Official Coolant Temperature sensor resistance value please

[Scooter]

Can someone tell me what the official coolant temperature sensor resistance values are supposed to be? (Room temperature preferred)

I've had about a 1.5 to 2 MPG drop off in fuel economy over the last few years NOT attributable to the thermostat so I was thinking of replacing the coolant temperature sensor. I did acquire a used one (no cost) that measures roughly 850 ohms from the terminal to the sensor housing at 68 deg F. My 2003 manual on page 22-16 says that the resistance should be within 2100 - 2600 ohms (+/-10%) at 68 deg F. It does list an incorrect 80 deg C for that temperature which I believe was an error and should have said 20 deg C instead. So the value on this part sounded suspect.

When I finally pulled out the original part a couple of weeks ago (204K miles of used) I was measuring 21.3K ohms with it which might have explained the slight loss in fuel economy since the engine is going to think that its colder than it really is.

So, I went and ordered a new part from Ron Ayers which just arrived on my doorstep. It certainly looks legit but when I measure the resistance of the new part I get about 20.4K ohms on it. *****???

I'm going to assume the following:

• That the resistance value listed in my manual is incorrect.
• That the used part that I got is measuring lower than normal. Likely much lower than normal.
• That the new part probably is not going to help my problem much.

Honda part number is 37870-MBG-003

STeed is a 2004 ST1300A

Thanks...

 

[Paul]

My book says 2.3-2.6 K Ohms across the adjacent terminals @ 20c/68 F in the Fuel system section on page 5-21
It has the same numbers you quoted on page 22-16 for the immersion test. You have to check the sensor from the single terminal to case ground while heated to 80c 178 F. Not sure how you can attain the 120c called out.

My thought is maybe there are two resistors in the sensor, one for the ECU and the other for the temp indicator??

 

[Igofar]

What made you settle on checking the sensor?
There are a lot of other things that can cause those symptoms....
Brakes dragging slightly, Failing/Failed SMC, Dirty air filter, worn out spark plugs, Tire pressure, or general overall tune of the engine.
Just wondering why you chose to target in on electrical sensor?

 

[Scooter]

My book says 2.3-2.6 K Ohms across the adjacent terminals @ 20c/68 F in the Fuel system section on page 5-21
It has the same numbers you quoted on page 22-16 for the immersion test. You have to check the sensor from the single terminal to case ground while heated to 80c 178 F. Not sure how you can attain the 120c called out.

My thought is maybe there are two resistors in the sensor, one for the ECU and the other for the temp indicator??

You are correct about the two resistors in the sensor. I don't know why I didn't just trace out the wires from the connector in the schematic first. Duh. So the thermistor that the ECU uses is connected to wires Yellow/Blue (+5V) and Green/Orange (sensor ground). In the 2003 manual, on page 5-18 for test #2, ECT Sensor Resistance Inspection, they give out a value of 2.3 - 2.6 ohms. Obviously they forgot the "k" symbol in this edition. They also refer to the Yellow/Blue wire as Pink for some damn reason???

So for the three ECT sensors that I have on hand I get the following values: (@68 deg F)

• New: 2018 ohms
• Original: 2197 ohms
• Free used unit: 2215 ohms

All a little on the low side but within a reasonable margin of error. I guess this means that I should not expect any difference in my mileage using the new sensor.

The second thermistor is used for the Coolant Temperature Gauge. The return electrical path is through the sensor body as you mentioned. This is the element that I am seeing the huge difference in value between the three units that I have and the spec given in the manual. My measurements are:

• New: 23.8K ohms
• Original: 23.0K ohms
• Free used: 940 ohms

Seems to me that Mother Honda screwed the pooch for this one. Either they sold me a bad ECT (unlikely) or else the resistance value shown in the table on page 22-16 (2003 manual) is incorrect. I'll have to plug in the free used sensor I got just to see how many bars show up on the gauge cluster when I first turn it on. I'll bet it's more than three...

 

[Scooter]

What made you settle on checking the sensor?
There are a lot of other things that can cause those symptoms....
Brakes dragging slightly, Failing/Failed SMC, Dirty air filter, worn out spark plugs, Tire pressure, or general overall tune of the engine.
Just wondering why you chose to target in on electrical sensor?

A few factors. First, everything on your list has been looked at many times previously. Bike has always run fine and still does. It has been well maintained for all 204K miles of its life. I had seen at least one post in the past where the replacement of the ECT had resolved a similar issue. When I had the opportunity to acquire a "free" ECT last year I took advantage of it. It wasn't until recently that I had the chance to install it.

When I removed the original one out of the bike I measured the resistance of it and compared it to the free one that I had received. That is when I noticed the discrepancy in the manual for the value. I ordered a new part thinking that both the free one and the original part was out of spec. And it seemed to make sense. My part was measuring at a value where the engine would have thought that it was much colder outside than it was.

As it turns out, my mistake was that I wasn't looking at the correct chart in the manual. No thanks to whomever setup the index, but it pointed me to the test for the thermistor that controls the temperature gauge and it didn't sink in my mind that there is two elements inside the ECT. I should have known better since I've seen the section in the manual for the Fuel system diagnostics before. Live and learn...