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Have any of you tested the ECT sensor/sender resistance at different temperatures, per the FAQ?
Am putting the head back on my 1991 Turbo 745. Could not locate the original ECT and temp gauge sensors - don't remember where I put them when I took them off back in mid-July.
I do have another pair, from an old head I bought from a pick-n'-pull, and I tested the ECT sensor per the FAQ. Below are the readings the FAQ says I should get, and the ones that I actually got at three different temperatures:
FAQ says 1300 ohms at 40 deg. C, my sensor gave 1240 and 1165. Both are too low (computers would think the engine is hotter than it really is), one side of the sensor is within 10%, the other is just barely over 10% off.
FAQ says 600 ohms at 60 deg. C, my sensor gave 716 and 674. Both are high (computers will think the engine is colder than it really is), approx. 12% and 19%.
FAQ says 300 ohms at 80 deg. C, my sensor gave 369 and 360. Both are too high, 23% and 20%. Again, the computers will think the engine is colder than it really is.
Do any of you think that these sensors will work properly? I am leaning toward just replacing them with new ones, but am concerned that the new ones might be just as far off. Any experience with this stuff guys?
--
1955 Human, Scott; 1991 745 Turbo, Brunhilde; 1990 745GL, Snuggle Bunny
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Some of the advice is way too complicated for this test.
Simple, remove the ECT from the head, and with the sensor comnnected to the cable in the harness, prepare a beaker or cup of ice water, and put the sensor in. Measure the resistance at the connector for the ECU. Repeat with a cup of boiling water, measure resistance again.
As a third point of comparison on the curve, measure resistance at the ambient temp.
Compare the three points to the curve of resistance in the Greenbook or Bently's.
Any small probe measuremt difference such as the ambient or boiling water will show small offsets to the curve, atmospheric conditions, elevation etc, will not change the shape of the curve, so don't worry about it. Any mercury thermometer or TC probe on a DVM will be more than adequate for reference, but the true reference is the hot/cold junction point, not the probe or measurement instrument.
If the resistance measured at the ECU connector shows a uniform offset in the direction of increased resistance at all three points then the wiring or connector may have corrosion, causing a decrese in continuity.
If the curve made by your three measurement points show the low temp and mid point temp offset in the higher resistance direction, then the sensor is fading.
Double check the temps with the sensor immersed into each bath without being connected to the harness. Subtract the two sets of data and the result is the wiring and connector component, which is usually 90% of the problems with these sensors, IMHO.
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posted by
someone claiming to be ibs740
on
Tue Oct 12 02:30 CST 2010 [ RELATED]
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What I do to check this type of sensor is to 1st get an old female connector from the wrecker. Strip the wires & make a couple of loops & solder. Plug the sensor on to the connector, tie both the thermometer probe & sensor together to an old welding rod, coat hanger, something firm & vertical. Place in an old soup can (clean) place a lid w/ a hole in it (so the leads pass thru. & fill w/ water so both sensor & thermo are immersed but not the connector. I use an old lab 115V hot plate (cheap & great control, good for thermostats also). With the connector loops out of the can you can quickly test all resistances.
Total cost for me to setup was about $10.00 & good for life.
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Hello,
What was the calibration date on your thermometer?
Goatman
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Don't know, it is a Hewlett-Packard digital multi-meter, model E2378A. I have had it at least 16 years, until now it has been in perfect agreement the digital thermostat in our house, and ice and boiling water. However checking it against the thermostat this time gave a discrepancy of about 3.89 deg. C., with the meter reading higher than the thermostat. This time I was in a hurry, and did not calibrate it against ice and boiling water as I usually do when testing a sensor.
If I assume that the thermometer is the one that is reading the full 3.89 deg. C too high, then the readings at 60 and 80 deg. C could be right, but the reading at 40 deg. C is still too low indicating to the computers that the engine is warmer than it really is, although the resistance is only off by about 10%. Of course if the thermometer is only off by, say, half of the discrepancy, then the ECT sensor looks like it is a little out of whack, but not as much as if the thermometer is exactly accurate.
