1985-1988 Honda CH250 is a liquid-cooled four-stroke 249-cc engine. The engine
is primarily cooled by a radiator in which heat from the engine is carried
to the radiator by antifreeze/coolant. Air passing over the radiator coils
removes the heat from the engine. A radiator fan assists cooling at high
The switch on the radiator is the thermostatic fan control switch. When the
coolant gets hot enough, the fan comes on. To check its operation, remove fan
plug from radiator switch and jumper wire the plug with the ignition on and the
fan should run. That will tell you if your fan is good. The thermostatic fan
switch has continuity at 208 degrees which means the fan comes on at that
Your thermo sensor housing is on the throttle side of the bike just above the oil
drain plug. The thermo sensor, which sends information to your temperature gauge,
has a single wire plug attached. To check your temperature gauge operation,
remove wire and ground it with the ignition on. Your temperature gauge will move
all the way to hot. That will tell you if your temperature gauge is
The thermo sensor is a switch which reads the temperature of the coolant at the
engine. The thermo sensor is checked by removing it from the engine and placing
it in hot oil and reading the temperatures.
Your thermostat is in the housing below the thermo sensor. Remove it for checking
and replacement. Be advised that the thermostat begins to open at 208 degrees,
which is considerable. It can be very hot and the fan will not always be
Honda’s Service Manual recommends that coolant should be changed every
years. As coolant ages, it loses its ability to resist boiling and conduct heat.
But, more importantly, it also loses it anti-corrosion properties, and this will
allow the build up of scale and residue in the cooling system.
This build up will reduce the cooling system's ability to do its job and could
eventually render the system useless. To prevent this oxidation of the aluminum
cooling passages, you should replace your engine coolant at least once every 2
Motorcycle Consumer News investigated a rash of water pump seal failures in
Honda Gold Wings and concluded the cause was from silicates found in regular
automotive antifreeze. Honda recommends using their own green coolant which is
free of harmful silicates.
Conventional automotive antifreeze contain silicates (sand) which scrubs the
radiator core to keep them flowing and free from obstruction. Automotive water
pumps contain seals which can withstand the silicates. Honda’s motorcycle and
scooter water pumps do not have that luxury. Be certain to use Honda motorcycle
coolant or automotive coolant equivalent that is silicate free! All coolant
should be mixed at a 50-50% ratio of coolant
and distilled water.
Tap water, which varies dramatically in areas, contains ionic species of
calcium, magnesium and chlorine. If there are any significant ions present, you
can generate bimetallic corrosion within the metallic components of the
radiator. Distilled water is a poor ionic conductor (ions removed), as is the
components in the antifreeze. So bimetallic corrosion is minimized (weaker
electrolyte). Most antifreeze will have additives in it to reduce the corrosion.
If you flush the coolant at the recommended intervals (even with tap water), you
are unlikely to have any problems.
The coolant check procedure should be done with a cold engine! Place the
scooter on its center stand. To remove the front cover lid exposing the radiator
cap, use a Phillips screwdriver to back off the cover screw while using your
fingernail or other small pry underneath the screw lip. Slide the cover up
toward the headlight and remove it. Start the engine. Check the radiator coolant
level in the reserve overflow tank next to the radiator. At normal operating
temperature, the level of the coolant in the recovery tank should be between the
“upper” and “lower” line marks.
The coolant change procedure should also be done with a cold engine. Place the
scooter on its center stand. Unscrew the trim clip screw and slide the front
cover lid toward the headlight and remove. Loosen but do not remove the radiator
cap. Place a drain pan under the water pump and remove the drain bolt. The drain
bolt is the 10mm bolt with the aluminum washer nearest the bottom of the pump
cover. Coolant will exit the drain bolt hole slowly. By unscrewing and slightly
lifting the radiator cap, coolant discharge control can be maintained.
To flush the system, replace the water pump drain bolt. Fill the radiator with
water and start the scooter. Run scooter until warm. Stop the scooter. Remove
drain bolt and release water. Replace drain bolt.
To refill the radiator, Fill the system through the radiator cap hole with a
50-50% mixture of distilled water and ethylene glycol coolant. Do not use
alcohol based coolants. Start the engine and run until there are no air bubbles
in the coolant, and the level stabilizes. Stop the engine and add coolant to the
proper level if necessary. Reinstall the radiator cap. Check the level in the
reserve tank and fill to the proper level if necessary.
Take the scooter for a ride to get it up to normal operating level. Check for
Note: Dispose of the coolant in a safe manner. Ethylene glycol tastes sweet but
is very deadly to animals which may come along and drink it if you drain it onto
the ground. It is equally toxic to children which are attracted to its lovely
Randy Pozzi (Rev. 08/2006)