not necessarily share the same fan hardware or fuse block assignments. Duramax diesel trucks also carry two batteries, which matters when you isolate the electrical system. Two trucks from the same model year can show different labels on the fuse block lid, so check the lid on your own truck rather than relying on another owner's photo.

Operating conditions matter too. An electric fan is meant to cycle on and off, so a stopped fan on a cool engine or at highway speed is normal, because moving air does most of the work. A viscous fan that roars briefly after a cold start and then quiets down is also working as designed. Judge the system when the engine is fully warm and idling with the air conditioning on, since that is when demand on the fan is greatest.

Finally, keep the electrical context straight. Every 2014 and 2015 Sierra relies on a conventional 12-volt starter battery for the fan circuit and its controls. No high-voltage traction battery is involved. The earlier Sierra Hybrid, sold through 2013, used a separate high-voltage system with its own isolation requirements, and the steps here do not apply to it.

Checking the relay and fan circuit yourself

Start with the simple checks. Find the fan fuse on the lid map, pull it and inspect the element. Confirm that the fuse rating printed on it matches the rating specified on the map. A blown fuse points to a short circuit or a seized motor, so replacing it repeatedly without finding the cause only buys time. Next, if the block holds another relay with the identical part number printed on its case, swap the two and see whether the fault moves with the relay. With the engine warm and the air conditioning on, you can often feel or hear the relay click when the module calls for the fan.

A digital multimeter gives you more detail. With the relay removed, the feed terminal in its socket should read close to the system voltage measured across the battery terminals. A low reading suggests corrosion or a poor connection upstream. With the fan running, a voltage drop test across the power and ground sides of the motor circuit reveals resistance that a simple continuity check misses. A weak battery or low charging output can also slow an electric fan, so confirm the battery and charging system are healthy before condemning the motor. Do not bridge relay socket terminals with a jumper unless the wiring diagram confirms which pins carry the load circuit, because bridging the wrong pair can damage the control module.

The opposite symptom also deserves attention. If the fan keeps running long after every shutdown, the relay contacts may have welded closed. That can create a parasitic draw that leaves the battery flat overnight. Pulling the relay and seeing whether the fan stops is a quick way to confirm it. For an engine-driven fan, check with the engine off and cold. Excessive wobble, oily residue on the clutch, or a fan that spins with almost no resistance all point to a worn clutch rather than an electrical fault.

Isolating the 12-volt circuit safely

Switch the ignition off and remove the key before pulling or installing relays and fuses. Before unplugging the fan motor or working near the fan, disconnect the negative cable from the battery so the circuit cannot be energized. On a Duramax truck, disconnect both batteries, since either one can power the system. An electric fan can start without warning while the engine is hot, so keep hands, clothing and tools clear until the circuit is isolated. Never reach toward an engine-driven fan with the engine running. Reconnect the negative cable last, and expect that some settings, such as radio presets, may need to be restored.

Limitations and next step

Fuse and relay assignments vary with engine and option codes, so this guide cannot give the exact cavity for a specific truck. The printed lid map and the owner's manual are the first reference. A factory wiring diagram matched to the vehicle identification number is the authority when they disagree or when you need pin-level detail for testing.

Get professional help if the temperature gauge climbs, the dash shows a cooling system warning, or a scan tool reports a fan control code such as P0480. Diagnosing an electronically controlled fan clutch, or a module that never commands the fan, usually requires a scan tool that can read live data and run output tests. If the truck is overheating, stop driving it. Continued operation can cause engine damage that costs far more than any relay or fan part.