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Unread 12-02-2009, 09:28 AM   #1
maxgordon7
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Coolant Temperature Sensor Ohms

I would like to find a chart on the ohms of the coolant temperature sensor. Like say 700 ohms at 210F and 18,600 at 30F. I'm searching the web with no luck
This is for a 4.0 96 jeep cherokee

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Unread 12-02-2009, 09:38 AM   #2
ehall
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I've never double-confirmed but from what I understand the ECU sensors for RENIX and HO used the same scale. This link has the RENIX scale--Jeep Cherokee Engines - RENIX (non-HO) Engine Sensor Diagnostics
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Unread 12-02-2009, 09:49 AM   #3
tjwalker
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4.0 COOLANT TEMPERATURE SENSOR: Temperature-to-Resistance Values (Approximate)

212 farenheit 185 ohms
160 farenheit 450 ohms
100 farenheit 1600 ohms
70 farenheit 3400 ohms
40 farenheit 7500 ohms
20 farenheit 13,500 ohms
0 farenheit 25,000 ohms
-40 farenheit 100,700 ohms
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Unread 12-02-2009, 09:57 AM   #4
maxgordon7
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I found this just seem to be a bit out dated and sensor seem to not be in the same spot. This all started with i switched in analog gauges both 96. I was able to use oil sensor from old truck but had to get a new temperature switch(borg warner part or BW) So at first im reading cold from gauge with little heat. So i replace thermostat. Have heat now but gauge reads from over 210 to 230. Someone already put in a new radiator, water pump is working great, fresh coolant and i get heat. So I just want to check gauge to coolant sensor ohms. When i unplugged the sensor the fan kicks on right away but it never turns on when the truck says its over heating. I got like 18,600 cold and got 880 hot before it started to go back up. The gauge said way over 210 when i took the 880 reading. Seems like 185 ohms from that chart is at temperature. I could care less if the gauge reads a little high but i want to be sure of that.
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Unread 12-02-2009, 10:35 AM   #5
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Got 880 again, would be nice for something to ohm match
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Unread 12-02-2009, 10:42 AM   #6
ehall
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You asked for sensor resistance but now you are talking about gauge readings, light switches, and the fan turning on so I don't know what you are asking for. The table data you were provided is for the sensor.
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Unread 12-02-2009, 10:49 AM   #7
maxgordon7
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Just looking for others input on a 880ohm coolant sensor at running temp
From the chart 185ohms at 212F so ideally the sensor should test at 200 ohms or so when your motor is fully warmed up. This sensor will send the ohm reading to the ecu, which the ecu uses this reading to determine how much fuel to use. So more ohms more fuel. So a good running sensor at 185 will cause the motor to use less fuel. I hate to replace parts that are good. If i got a new sensor and it worked at 185 I would be happy but if I replace and still get the same readings i will be pissed i wasted my time and money for nothing.

Last edited by maxgordon7; 12-02-2009 at 11:12 AM..
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Unread 12-02-2009, 11:26 AM   #8
ehall
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Looking at the progression there it seems that resistance essentially doubles for every 30 degrees F so resistance in the neighborhood of 800-900 ohms would be 130f

Go get a temperature gun and measure the thermostat housing if you want another reading. If you have a buddy with a shop he can probably hook up a scanner to the OBDI port (next to the washer fluid) and tell you the actual temperature. Also remember the sensor in the housing measures coolant temperature not engine temperature. Once you have the sensor calibrated go find a gauge sender that reports the correct value.
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Unread 12-02-2009, 11:45 AM   #9
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Quote:
Originally Posted by maxgordon7 View Post
I found this just seem to be a bit out dated and sensor seem to not be in the same spot. This all started with i switched in analog gauges both 96.
Does your model have a gauge temp sensor at the rear of the head or is the gauge operated by the ECM? On the models without the head sensor, the gauge is not connected directly to a sensor; the sensor reports to the ECM and the ECM communicates with the gauge.
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Unread 12-02-2009, 12:10 PM   #10
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Put the given values in an excel file and plot them, then do a trend line and find out what would match according to the trend...
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Unread 12-02-2009, 12:35 PM   #11
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I think you need to know what temp you're running at first. Get an infrared point and shoot thermometer and point it directly at the thermostat housing. That will get you pretty darn close to your coolant temperature. Without an accurate running temp number, it's impossible to tell if your coolant temperature sensor is in spec.