Before you ask, I checked the ohmmeter function against two known resistances today, high precision (+/- 1%) non-inductive 300 watt, 8 ohm resistors, and it was exactly right on. I do have some other +/- 5% resistors of higher resistance I could check the readings in the 300 to 600 ohm range and around 1300 ohms with, but for now I trust the meter.
--
1955 Human, Scott; 1991 745 Turbo, Brunhilde; 1990 745GL, Snuggle Bunny
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Hi Scott,
My instinct based on experience tells me both your sensors are OK.
If you notice the deviation of one from another, they're within a few percentage points, but the deviation from the FAQ is what you're concerned about - tenfold of that.
Remember FAQ is not FACT. And those readings, which I suspect are approximated by someone running his finger up the semilog plot in the green book and calling off easy to remember round numbers, are sufficient for blessing the FI equivalent of the choke bimetal.
What I know of NTC thermistors is they don't normally degrade out of tolerance, just the real world creeps up on them in the form of opens and corrosion, problems you'll discover easily comparing with those numbers quoted in the FAQ, if a little common sense is applied.
How did you thermally couple your thermocouple meter probe with both sensors and what did you do to mitigate time differences switching to resistance probes on two separate devices under test?
--
Art Benstein near Baltimore
There is no psychiatrist in the world like a puppy licking your face.
-Ben Williams
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Thanks for the info Art. The thermocouple probe was not coupled with the device under test, and I checked the temp of the water before and after testing the sensor. By the way, there was only one device under test, it has two NTC thermistors in one housing. That is what the two numbers are for.
Because I only have the one meter, I just pulled the thermocouple out of the water, changed leads in the meter (the other end was already clamped to the sensor to save time) and lowered the sensor into the hot water. Pulled the sensor out and changed contact quickly to check the other thermistor. When done, I switched leads back to the thermocouple and re-checked the water temp at about the same location in the jar as where the sensor was sitting. In all cases, it did not change.
I used a full (of water) quart canning jar sitting a 3 quart sauce pan that was about 2/3 full of water. That was to protect the pan from getting any car crud in it. I guess that because of the relatively large mass of water involved, about 3 quarts, it stayed the same temp. during the actual testing of the sensor.
Thanks to you both, and I guess I'll just put it back in and hope for the best.
--
1955 Human, Scott; 1991 745 Turbo, Brunhilde; 1990 745GL, Snuggle Bunny
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Ahh, just the one sensor, but bearing the "matched pair" of thermistors.
Your methods are certainly more careful than my ad hoc testing of a new gauge sender as related in this recent thread: 123 ohms at 9 o'clock position
--
Art Benstein near Baltimore
A dog teaches a boy fidelity, perseverance, and to turn around three times before lying down. -Robert Benchley
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N7SC,
I’m glad you have a quality instrument. Since the temperature of boiling water is affected by the pressure of the atmosphere, be sure to factor that into your calibration. Be aware that the accuracy of instruments can drift along the scale. Unless you have a calibrated thermometer, I would not rely on it's readings for precise measurements.
I would install one of the ECTs and see how it runs. If it runs poorly and fails driveability checks, THEN order a new one.
Goatman
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Makes sense, and replacing it if it fails driveability is not that much work. BTW, I only have one ECT sensor, the double readings are for the two different ntc thermistors in the one housing, one for the fuel injection computer and one for the EZK 116 ignition computer.
Am aware that accuracy can drift, sometimes wildly, along the scale. Have just been okay with using it as is with ice/boiling/thermostat checks because I don't want to be without it or to pay a probably exorbitant cost for calibration.
--
1955 Human, Scott; 1991 745 Turbo, Brunhilde; 1990 745GL, Snuggle Bunny
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