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Discussion Starter · #1 ·
My '02 ST4s is getting it's annual in my workshop and i noticed the temperature was flashing 248'f after a short time at idle, I used my infra-red pyrometer and shot both temp sensors to get a reading of 195'f.
I'm thinking the instrument cluster is "going south".I know the fan works because I have an aux. switch.
Has anybody tried to fix a cluster or know a remedy for this?
 

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Your point and shoot pyrometer measures surface temperature. It's not a good indication of the actual coolant temperature. That being said, the temp senders have been known to go south quite frequently. One of the sender sends a temp reading to the gauge, the other to the ECU. You can swap them and see if the reading changes. There's a aftermarket replacement (see the "alternative parts" sticky) one of which is from NAPA TS5520. It's about $20 dollars. I would suspect the sender before the instrument cluster.
 

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Consider yourself lucky that the readout sender is skewed. My ST2 was the other way around, where the fan wasn't coming on until the temp was actually OVER 248F.
And good move with the opt. pyrometer to help verify reality.

The French made Jaeger brand senders are crapolla. Use the Bosch or NAPA replacements as Mike L suggested.

If you'd like to check your readout for accuracy, a 220 Ohm resistor inserted into the connector in place of the sender (on the H cylinder) should force the reading to be 207F.

The H sender is for the readout, the V sender is for the ECU.
 
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Discussion Starter · #4 ·
Stick, I replaced the Jaeger junk several years ago with Bosch units.

With the disappearance of Radio Shack I don't have easy access to resistors, transistors or diodes beside Amazon or E-bay.

I'll find a resistor and check it out.

But first I'll check the resistance of both sensors for diagnostic principles.

Ride On
Dave
 

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Check if the temp changes when you turn on Electrics, if so could be Earth issues or the terminals need cleaning
 

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Stick, I replaced the Jaeger junk several years ago with Bosch units.

With the disappearance of Radio Shack I don't have easy access to resistors, transistors or diodes beside Amazon or E-bay.

I'll find a resistor and check it out.

But first I'll check the resistance of both sensors for diagnostic principles.

Ride On
Dave
If you google the Bosch replacement, the IR curves are available.
And don't be surprised if/when you look at the senders, that the readings sometimes go whacky - like open circuit, you'd think there's something wrong with UR test leads....

Nope, just VERY flakey senders!

This usually happens if/when you heat them. I had mine on the kitchen stove. Room temp to 212 F. What POS's they are!
 

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Stick, I replaced the Jaeger junk several years ago with Bosch units.
So you're saying that the newer Bosch replacement is showing the high reading? I guess anything can "go bad" ...

As Johnny mentioned it "could" be a lousy connection in the readout. They've been known to be flaky on the older machines.

Note that for a hi reading, the resistance needs to be pretty low. like less than 100 Ohms (estimate). They start out very high - several thousands of OHMs when cold. So maybe a short in the harness could produce this.

Have U checked your "big connector" for green-corrosion?
It's the one on the left, below UR fuse area, just above the radiator.
 

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The H sender is for the readout, the V sender is for the ECU.
Is there any remote possibility of mixed water sensor wire connectors? Hz wire connector plugged to the Vert and Vert connector plugged to Hz?

If so, easy to check: Just pull the Hz cylinder connector to see if the temp display goes blank. If it does go blank, then connectors are where they should be. If not and it continues to show temp, then bas ackwards.

S
 

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Discussion Starter · #9 ·
If you'd like to check your readout for accuracy, a 220 Ohm resistor inserted into the connector in place of the sender (on the H cylinder) should force the reading to be 207F.

The H sender is for the readout, the V sender is for the ECU.
Well I found a local source for a variety of resistors ($0.25 each) to check the accuracy of the readout and installed them one at a time into the H sender the one in the front and found it did not register 207' @ 220 ohms as I was told.

What I did find was that when said resistor was installed to the V sensor the one in the rear the readout went to 210'.

I went back and re-read this post to double/triple check and my 2002 ST4s is different than all of yours, bass ackwards!

I'm the second owner, did most of the maintenance for the PO and know the complete history of this little tart. The engine has never been out or the wiring harness fooled with except by me. But on this machine the vertical sensor controls the dash readout and the horizontal sensor controls the fan.

I replaced the vertical Bosch sensor with a Standard unit $15 US and all is well.

I'm thinking of leaving out the grey colored panel on the left side panel to let more air flow through the body work. Any thoughts on this?
Ride On(cooler this time)
Dave
 

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Great that you got it sorted! I don't think you'll see any benefit from removing the side panel insert. That being said, now that you have a more accurate temperature you can monitor the coolant temp with and with out the side panel.
 

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Well I found a local source for a variety of resistors ($0.25 each) to check the accuracy of the readout and installed them one at a time into the H sender the one in the front and found it did not register 207' @ 220 ohms as I was told.

What I did find was that when said resistor was installed to the V sensor the one in the rear the readout went to 210'.

I went back and re-read this post to double/triple check and my 2002 ST4s is different than all of yours, bass ackwards!

