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Dodge Repair/ECT Sensor inaccurate?


QUESTION: Mr. Finston,

2004 Ram 1500 4.7.  Truck idle is slightly erratic when first started cold.  No stalling or running rough or anything of that sort.  It's just that the idle speed is up-and-down like a pea on a drum for a few moments and then straightens up and idles steady (@625 rpm).  

Now, when up to operating temperature the truck has a tendency to not start after shut down for say ten minutes. I'm suspecting (from past experience) that the engine coolant temperature sensor is bad.  I'm at about 1,800 ohms, hot. Most of your posts indicate 700-1000 ohms hot vs. 10-20,000 cold.  Could you please confirm the correct resistance specs for this application along with any additional thoughts.  Thanks for your time.

ANSWER: Hi John,
The resistance when cold should be in the range of 3-4.000 ohms, and typically in the range of 1,000 ohms when at operating temperature. So 1800 ohms would seem to high and possibly causing a too rich situation when at operating temp. It may not be so far off as to set a fault code but if you can get a free code readout at a nationwide auto parts store such as Autozone that would be worth a try. You could check the voltage on the violet/orange wire at pin 20 of the pcm plug (or at the sensor proper) and let me know what that is when the engine is cold and at operating temp. The manual doesn't have the resistance readings (I am basing my numbers on other vehicle manuals) but I do have the voltage table as a function of temperature.
Thanks for the rating and nomination.

---------- FOLLOW-UP ----------

QUESTION: It's me.  ECT sensor cold resistance: 6,800 ohms.  At operating temperature: 8-900 ohms.  Five volts is available at the sensor connector when checked cold, or at operating temperature.

ANSWER: Hi John,
6,800 ohms seems high unless you are in below 0F weather, but it is possible that the later year sensors are higher ohm than my manuals show. About the voltage reading you may have been reading the 'supply' voltage (which is 5v); what does the other wire show (of course when hooked up to the sensor)?  Make your measurements of each of the wires as compared to chassis ground. In any case, a new sensor is not very expensive so it might be worth replacing yours.
PS Feel free to rate my answer, and you are entitled to do a second 'nomination' of me if I merit that in your eyes. Thanks

---------- FOLLOW-UP ----------

QUESTION: "1500 RAM ... 'TAKE THREE!'"  

Mr. Finston ...Okay, here's the rest of the story.

At the parts house, I checked the resistance on a new-in-the-box ECT sensor: 10,300 ohms.  

A few hours ago I performed the test as you described.  

COLD: ECT sensor resistance 8,500 ohms.  Orange/violet wire: 3.37v.  Other wire 0.02v.

OPERATING TEMP.: ECT sensor resistance 800 ohms.  Orange/violet wire: 0.83v.  Other wire 0.02v.

Now, out for a test drive.  The third time that I stopped--and shut the truck off (for about ten minutes)--whatever is breaking down broke down and the engine wouldn't start.  Here's the readings during the no-start condition:

ECT sensor resistance 800 ohms.  Orange/violet wire: 0.90 - 1.04v.  Other wire: 0.00 - 0.01v.  

Finally, here's the readings 15 minutes later after whatever is breaking down corrected itself, (i.e., with the engine running):

ECT sensor N/A.  Orange/violet wire: 0.87 - 0.94v.  Other wire: 0.04v.

What are your thoughts about all of this?  Thanks for your time yesterday to discuss this matter.

Hi John,
Thanks for all that information about the ECT readings. My thoughts are that the ECT is not the reason for the no start after warm shutdown. The readings are consistent with the temp and the temp is consistent with the operating history.
So my next suggestion would be to try for a fault code readout. It is not clear in the manual whether using the ignition key will result in a readout on the odometer window but give it a try: "on-off-on-off-on and leave on" doing that in 5 seconds or less elapsed time. Then notice whether the mileage reading is replaced by any 4-digit numbers preceded by a P. If so, let me know what they are. If not, then try for a free code readout using a plug-in code reader at a nationwide auto parts store, such as Autozone. If you can't get a free readout then an independent shop will usually charge in the range of $40. Ask for the numbers, what they mean, what they recommend. Then let me know and we'll compare notes with the shop manual "powertrain" volume.
It may be that another sensor heats up due to residual heat in the block when you shut off the engine, then cools down in about 15 minutes enough to run properly again. I'm thinking that the cam or crank rotational position sensor is doing that, and if so there would be a fault code stored for that.
If there are no codes, then we have a problem. You would want to verify whether you have spark, and that when you turn the key to the run position that you hear the fuel pump run for about 1 second when you are in that no-start situation.
I look forward to your next report.
If you would be so kind as to offer another rating and a nomination for 'volunteer of the month' (you are allowed to make 5 such nomination in any given month) I would be most appreciative.

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Roland Finston


Free fast answer to your repair question about a Chrysler/Dodge/Plymouth car, minivan, SUV, or truck. Problems with electronically controlled engines and transmissions as well as body wiring problems are my specialty. This free troubleshoot advice forum helps you diagnose faults, minimize repair costs or do-it-yourself.

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