I looked in a couple of manuals and did find a different chart for non Renix coolant temperature sensors. Resistance data for a 1991-1998 4.0 coolant sensor is included below.

Keep in mind that the numbers are for the sensor removed from the engine, in a beaker of water, and using a calibrated thermometer. Ballparking ambient air temperature and measuring with the sensor still installed will NOT be as accurate. That being said, if you get an 880 ohm reading with a fully hot engine, using below chart and assuming you are testing with sensor still in the engine, your sensor is probably okay.

176 degrees farenheit has a spec between 1,170 ohms and 1,340 ohms
194 degrees farenheit has a spec between 860 ohms and 970 ohms
212 degrees farenheit has a spec between 640 ohms and 720 ohms
230 degrees farenheit has a spec between 480 ohms and 540 ohms
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Unread 12-02-2009, 01:35 PM   #12
ehall
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where did you find that chart? there's no info at all anywhere in my 91 FSM for testing the sensor
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Unread 12-02-2009, 01:42 PM   #13
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That information is right on par to 880ohms. Thanks for your time Keep JeePin
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Unread 12-02-2009, 02:50 PM   #14
tjwalker
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Quote:
Originally Posted by ehall View Post
where did you find that chart? there's no info at all anywhere in my 91 FSM for testing the sensor
Found it in my 84-98 Haynes XJ manual. I didn't see anything about it either in my FSM.

Imagine that. Something in a Haynes manual that is missing in an FSM!
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Unread 07-05-2012, 07:23 PM   #15
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Quote:
Originally Posted by tjwalker View Post
I think you need to know what temp you're running at first. Get an infrared point and shoot thermometer and point it directly at the thermostat housing. That will get you pretty darn close to your coolant temperature. Without an accurate running temp number, it's impossible to tell if your coolant temperature sensor is in spec.

I looked in a couple of manuals and did find a different chart for non Renix coolant temperature sensors. Resistance data for a 1991-1998 4.0 coolant sensor is included below.

Keep in mind that the numbers are for the sensor removed from the engine, in a beaker of water, and using a calibrated thermometer. Ballparking ambient air temperature and measuring with the sensor still installed will NOT be as accurate. That being said, if you get an 880 ohm reading with a fully hot engine, using below chart and assuming you are testing with sensor still in the engine, your sensor is probably okay.

176 degrees farenheit has a spec between 1,170 ohms and 1,340 ohms
194 degrees farenheit has a spec between 860 ohms and 970 ohms
212 degrees farenheit has a spec between 640 ohms and 720 ohms
230 degrees farenheit has a spec between 480 ohms and 540 ohms
Old thread, I know. I've been troubleshooting my temp gauge and came across these resistance values. I took the sensor out and dropped it into a pot of boiling water for a few minutes. Then I measured the resistance while it was in the water. It measured a little over 140 ohms. Then I pulled it out, and measured the resistance immediately after pulling it out. It started at 140 and started to rise. The temp of the sensor as soon as it came out of the water was about 180. I let the sensor cool and watched the resistance climb. At 100 degrees the sensor was around 1k ohm. I put the sensor back into the engine and let it get to temp. I measured the resistance again and found 180 ohms at 180 degrees. These values don't come anywhere near the values listed above.

I have 2 questions based on these results. Did I do the testing correctly? If so, I definitely have a bad sensor. Brand new, oh well. Secondly, I think I have a bad gauge because lower resistance readings should red-line the gauge. Is there anything I could test on the gauge itself so I can isolate the gauge or wiring? Or am I way off on all this crap?

Guess I had 3 questions! Thanks!
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