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Motorcycle Repair/1984 Honda Ascot VT500FT Thermostat


Hi. I got my hands on a 1984 Honda Ascot VT500FT. Originally it looked to be in pretty poor shape. I had no way of knowing whether it would ever run again.. Got it home and ordered a battery.. Put the battery in, checked the engine for oil, and cranked it over.. The front cylinder was seized.. Perfect.. Long story short, I got the cylinder freed up, had to do lots of other repairs, and the bike runs.. I have not ridden it yet, but I cant wait. Here's my issue.. It is overheating.. The fan is not coming on.. so after 10 minutes of idling, it got so hot that the boiling antifreeze pushed its way into the overflow reservoir with so much pressure that it blew the lid off the reservoir itself.. never seen that before. I immediately started scouring the Clymer to better understand the coolant system.. It helped a little.. The radiator has a 2 wire fan and a 2 wire thermostat switch.. Both of these are located at the base of the radiator. The thermostat is located in the top of the frame just under/behind the head.. Both the thermostat switch and the fan seem to be wired correctly. Clymer says to remove an electrical connector from the thermostat itself.. There is no trace of a wire for that spot.. the wiring diagram does not show a wire there.. What do I do? Where is the wire supposed to connect to? Where does it run from? Please help in any way you can. Thank you. Rian

Hi Rian,

You can unplug the thermo switch on the radiator
and connect the two wires together.
If the fan circuits are okay the fan will run.

If not, try checking if there is battery power on the wires leading to
the thermo switch and fan. I think postive power might be on a black
wire and black/blue is negative.

If the fan runs with the thermo switch wires connected
then the thermo switch may be faulty.

If the fan won't run try checking for power as above or
try connecting a battery directly to the fan to see if it works then.

The thermo switch turns the fan on between 176 and 216 degrees
farenheit. You can test the thermo switch in some hot water
to see if it has continuity at those temperatures.

If the fan works or works when you bypass the thermo switch
then just check the thermo switch.

If the fan works with a battery then check your fuses
and check if power is getting down to the thermo
switch and fan area.

Once the fan is working then you still have to make sure
the radiator cap is holding pressure.
Also you may have to run it a bit with the cap off
to get any air out of the coolant system.
Even rock the bike a bit to get the air out.
Top it up with coolant until it stays full.

Use 50/50 coolant and distilled water or
premixed coolant.

If it still has excess pressure check your oil
for coolant and don't run it if you see any
coolant in the oil.
Change the oil and get some Rislone head gasket
sealer since it could have a leaking head gasket.

Hopefully you can get it going to give it a good run.
The wire on the thermostat is just for a temperature
light or gauge so you don't really need it if the sensor
is missing or removed.

Wayne S

Motorcycle Repair

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Wayne S.


Licensed Motorcycle Mechanic, Knowledge of motorcycles from 1960 up, Japanese, British and most other brand motorcycle repairs.


Worked for Yamaha, Honda, Suzuki, Kawasaki British: Triumph, Norton, BSA Other: Most Scooters and Mo-Peds

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