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Discussion Starter #1
I have (as mentioned) a 98 ST2 which is new to me. My other bike is an R100RS and my other OTHER bike is a Honda ST1300 for the bride and I to ride on. And I like side cases!
Anyway, I wonder how the ST2 thermostat is supposed to behave. The bike, on a cool day, will slowly warm to 150/160 and stay around there as long as I am moving OK. I would think that is should get to 180 or so rapidly and stay between that and 220 or so.
I wonder if my weird looking thermostat is stuck open, and what that implies RE: gas mileage etc.
I have MANY questions and this is the first.
Thanks
Jim in CT
 

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I have a 03 ST4s and it runs about that on a cool day down the highway 160 to 170 or so. Hot day on highway 180 to 190 and in town will easily go up to 210 and past until the fan turned on.
Installed an auxiliary fan switch from Desmotimes to keep her cool. Don't like going over 200 F myself. They're prone to getting hot riding around in town.


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The thermostat should open around 150 degrees or so. Start the bike cold and feel the radiator. It should stay ambient temp, feel cool, until around 150. At that time you should feel it get warm / hot pretty quickly. You can feel the heat move across the rad from one site tot he other as it is happening. If that is what you find, your thermostat is good to go. If it starts building heat right away, yeah, maybe it's stuck open. Fan is supposed to kick on around 220. My temps are about the same as yours when moving on the open road. Traffic, in town or at a spot light it comes up pretty fast, but also goes back down pretty fast once moving again.
 

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Discussion Starter #4
I guess it's not a very efficient cooling system. Not used to thermostats opening so early and fans going on so late. So OK..she's acting normally. Not sure I can easily get a hand on the radiator inlet or outlet. But I haven't tried yet.
Thanks guys. Moving on.
Jim
 

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Well yes, guess that's true it's not that great. But in defense of this great all around motorcycle, It wasn't built to cruise around downtown but for the open road. I've learned to live with it.

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Discussion Starter #6
Pacabada, a great "all around" motorcycle SHOULD be built to cruise around downtown. And highway, and twisties etc.
Actually I am not saying it isn't. Just that the cooling system could be dialed in a bit better. Looks like it doesn't matter though.
But for instance, my lard ass honda ECU would think it was not warmed up yet at 170 degrees and it would be running very rich.
I'm just getting to know the thing. And I think it's fantastic! Got my new plate today!
 

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If you search in here, there are many that have added a "fan switch" to manually turn on the fan pre-maturely, when in town.

The ECU controls the fan. If you find the correct pin, you can switch that wire to ground (the same thing the ECU does), and it will energize the fan relay (which is under the seat). The other relay next to that one is for the FI pump+injectors+coils power.

And they can be swapped if you ever need to trouble-shoot,,,
 

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Discussion Starter #8
If you search in here, there are many that have added a "fan switch" to manually turn on the fan pre-maturely, when in town.

The ECU controls the fan. If you find the correct pin, you can switch that wire to ground (the same thing the ECU does), and it will energize the fan relay (which is under the seat). The other relay next to that one is for the FI pump+injectors+coils power.

And they can be swapped if you ever need to trouble-shoot,,,
I have yet to definitely hear the fan turn on, BUT on my test ride which wasn't very long and not very hot, I THINK the fan turned on at my return to the guys garage. I paid no attention. I must bring it in warm and let it heat up till the fan turns on to be sure.
I had hooked up a manual switch on my 60 something Fiat 124 about 30 or 40 years ago. Must be an Italian thing.
In my mind the thermostat should open at 180 and fan on at 220 and off at 200.
I wonder what temp the ECU thinks it needs no extra fuel. Must be low. 150 or so.
I have a complete spare wire harness with all the electrical bit's attached (except ECU). I reckon I can find the right wire.
 

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The OP was questioning why the bike runs so cool given that it's got a thermostat. A thermostat's job is to maintain a MINIMUM operating temp. I've never read an adequate answer as to how these bikes run so cool.
 
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2 out of 3 isn't bad.

Enjoy the ride!

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Discussion Starter #11
SO, I ran the bike in the garage while I did other things. It warmed up to 237 degrees and I pussied out. I neglected to have a plan to cool it down if it got too hot waiting for the fan to turn on.
I think this is not normal. Is this normal? Seems like the fan should have turned on.
Educate me.
 

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Your fan should have kicked on between 210 and 220. Either your sending unit is bad, fan or a connection. There's a stickie on equivalent parts for the ST's under the Sport Touring forum if your sending unit is bad.

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That is too hot.

The ECU turns on the fan through a relay. The relay is powered through a 7.5A fuse that is not in the fuse box. I believe it is under the seat on that bike. I'd look at the relay and the fuse first. Fan too, I guess.

Another possibility is the temp sensors. There are two on the ST. One for the ECU and one just for the temp display. These temp sensors have been known to fail. If the ECU's temp sensor was off, I'd think you have some running issues with the bike as the fueling would also be effected. A faulty temp gauge sensor would just give an erroneous reading at the display. Easy to verify if you have one of those infrared thermometers handy, and if not they are around $20.00 at Home Depot, Lowe's, Harbor Freight etc.
 

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Discussion Starter #14
That is too hot.

