Thermal Switch Retrofits Car Horn

We recently had a customer contact us whose car horn was going off in the middle of the night caused by below-freezing temperatures. The horn sounds constantly under cold conditions, estimated below 25° F, until the fuse blows.  That fuse is rated at 15 amps and can run for many minutes, depending on the current draw of that particular vehicle.  The problem exists in the horn-air bag-assembly which cannot be disassembled and repaired.  A new air bag assembly may work but the problem is inherent in the design of the molded assembly, so replacement of the bag does not guarantee a fix. Instead of risking the new airbag assembly, integrate a bimetal thermal switch into the retrofits, turning off the car horn at temperatures slightly above the estimated malfunction threshold of around freezing or 32°F. For more information about our snap disc thermostats, click here.

The switch found to work under these conditions was 2511F008-544. To purchase, contact us.   The horn will now work 100% of the time under warm outside weather and will only be out of commission for a short time until temperature in passenger compartment rises above the temperature setting of the installed thermostat switch- in this case around 55°F. Our temperature switches work by using a bimetal disc. The bimetal disc in the switch reacts to heat, and opens or closes a set of contacts based on what the ambient temperature is. This is works because the different metals in the disc react to heat in different ways, which causes the disc to bend.

Note: Air bags are dangerous and should not be handled by anyone not familiar with their operation or how to disarm them safely.

Thermal Switch Retrofits Car Horn

Installing Thermostat Switch in air bag assembly

Removal tool and backside view of steering wheel.

The thermostat switch is installed under the air bag. To disassemble the air bag, remove the negative battery terminal and wait a minimum of 10 minutes to allow reserve energy supply to be depleted.  The paint can- bottle opener tool below was bent as shown so as to release the air bag retention wires or leaf springs located on the back side of the steering column (see mirror view).  The leaf springs have to be moved outward, or away from the center, of the steering column.  At the same time, pull on the bag and a slight movement in the bag will show that one of the four springs have been released.  After all four of these are release the bag will come loose.

There will be red and black wires for the horn switch showing along with the yellow wires for the air bag.  The black wire terminal shown below is the ground for the horn.  The thermostat switch will be installed in this line, keeping the horn from working when the temperature drops below the switch setting.   The second photo is showing the thermostat switch with white terminals attached for reference only.  The thermostat switch was rotated in installation to allow room below air bag assembly.

Wiring thermostat switch into steering wheel

As stated above if you do not feel comfortable working with air bags please get someone that is familiar and can safely remove the bag to install the switch.

Thermostat Switch Installed

Temperature switch in final position, nested with connection plug for cruise & radio controls.

In this case the temperature range (Close at 55°F, Open at 45°F) was determined through testing to be the appropriate range for a customer’s application.  In some cases this same problem may be best solve with a different range; perhaps shifting 5°-10° up or down.  Senasys will be happy to work with you to find the best temperature solution for your particular environment.

Senasys does not endorse tampering with safety systems.  Do so at your own risk- and please don’t hurt yourself.

Contact our technical support today!


The superior controls manufactured by Senasys have a history of solid performance in many industries. From 22.5 mm selectors, to the four quadrant lighted feedback CMC controls, almost any switching need can be, and has been utilized in a variety of applications. Select a group below for more information. When you have found a switch that fits your needs, simply fax or email your specs to us and let our highly trained engineers find the most efficient configuration for your applications.

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Senasys products are designed & manufactured in Eau Claire, Wisconsin, U.S.A.