Radiator Fan Troubles

ejf012

Joined
Jan 14, 2026
Messages
27
Age
26
Location
Central NJ
Bike
93 Pan
Hi all,

For the past couple months I've been struggling with cooling problems on my 1100. I just recently fixed a significant leak, not at the coolant tubes under the carbs like lots of folks experience, but in the metal connector between them and the block. However, through all of that the core issue has remained that my radiator fan won't turn on. It repeatedly stays off despite my gauge reading middle, above middle, all the way into the red, even. I'm hesitant to keep trying solutions at this point because I'm working my engine so hot so often. My most recent attempts at diagnostics yielded the following information:

  • The wiring and fan itself is all good. When the fan switch connector is shorted to ground the fan turns on just fine.
  • The fan switch is good. When tested in a pot of boiling water my old (replaced a few months ago, $9 on Amazon) switch shorted itself after a while as it should, the exact same as an OEM example I bought thinking it would be the issue.
  • The radiator is grounded. The threads on the radiator test continuous with the negative terminal of my battery.
  • There is plenty of coolant. The level is in between the two lines on the reservoir towards the rear, and was over-full for a while.
  • Coolant should be getting to the switch. This one I'm less sure about. When I removed the fan switch, coolant came out into my drain pan. I also replaced the thermostat a while ago thinking it was the issue.
This leaves me pretty darn stumped. Everything seems to be okay but without fail the fan will not turn on and my bike overheats. Please, any ideas or tests I will gladly take! Help me!
 
Larry Fine said:
Sounds like low coolant level in the engine itself, not the overflow tank.

Check it when cool, and check the radiator-neck end of the overflow hose.
Click to expand...
Interesting. Last time I checked it, the thermostat housing and that whole area up there was dry. Should it be full up to the filler neck and the reservoir should be at the proper level? I was wondering how it ends up pulling coolant from the reservoir. My guess was the smaller line between the thermostat housing and the block pulling a vacuum?
 
One of the known problems with the 1100 is the radiator overflow hose splits and the coolant fills the overflow bottle and empties the radiator. The coolant is heated and flows to the overflow bottle. The split hose causes the radiator to loose the vacuum that pulls the coolant back into the radiator. You may not find coolant in the area or smell it boiling off the hot engine. If the overflow bottle is full, check the radiator at the radiator cap. If it's low, check the hose.

The overflow hose usually splits, on the underside, right at the point where it leaves the radiator. You may be able to cut the end off past the split and use the same hose. If it's the factory original hose or you don't know the age, you may want to replace it. I don't think there is a part number for the hose. The hose length is cut from a stock, bulk hose reel. Don't know about the non-ABS bikes, but the ABS II takes about 6-7 feet of hose for a total replacement.
 
ejf012 said:
Should it be full up to the filler neck and the reservoir should be at the proper level?
Click to expand...
Yes. That's what I meant by checking it when cool and safe to open. It works just like a car.

ejf012 said:
I was wondering how it ends up pulling coolant from the reservoir. My guess was the smaller line between the thermostat housing and the block pulling a vacuum?
Click to expand...
No, the entire cooling system is one closed loop. Heat pushes coolant out; vacuum draws it back in.

If the recovery bottle hose leaks, it'll pass coolant down to the bottle, but not draw it back up and in.
 
Ron said:
One of the known problems with the 1100 is the radiator overflow hose splits and the coolant fills the overflow bottle and empties the radiator. The coolant is heated and flows to the overflow bottle. The split hose causes the radiator to loose the vacuum that pulls the coolant back into the radiator. You may not find coolant in the area or smell it boiling off the hot engine. If the overflow bottle is full, check the radiator at the radiator cap. If it's low, check the hose.

The overflow hose usually splits, on the underside, right at the point where it leaves the radiator. You may be able to cut the end off past the split and use the same hose. If it's the factory original hose or you don't know the age, you may want to replace it. I don't think there is a part number for the hose. The hose length is cut from a stock, bulk hose reel. Don't know about the non-ABS bikes, but the ABS II takes about 6-7 feet of hose for a total replacement.
Click to expand...
That sounds very likely as to what it may be. I noticed while originally fixing my leak that hose bulging fairly significantly and did have it in my cart with the replacement hoses I bought, but foolishly took it out. It does have an OEM number and show up in the parts fiche. I'll grab some hose tomorrow after work and fill the radiator and give it a try, I hadn't thought of it killing vacuum by having just a tiny leak. 1781576072618.png
 
That certainly looks like the smoking (steaming?) gun right there. The bulge is just due to the flare on the spigot under the hose, but I can tell you that my 1990 had a tiny leak in very nearly that exact location. As Larry says, a pinhole there may dribble a little coolant out when the system heats but absolutely will prevent coolant returning from the reservoir, which means the radiator will be part-empty.
 
TerryS said:
As Larry says, a pinhole there may dribble a little coolant out when the system heats but absolutely will prevent coolant returning from the reservoir, which means the radiator will be part-empty.
Click to expand...
And that's because the bottom of the overflow bottle is several inches below the top of the radiator.
 
ejf012 said:
It repeatedly stays off despite my gauge reading middle, above middle, all the way into the red, even.
Click to expand...
Same symptoms on my (prev) '92 ST1100...
slight loss on coolant (leaking w/pump) exposing the temp probe to nothing but steam, hence temp gauge showing red, while the fan remained off as the fan/stat was still submerged thus nicely cool...

Giving the age of your ST (and what's showing in your pic) I'd give the whole cooling system a throughout check and probably replace hoses, elbow fittings + O-rings, rad/cap...
 
The radiator will not get filled from the coolant tank. Run the bike radiator cap off make sure the coolant is full. Assuming the rad was full then I would have a ground wire ready to ground the switch. Get the bike hot, ground the fan all while the bike is running. 1, There should be hot air coming from the radiator fan and the temp should start coming down. 2, If none of that happens the coolant is not circulating, hot coolant is not getting to the switch, stuck closed t'stat or water pump impeller. If the first part is true try running a separate ground to the radiator. You did state there was continuity from rad to ground that should of been checked with a ohm meter which sounds like you did.
 
Last edited:
When I bought my ST, it had a switch in the left side fairing, which activated the fan.
It can be seen on the enclosed picture, to the right of the 2 screws.
The previous owner may have experienced the same problem.
So I decided immediately to renew all hoses, elbows, O-rings, thermostat and radiator cap.
The bike has been running fine since 2023 (without using the fan switch).
 

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Al st1100 said:
You did state there was continuity from rad to ground that should of been checked with a ohm meter which sounds like you did.
Click to expand...
Wrong way to test. A high wattage test light is a better way test if a circuit is intact. An led test light won’t put enough load on a circuit nor will an ohm meter.
 
Looking at the pic in #6 there seems to be two small spots on the overflow hose that looks like small holes in the tubing. But that's a lot of old coolant around the area. A fair amount of TLC for coolant levels and hoses seems in order.
 
Hey all, glad to report that some of the solutions shared within worked! I replaced the hose between reservoir and radiator, and made sure to fill extra-well both the reservoir and radiator. With the confirmed working fan sensor in, the fan turned on A-OK on its own and I couldn't even get the temp needle to move above halfway while stationary and revving! I appreciate all of y'all's feedback. My cooling system is for sure pretty crusty: every time I take off a hose to replace it there's a decent amount of old coolant crust left behind on the fitting. Glad to report this should conclude a quite thorough refresh of everything but the water pump. Thank you for all the help!!
 
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