ST1100 Cooling fan

pathfinder7377

Joined
Jun 7, 2020
Messages
50
Age
66
Location
Ohio
Bike
1999 ST1100
My cooling fan on the radiator is not coming on during engine operation. I have done some research and it operates normally if I disconnect the wire from the thermostatic switch / sensor on the radiator and ground it, so I know that the fan motor itself is operational and receiving power.

I have replaced the thermostatic switch with no change.

I also confirmed that the body of the thermostatic switch has continuity to the radiator and to battery ground.

Any suggestions as to the next step will be appreciated.
 
Just off the top of my head is the radiator full or do you have an air pocket in it? Got to have enough coolant so sensor can read its temp. Also fan should come on a little pass halfway mark. Also did you do any painting of the radiator or frame where it mounts?
 
Let's first confirm that the engine is actually getting hot enough for the fan to come on. As red one says, it won't come on until the temperature gauge reads a little to the right of vertical. If your gauge is getting higher, or near the red line and still no fan, it could also be a faulty temperature sensor, located in the thermostat housing, giving you a false reading on the gauge.
 
red one said:
Just off the top of my head is the radiator full or do you have an air pocket in it? Got to have enough coolant so sensor can read its temp. Also fan should come on a little pass halfway mark. Also did you do any painting of the radiator or frame where it mounts?
Click to expand...
Radiator is full. Temp needle definitely past halfway. No painting, disassembly or other disturbance of radiator, frame or mounts.
 
Bush said:
Let's first confirm that the engine is actually getting hot enough for the fan to come on. As red one says, it won't come on until the temperature gauge reads a little to the right of vertical. If your gauge is getting higher, or near the red line and still no fan, it could also be a faulty temperature sensor, located in the thermostat housing, giving you a false reading on the gauge.
Click to expand...
Temp needle well past halfway.
 
Could be a faulty temperature sending unit in the thermostat then, giving you a false reading. Check the wire connection on that unit is clean and secure.
 
Normally, from a cold start, idling in place, it could take up to 10 minutes for the engine to get hot enough for the fan to come on. Out on the road, unless stopped in traffic, it may never come on. Are you seeing the needle actually reach the red line?
 
Bush said:
Normally, from a cold start, idling in place, it could take up to 10 minutes for the engine to get hot enough for the fan to come on. Out on the road, unless stopped in traffic, it may never come on. Are you seeing the needle actually reach the red line?
Click to expand...
Yes, temp needle approaches the far right side of the gauge when stopped in traffic.
 
pathfinder7377 said:
Yes, temp needle approaches the far right side of the gauge when stopped in traffic.
Click to expand...
So you're possible low on coolant (probe not submerged, measuring steam) causing lack of coolant flow over the rad & fan/stat...
Locate & eliminate leak, top off coolant at radiator, test again...
 
Troubleshooting is a process of elimination. Are you sure the thermal switch body is electrically grounded? I would use a 12v test light (not a voltmeter) with the wire clip connected to +12v to make sure it is.

Then, with everything connected, touch a jumper wire from the wire terminal on the thermal switch to the body of the switch, not a different ground point. If the fan runs then, the thermal switch must be bad.

Lastly, I would test the thermal switch by using the test light with the wire on +12v to see if the switch does indeed connect the wire terminal to ground when the engine gets hot enough that the fan should start.
 
If you are low on coolant, one likely source may be the hose going from the thermostat housing to the overflow bottle. The hose will crack a few inches from the housing where it makes a sharp turn to go down to the bottle. If not low on coolant, and since your fan works, sounds like a sensor or (looks like you have a new sensor) wiring problem to me.
 
If grounding the wire makes the fan operate, the circuit is fine - only two things can cause the fan not to operate. Either the new sensor switch is bad or calibrated to the wrong temperature or the coolant level is too low. I saw a weird problem once on a new fan switch in a vehicle and it was caused by the person who installed it used a lot of teflon tape on the switch and this isolated the switch from gounding so it could not work. Don't use teflon tape on these switches, it has a gasket.
Another thing to try may be making sure the ground to the radiator is good. If the radiator is rubber mounted, maybe it is not grounded correctly so the switch when closed can't complete the circuit.
 
I'm thinking some of the guys above are onto something. The radiator is in fact rubber mounted. I have one out right now, doing a timing belt, idler and tensioner. While it's out, I cleaned up the ground eyelet terminal located at the top right right fan mounting bolt. It was green, so I wire brushed and pasted everything before I put it back in.

This cheesy little eyelet terminal is the only thing that grounds the radiator. There is a black 2 pin plug just above the left camshaft cover. This is power and ground for the fan. At the fan harness, right next to the fan motor, the factory ground is spliced and the dinky 20G wire goes to the eyelet terminal. Have a good look in there. Potentially, this could be a problem for you...
 
pathfinder7377 said:
I also confirmed that the body of the thermostatic switch has continuity to the radiator and to battery ground.
Click to expand...
simple continuity isn't sufficient, what's the actual resistance value??

If you have too much resistance in that path, it won't be able to carry the current required to turn the fan. If the measured resistance is more than a few tenths of an ohm, then that's the first place I'd be looking.
 
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