2003 Sedona - Temp gauge showing overheat - ECT sensor says NOT!

I apologize in advance for this very long post. I didn't realize how long it was.

I would like some help to determine the root cause of engine temperature gauge showing high temperature with no other obvious symptoms of overheating.

I believe it is a bad ECT sensor but not 100%

I normally would DIY - but no longer have access to all my tools required. (Divorce sucks btw) (I am learning to "embrace the suck").

I would like to verify or refute my guess for the requirement to replace the Engine Coolant Temperature Sensor 2003 Kia Sedona VIN: KNDUP131436428078

Observations:

*Upon inspection noted that coolant reservoir was below cold minimum level. Topped up with 50/50 premix. No change.

* Upon inspection cooling fans do not come on unless A/C is turned on. (Different circuit to initiate I assume).

The car seems to run fine. No apparent operational issues. Heater works produces warm air (although not super hot - but reasonably warm). A very slight sense of coolant odor.

I would normally perform radiator/cooling system pressure test to identify the reason for low coolant level in the reservoir. Radiator cap? Leak? Whatever? But not going to do that at this point, because I see no obvious leaks - granted not under a pressure test.

So the question becomes:

Is the engine really overheating or does the sensor just want to say it is?

So retrieved my Harbor Freight OBDII scanner 60693 (rarely used except for this kind of stuff)

If we were to say at Key Off the Temp gauge sit at = -15%, at the Cold marking with the Key on/ but not running - Temp gauge = 0%, then at Normal operating marking - Temp gauge = 50% (center) and finally at Overheat temp marking - Temp gauge = 100% (pegged at top).

I observed (I also recorded) the data from the OBDII interface with some interesting discoveries.

Overall my observations would indicate (to me anyway - maybe not those of you that are experienced and experts) that there really is no overheat condition - the temp gauge is "lying to me".

* Key off - Temp gauge = -15%

* Upon Key on/ started but not yet running (all warning lights illuminated) beep beep beep ... And after sitting overnight (ambient temp down to 33 degrees F (Denver Colorado) - The cold start temp gauge = was at already at 45% or
already at the near normal operating range. I would have thought it should be at Cold marking or close to 0%?

*There are ZERO stored or pending DTC's and of course no MIL.

* I stared then let idle - Temp gauge increased to 100% fairly quickly (15-20 min). However, the scanner reported the corresponding ECT was at 140 degrees.

*Temp gauge stayed "pegged" at 100% during a short drive. Maximum ECT temp was 194 degrees.

*At this point cooling fans are NOT heard/thought to be on when at stop light etc.

*Turning on A/C (cooling fans heard to be started) gauge remained at 100% but ECT lowered to 184-186 with the temp set to cold air (as I expected). Temp from vents was cold

*Gauge remained at 100% ECT lowered to 180-184 with the temp set to hot air. (I expected that setting the temp to hot would remove a little more heat - looks like it did).Temp from vents was warm air (fairly hot but not scorching).

*During drive ECT was fluctuating between 188-194. Temp gauge remained at 100%.

Van ran fine - no obvious issues, idle was normal and not erratic.

Acceleration seemed okay (for this van anyway). No hesitation or stuttering etc.

Upon return from the drive and turning the van off - sitting there ECT increased to a maximum of 205 degrees. No air flow, no
fans etc. Just sitting there "cooling off".

All of the above is why my guess is that it is a faulty ECT sensor (or circuit - I really don't want to go there).

Since the sensor is three wire - I assume one is ground, one is input to the ECU and the third is driving the temp gauge?

Have you ever seen an ECT sensor fail where the output to the gauge was at fault? Or ... the gauge itself is faulty, or the continuity of the wiring to the gauge is compromised?

To replace the ECTS it is my understanding that since it is in a "well" in contact with the coolant, the coolant must be lowered to a point below the sensor to avoid it pouring out during replacement. At that point might as well drain the whole system and replace with fresh coolant. Is flush really required?

Thanks in advance for any assistance.

Remember - I am a retired Electrical Engineer and not a mechanic. I am for sure smart enough to know that those skill sets are very different! Right about now I wish I was a mechanic.

Upon reading my old mitchell diagrams on this ECT it was drawn as a potentiometer, so i followed the wiper thinking that was the signal wire but it goes to ground. So now i'm thinking this picture is wrong. I'm flipping through the years and I'm up to 2006 kia sedona and the picture makes more sense. There's TWO variable resistors !!

I think this explains why your engine computer reads temp as normal and the gauge one is out of wack.

Try a bypass test. With KOEO, what voltage is on the orange wire? With it disconnected? Short it to ground and see what the gauge does in the cluster.

imgur.com/a/sjFx8O6

Hey poolsider! No worries about the long post, we like getting all the info we can.

I'm with BobbyM - if your coolant temp sensor has three wires, then there's absolutely two sensors in there. One for the gauge, one for the PCM. I also agree with a bypass test to establish if the circuit and gauge are good. If this gauge plays by the usual rules, then it's pretty darn straightforward.

KOEO, with the gauge reading 50%-ish, disconnect the sensor. The gauge should go all the way hot or all the way cold. Some gauges move slowly, so give it a minute. Now find the wire for the gauge. Orange, if BobbyM's diagram is correct. I'd look on BBB Industries, but they're updating their database right now. Disconnected, it'll likely read around 10-12 volts. Short it to ground and see if the gauge moves the other way.

If it does, you're done! Sensor time. The wiring integrity and gauge are good.

Drain the coolant if you want? Or just put a drain pan under the sensor, and have the new sensor close by to swap in. Flushing is a good thing, but you could probably hold off on that until you get the gauge working correctly. If you're gonna drain and fill or flush the coolant, you could also try flushing the heater core while you're at it. See if you can improve the vent temps.

Finally got a diagram.



BobbyM is right, this diagram is wrong. :lol: Blue/yellow is absolutely a ground. Black/yellow is ECT signal to the PCM. Orange is the gauge circuit.

In this case, it's not Mitchell's fault for producing a bad diagram. :silly:

I really appreciate your taking all the time to help me track this down.

I haven't had the opportunity to do the bypass test yet - but have confidence it will confirm what you think.

Again thanks.