The ECU turns on the fan through a relay. The relay is powered through a 7.5A fuse that is not in the fuse box. I believe it is under the seat on that bike. I'd look at the relay and the fuse first. Fan too, I guess.

Another possibility is the temp sensors. There are two on the ST. One for the ECU and one just for the temp display. These temp sensors have been known to fail. If the ECU's temp sensor was off, I'd think you have some running issues with the bike as the fueling would also be effected. A faulty temp gauge sensor would just give an erroneous reading at the display. Easy to verify if you have one of those infrared thermometers handy, and if not they are around $20.00 at Home Depot, Lowe's, Harbor Freight etc.
Without yet having enough time to dig into it, i checked the under seat fuses. All fine. The fan blades do spin by hand. I saw the coolant temp sensor on the forward cylinder. It exists. I wonder if it's the original. Has an aluminum washer as opposed to a copper one. I also found a hose dangling there with a bolt plugging it up. I pulled it out a bit for a photo op.
I also found under the tank a small diameter hose missing from the connection and no where in sight. Yes the big hose pops off when the tank is lifted and I pop it back on when lowering.
I would like to replace all the little bits that are inexpensive enough just for fun. The nappa bits are still NAPA TS5520? I will grab two. They are cheap enough. Where does the french relay live? I would to at least check it.
See the pics if I manage to attach then
Thanks Guys!

 

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The plugged hose is likely the result of a charcoal canister removal. The hose probably leads to the intake manifold. I guess it was easier to plug the hose than use the easily available screws at the actual manifold to plug the hole. Maybe it was the prior owner's fix for an easy way to connect throttle balance sticks?

The open nipple under the tank is your fuel pressure regulator. That needs to stay open.

Not sure on your temp sensors regarding washer type. You may be right, I think OEM sensors have copper washers. Not sure what kind of washers the NAPA sensors have.

I have an ST4s, which is not the same as your ST2 in regards to wiring, ECU and relay location. I'd be guessing there. An ST2 owner can answer that.
 

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I agree the cooling system does not seem the best design. I think the main issue is having the thermostat in the bottom radiator hose instead of the more usual top hose. Coolant cannot circulate through the radiator until the thermostat opens but the hot water can mix with cold radiator water which makes it harder to maintain temp when moving at hwy speeds on coolish days.
 

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Discussion Starter #17
The plugged hose is likely the result of a charcoal canister removal. The hose probably leads to the intake manifold. I guess it was easier to plug the hose than use the easily available screws at the actual manifold to plug the hole. Maybe it was the prior owner's fix for an easy way to connect throttle balance sticks?

The open nipple under the tank is your fuel pressure regulator. That needs to stay open.

Not sure on your temp sensors regarding washer type. You may be right, I think OEM sensors have copper washers. Not sure what kind of washers the NAPA sensors have.

I have an ST4s, which is not the same as your ST2 in regards to wiring, ECU and relay location. I'd be guessing there. An ST2 owner can answer that.
Dunno about that plugged hose. I will sort that next time the tupperware is off. My GUESS is the OEM temp sensor has the aluminum washer because the Nappa sensor pictures seem to show a copper gasket. I know none of this for sure though.
Thanks for helping me sort this stuff.
 

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The OEM senders are Jaeger, which is French. And they are crap. I bet if you search for Jaeger in here, you'll find my old posts. Sounds like the one feeding the ECU is scewed.

So, a 220 ohm resistor inserted in place of a sender should make your readout read 207 F. (+ or -, depending on tolerance!) This is the H cyl. sender. The Vert. sender feeds the ECU. If you insert the 220 and another 500 Ohm on parallel, it should make the ECU turn the fan on... (about 150 Ohms if ya wanna use 1 resistor).

Note that the NAPA, Bosch, Jaeger, etc. all use the same temp vs. resistance curves. And they are NTCs. Hotter = lower Ohms
 

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Discussion Starter #19
The OEM senders are Jaeger, which is French. And they are crap. I bet if you search for Jaeger in here, you'll find my old posts. Sounds like the one feeding the ECU is scewed.

So, a 220 ohm resistor inserted in place of a sender should make your readout read 207 F. (+ or -, depending on tolerance!) This is the H cyl. sender. The Vert. sender feeds the ECU. If you insert the 220 and another 500 Ohm on parallel, it should make the ECU turn the fan on... (about 150 Ohms if ya wanna use 1 resistor).

Note that the NAPA, Bosch, Jaeger, etc. all use the same temp vs. resistance curves. And they are NTCs. Hotter = lower Ohms
I purchased the nappa TS5520 sensors today. This is what you now recieve. Red box from Taiwan and Blue box from China.
So you are saying that if I insert a 150ohm resistor into the connector and turn the key on the fan should go on?
like one of these? This is not my area of ANY expertise.
https://www.amazon.com/Projects-Resistors-Watt-Choose-Quantity/dp/B072F77NL5
 

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Discussion Starter #20
It probably would have been best to use two sensors from the same supplier BUT I ran the bike this AM with the new sensors. I warmed it riding then I pulled over and let her tick over at a fast isle. The fan turned on (halafrikkinlooya) at an indicated 200 and off at indicated 190. I got back in the garage and my cheap Chinese point and shoot thermometer agreed perfectly with my cheap Chinese sensors. I am very much fine with that, and thanks you all for your valuable input!
 
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