I'm the second owner, did most of the maintenance for the PO and know the complete history of this little tart. The engine has never been out or the wiring harness fooled with except by me. But on this machine the vertical sensor controls the dash readout and the horizontal sensor controls the fan.

I replaced the vertical Bosch sensor with a Standard unit $15 US and all is well.

I'm thinking of leaving out the grey colored panel on the left side panel to let more air flow through the body work. Any thoughts on this?
Ride On(cooler this time)
Dave
I have a 2002 ST4s. I've been through this issue before. The wiring diagram for this bike certainly calls for the front/horizontal cylinder sensor and connector controlling the temp display. I don't have the wire colors as I'd have to get the diagram, but they match the plug wires for the horizontal cylinder in its temp display function. The rear plug attached to the vertical cylinder sensor provides information for the ECU and it has different wire colors: the fan is designed to come on at approx 220° and, to my understanding, some sort of fuel trim.

To my recollection, the front temp sensor is plugged into the water system after the thermostat. It is designed to give an accurate water temp display after warm-up and thermostat opens. Having the temp display wires on the vertical cylinder will probably not affect accuracy of display (maybe). BUT, as I see it, the horizontal sensor (with the complications of measuring water temp post thermostat) will not be providing proper and consistent flow of information to the ECU for fan operation and fuel injection trim(?). I associated the plugs per the wiring diagram and as noted earlier: horizontal providing display information and vertical providing information for the ECU.

If I am wrong, it will be the very first time in my entire life I've ever made a mistake! :smile2: If you need the wire colors for the two sensor plugs, please advise. Steve
 

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Bottom line, I applied my simplistic and idiotic fail proof mechanic's analysis. There must be a coolant temp difference between the two locations. Consequently, it must be important from the Duc engineers POV to have two separate sensors providing information to the proper locations: display and ECU.

Possibly the difference between the two locations may be the actual temp, how quickly the temp rises or consistency of the ongoing coolant temp. Otherwise Ducati would certainly save money with one sensor for the different functions of the currently installed two temp sensors.
 

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There must be a coolant temp difference between the two locations. Consequently, it must be important from the Duc engineers POV to have two separate sensors providing information to the proper locations: display and ECU.

Possibly the difference between the two locations may be the actual temp, how quickly the temp rises or consistency of the ongoing coolant temp. Otherwise Ducati would certainly save money with one sensor for the different functions of the currently installed two temp sensors.
I think that's giving the Ducati engineers a bit more credit than they deserve... The 916 series also has two sensors, one for the gauge and one for the ECU. They are located on the same water union at the horizontal cylinder. That same union also houses the fan temp switch. Further, my old 900SSie had two sensors to read oil temp. The one for the ECU on the horizontal exhaust valve cover and the one for the oil temp gauge on the sump screen cap. In the case of the SS and SBK, the sensor supplying input to the ECU are the same style sensor and connector as used on the ST. Both temp gauge inputs come from a smaller simple thermistor with a single wire spade connector.

If I had to make a guess as to why two, I'd say it was simply an evolutionary thing. Temp sensors existed in the carb era in the form of that simple small thermistor. Likely when they started with FI bikes, rather than reinventing what they already had for temp readout, they just added a separate sensor for the ECU. Doesn't fully explain the ST set up or the SBK set up which were always FI, except maybe that the ST2 engine was an evolution of the carbureted and oil cooled Paso. Maybe the ST needs the "better" sensor for the gauge because it feeds a digital display? Both the ST and SBK use a simple analog dial.
 

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I agree, the single terminal sensors are not accurate because you are rellying on a thread with sealant on it for your earth. This is ok for a gauge but not good enough for a ECU. Often manufacturers let old habits go rather slowly so we see a lot of early EFI vehicles use one sensor for the gauge and one for EFI. Later stuff uses CAN bus so has one sensor for each task and then shares that information with all the other systems.
 

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Discussion Starter · #15 ·
I checked and double checked the wiring diagram and the wiring on the Ducati and I can verify that;
HORIZONTAL temp sensor with the white and violet/black wires attached to it goes to the ECU.
VERTICAL temp sensor with the grey/black and orange/black wires attached goes to the cluster.
All the info I could source from this forum states the opposite. Perhaps this was one of those bikes made late Friday afternoon just before a long weekend or the first bike made on Tuesday after a long weekend.
I'm waiting patiently for a set of silicone water hoses to arrive then I will finish cleaning the coolant system replace the hoses and refill with 65/35% mixture and then ride the snot out of it like it was meant to be.
Ride On
Dave
 

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I checked and double checked the wiring diagram and the wiring on the Ducati and I can verify that;
HORIZONTAL temp sensor with the white and violet/black wires attached to it goes to the ECU.
VERTICAL temp sensor with the grey/black and orange/black wires attached goes to the cluster.
All the info I could source from this forum states the opposite............
Dave
Possibly, there was a change in the wiring harness for the ST4s models. Maybe someone with the left side fairing off for a tune-up could remove the Horizontal sensor plug and report if the display is blank. If it remains displayed (and feeding the ECU) then such may be an anomaly in the wiring diagram.

BTW, anyone have an opinion as to whether it matters? S
 
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