Radiator Fan Controller with Overheating Alarm


  • Uses the stock temperature sensor for reliable and accurate operation
  • Standalone Fan Controller with active Overheating Protection
  • No external gauges, adjusts through the coolant temperature gauge in the instrument cluster
  • Full range temperature adjustment for fan and alarm thresholds
  • Retain factory fan control
  • Built in 30 amp relay to power 12 volt fan
  • Allows for 120 second fan operation after vehicle shut off
  • External buzzer option for noisy environments
  • 30 amp fuse to provide short circuit protection
  • Reverse wiring protection
SKU: Grotto-Relay Category: Tags: ,



Grotto is a fan controller and engine overheating alarm. It relies on the OEM temperature sensor. The fan and alarm activation levels are adjusted through the original dashboard gauge. There are no additional displays to mount and monitor in your passenger cabin which makes for a stealthy, out of sight, OEM-like installation. It operates in tandem and doesn’t interfere with the factory cooling fan controller.


What does Grotto do?


Grotto is a full feature fan controller used to expand the factory radiator fan function. It is a fully user adjustable electric fan controller with 2 set points. The first set point activates a 30 Amp built in relay. The relay is capable of handling up to 400 Watts to power single and dual cooling fan setups. The set point is manually adjustable and is capable of turning the fan on at any temperature. The second set point is an overheating alarm. If the temperature rises above a user adjusted temperature, the alarm will alert the driver. Extensive engine damage is preventable by activating the fan at a low temperature and through the use of the overheating alarm.


Grotto is a thermostatic fan controller that uses the original coolant temperature sensor. It relies on the accurate sensor installed on the engine by the factory. The thermostatic fan controller turns off the fan when the temperature is below the threshold. Because it uses the OEM sensor, adjusting the Grotto cooling fan switch operation is done through the coolant temperature display in the dashboard. Install the fan controller out of sight and then enjoy the peace of mind.


Grotto overheating alarm signals by blinking a factory dash warning light. It can be set to blink the low coolant or low washer fluid light. The overheating alarm also has an audible alarm to alert the driver. When the alarm is activated, a tone will sound 3 times, and the dash warning light will keep flashing. For noisy applications, the option to wire an external 5V buzzer is made available. The alarm temperature is user adjustable through the manual override of the coolant temperature display in the dashboard.


The factory cooling fan controller can remain in use. Grotto won’t interfere with its function. Under certain operating conditions and if set correctly, Grotto will activate the fan before the factory radiator fan controller. However, when using the A/C, the factory radiator fan controller might request to operate the fan at all times.


Install Manual


Who should install Grotto?


The Grotto radiator fan controller can be installed on any vehicle that uses a relay to power the cooling fan. There are many variations to radiator cooling fans installed by the factory. Typically there are 2 fans, a main and a secondary. When the main cooling fan is powered through a mechanical fan clutch, wire Grotto to the auxiliary fan. On applications with dual electric fans, Grotto can power either fans. Meanwhile on cars that have a single fan with dual fan speeds, Grotto could augment either settings. It’s important to analyze the vehicle wiring diagram, and determine where a more aggressive electric fan controller is best used.


The factory fan relay can remain connected and function as intended. For most applications, Grotto should be adjusted to turn on the fan before the factory cooling fan controller. However, running the A/C might request constant use of the radiator fan. The Grotto radiator fan controller has its own built in relay and fuse. It doesn’t interfere with the factory fan relay function. So either cooling fan modules can activate the radiator fan. Once activated, powering the fan from both modules won’t overpower or damage the fan.


The purpose of Grotto is a second layer of protection against engine overheating. It provides more aggressive use of the radiator cooling fan and helps recognize cooling system issues in time. This is especially useful in hot climates and for vehicles where the manufacturer set the secondary fan temperature too high. By turning the fan on earlier, the peak coolant temperature is reduced. Also, since the auxiliary fan is rarely activated, it’s more prone to be rusted, or seized. Use the fan more often with Grotto and recognize overheating before damage occurs.


Certain engines are more sensitive to elevated temperatures than others. These are engines made from dissimilar materials for the crankcase and the cylinder head. As these metals have a different rate of expansion, the headgasket, which is sandwiched and seals the two, can become distorted and torn. Also, the moving parts in the engine are designed to operate at a certain temperature. At higher temperatures, the clearance becomes tighter. This increases friction and accelerates wear. Applications such as racing, towing, and spirited driving demand elevated power output. As a side effect, the temperature increases.


Grotto can also be used in Hot Rods and custom builds. It is a self contained unit with its built-in relay and fuse. As long as there is a temperature sensor present, Grotto can be calibrated to nearly all 12 volt applications. Grotto is a standalone electric fan controller.


Fan Overrun Option


Fan Overrun is the option to operate the fan after the vehicle is turned off. After a vehicle is powered off, the last of the combustion heat remains trapped in the engine bay. This is called heat soak. At times, heat soak can raise the engine temperature as much as 25C after shut off. In some cases, this is enough to boil the coolant at stand still. It is harmful as it can damage engine components, but also render plastics brittle and overtime damage the hood paint. It can be a good idea to use the radiator fan to extract heat from the engine bay and reduce the temperature spike. However this is not always necessary, so this option is only active if the fan is already on, otherwise, the fan will remain off.

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