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How To Replace the Thermostat Fan Switch on a 1978 Honda Goldwing GL1000

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I am an IT professional from Cleveland, OH who enjoys electronics, vintage radio restoration, air guns, and working with model trains.


Replacing the thermostat fan switch on a 1978 Honda Goldwing GL1000 is easy with these simple instructions.

Just be sure that you have plenty of time to work on the project and a large outdoor workspace (or a well-lit one indoors).

Why Was My Cooling Fan Running All the Time?

One spring morning I was about to take my vintage 1978 Goldwing out for a ride. When I turned the ignition switch to the "Run" position, the cooling fan behind the radiator sprung to life. This was odd considering that the bike was stone cold and had not been ridden that day. While the cooling fan problem didn't seem critical, I knew that I would have to investigate the problem upon my return, after letting the bike cool for a while, because the cooling fan in the constantly running state was causing an extra load on my several-year-old battery.

Upon consulting my trusty Goldwing service/repair manual, I determined that the problem was the thermostat switch, which is an electrical switch that is inline between the battery power when the key is in the run position and the cooling fan. The thermostat switch is attached by the thermostat housing and senses the coolant temperature. When the coolant temperature reaches a preset temperature, the contacts in the coolant switch close, energizing the cooling fan.


Safety Warning

Before doing any electrical work on your motorcycle, remove the positive battery cable from the battery.


Remove the Connector

The thermostat switch is connected at the thermostat housing. I had to remove part of my Vetter fairing to get access to it. Carefully pull the connector boot from the thermostat switch. The connector boot and thermostat switch are circled in the picture above.


Closer View of the Thermostat Switch

Here is a closer look at the thermostat switch. The thermostat switch is circled in the picture above.


Double-Checking Using a Multimeter

Connect a digital multimeter across the two terminals on the back of the thermostat switch. Set the meter to read resistance in Ohms. You should see a really high resistance on the multimeter. If you see a really low resistance, as shown in the picture, there is a problem with the thermostat switch. The resistance should only be low when the motorcycle is above operating temperature and the fan needs to run to bring the motorcycle back to operating temperature.

OK, so we verified that the thermostat switch needs to be replaced.

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Drain the Coolant

Before removing the thermostat switch you must drain the coolant in the radiator. The radiator drain bolt is at the base of the water pump right under the radiator.


Remove the Radiator Cap

While the radiator is draining, remove the radiator cap, so as to prevent a vacuum buildup that could slow down the draining of the coolant.


Clean the Coolant Reserve Tank

Now that you have the coolant drained from the radiator it might be a good time to rinse out the coolant reserve tank with water. The coolant reserve tank is the white container in the picture. It is held in place by one bolt toward the top of the tank. Once the bolt is removed, simply pull up to remove the coolant reservoir from the motorcycle. To re-install the reservoir, follow these steps in reverse.


Remove and Replace the Thermostat Switch

Use a large socket attached to the ratchet to unbolt the thermostat switch from the thermostat housing. Compare the old thermostat switch with the new one to make sure they are identical. If they are, place a small tab of RTV on the threads of the new thermostat switch and hand-thread it into the thermostat housing until it is snug. Hand threading will prevent you from accidentally stripping the thread of the thermostat housing. Give the thermostat switch about a quarter turn with the ratchet to snug it up.


Here is a picture of the new thermostat switch in place.

Refill the Coolant

Time to refill the coolant. Make sure you replace the radiator drain bolt that is at the base of the water pump before adding coolant. I recommend using new coolant, but if you drained your used coolant into a clean container, you can re-use it in the motorcycle. Place a funnel where the radiator cap goes and slowly pour the coolant into the radiator. After the radiator is full up to the radiator neck, let it sit for a couple minutes to allow any trapped air in the radiator to escape, then top off the radiator to the right coolant level. Also fill the coolant reservoir to the right level.


Reattach the Connector Boot

Attach the connector boot to the new thermostat switch, making sure it is firmly in place.


Reconnect the Battery

Don’t forget to reconnect the battery cable to the positive terminal of the battery, otherwise you will be going nowhere fast.


Road Test

Start up your motorcycle in a well-ventilated area and check for coolant leaks. You will notice that the cooling fan does not turn on when the ignition switch is in the run position; this is a good thing. If no coolant leaks, take it out for a test drive, but don't go too far in case your motorcycle experiences problems. Allow the bike to reach normal operating temperature. At some point the cooling fan should kick on when the coolant exceeds normal operating temperature. Make sure that the cooling fan shuts off when you are done with your ride and you put the ignition switch in the off position.


Armed with your trusty Goldwing repair manual, you too can fix common electrical problems, such as a faulty thermostat fan switch, with just a little mechanical and electrical know-how.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

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