Radiator fan doesn't work

STraymond · Jul 20, 2016

John OoSTerhuis said:
Yes, check the coolant level at the radiator fill cap (yes, some Tupperware to remove, you own a fully faired motorcycle, get on with it). Top off with coolant if necessary. The next step is to run the bike until the gage gets to vertical or a bit more and listen-to/watch-for the fan to run. Report back.

+1 on checking and topping off the radiator fluid. My bike just started overheating. The fan was not coming on while the engine was running. The fan would come on when using the kill switch to turn off in the hot mode. Also, the coolant reserve tank level would go from low when the engine was cool to max when the engine was warm/hot - so it was acting normally. After waiting for the engine to cool, the coolant level in the radiator was well below the top of the neck. Topping off the radiator and retesting solved the problem. The fan now comes on at the appropriate time and maintains the temperature within a narrow band.

ST1100Y · Jul 20, 2016

STraymond said:
After waiting for the engine to cool, the coolant level in the radiator was well below the top of the neck.

Check the overflow hose for leaks and the rad cap for cracked seals; the system obviously draws air when cooling off/contracting...

STraymond · Jul 20, 2016

ST1100Y said:
Check the overflow hose for leaks and the rad cap for cracked seals; the system obviously draws air when cooling off/contracting...

Will do. Thanks!

Aulia · Jun 28, 2017

dwalby said:
the fan won't come on at exactly vertical, it has to go a bit past vertical. You never did say if the gauge keeps climbing well past vertical, should we assume that it does, and the fan still doesn't come on?

If so, it sounds like you need a new switch, but $61 sounds way too high, $36 is more like it.

Does anyone have a part number for a generic temp sensor switch? I've been using a manual over-ride toggle switch for several years. Might be nice to have it function. Currently temp gauge is pinned to the red zone before fan comes on by itself.
TIA

dwalby · Jun 28, 2017

Aulia said:
Does anyone have a part number for a generic temp sensor switch? I've been using a manual over-ride toggle switch for several years. Might be nice to have it function. Currently temp gauge is pinned to the red zone before fan comes on by itself.
TIA

Don't know about generic part, but for cross referencing the Honda part is 37760-MR1-003

So I'm guessing from your wording that you've already made sure everything else in the cooling system is in proper working order, and your thermo switch is faulty?

Oldbikefixr · Jun 28, 2017

bikerdrumr said:
The rad had plenty of coolant and the system had no air in it when I went to burp it. However, the overflow reservoir was dry. I added fluid up to the specified level. There is no evidence anywhere of leaking. When the bike's gauge goes to vertical, the fan doesn't turn on. At this point, when I flush the system in a couple weeks I think I'm just going to spring for a new fan switch. The temp sensor is only $8 so I might change that as well. Didn't someone say that you should change your radiator cap every several years or so? This bike is 17yo and has 72k miles. Am I due?

Unfortunately, I don't have time to sort all of this out at once. So every chance I get to work on it, I do.

Check the condition of the tube that runs from the thermostat housing back to the overflow reservoir. I suspect it is cracked on the forward end or plugged...otherwise there should be some fluid in the reservoir.

Radiator fan doesn't work

STraymond

ST1100Y

STraymond

Aulia

dwalby

Oldbikefixr