Just replaced my waterpump and thermostat housing, including both ECT sensors. When buying ECT Sensors the part store gave me two different sensors that were similar, but did not have the same part numbers and the probes weren't exactly alike. The old sensors looked identical. Should have stopped there but figured they were close enough and took the same harness.
The initial start up was rough. I had cleaned the throttle body and replaced the IAC while I was working on it so figured it just needed to adjust. Wrong. D@** thing will not idle for s$!t. When it isn't stalling out its running 2500 RPM's. The temp gage is functioning properly so I know one sensor is at least good. Put a meter on the IAC and it seemed to be getting the right voltage. Its getting around 6 volts when it drops below 1000 RPM and maybe 2 volts when its idling @ 2500 RPM. is going on??? It has to be something I caused during the install of WP and Thermo Housing. I have retraced all my steps and checked every vacuum line I can find. Even put the old IAC on it just in case, same results. Sorry to be a newbie asking questions, but at my wits end at the moment.

 

Welcome to the forum

We assume a Ford Ranger but what year?

And I assume a 4.0l because of the sub-forum.

There is only one ECT Sensor, the other one is an ECT SENDER, use for dash board temp gauge.

Sender has a cylinder end
Sensor has a cone end

ECT Sensor is only used by the computer, it is used to set "Choke mode" when engine is cold, Choke mode is Richer fuel mix, higher idle(1,100), and advanced spark timing.

Anyway the voltage you describe is incorrect.
ECT Sensor should get stable 5volts on one wire, it comes from computers 5v power supply.
Other wire should show about 3volts cold, .7volt fully warmed up
So the warmer the coolant the lower the voltage will get.

The same 5v power supply is used by the TPS(throttle sensor), MAF(air flow) sensor, and IAT(air temp) sensor.
This is called the Reference voltage, so is very stable, if it isn't then the "Return Voltage" would be meaningless

The Grey/red stripe wire on the ECT should be the stable 5volts
Green/red the Return voltage wire

 

Thanks for the response. Its a 2005 Ford Ranger XLT 2wd Automatic with a 4.0
https://goo.gl/photos/7EViRBYd7Y8F93oT7


The two sensors (picture linked) that came out of the truck (sensor & sender) are factory original and just alike, probes and all.
https://goo.gl/photos/gpaFBPhJp8VwmPuS8


Pulled the thermostat housing out because I just had a feeling something had to be wrong in that area. Don't know about volts... but I can tell you at ~75F the ohms read 30 on the original parts (BOTH). One minute in the fridge and the ohms climb so they are functioning correctly. On the replacements the ohms read 30 on one @75F and the other reads 2.1 at the same temp. The dash temp reads correct, so the other "low ohm" one must have been going to the computer and suggesting that the truck is actually just over 200F even at cold start.


I must be confused on the IAC function. Multi showed harness was feeding it at 6 volts when it dropped below 1000 RPM (more air) and 2 volts when it was running higher RPM's (less air). Maybe it needs to be calibrated, or I do, or that is the wrong way to check even though voltage output is logical correlation to RPM.


This is completely my fault as I should have checked the new parts with a multi; it would have saved me a lot of trouble. Hopefully, if anyone else makes a bonehead mistake like this in the future they come across this and it helps them out. The truck is running fine now after replacing the low ohm with the correct sensor/sender

 

Sorry my mistake, I missed the "IAC", and was thinking ECT voltage

IAC Valve gets 12volts with key ON on one wire
The computer is the Ground wire for IAC Valve, the other wire

With key ON IAC Valve should open all the way for starting
After startup computer will start Pulsing the Ground, which in essence lowers the voltage, and IAC Valve should start closing.

You need to measure voltage on the two wires, so don't Ground the Meter or hook it to Battery 12v, as you will get false readings

So higher voltage = more air = higher RPMs

On some analog meters you can see the Pulsing, quivering needle, with digital you won't but will see the voltage change as computer adjusts RPMs

2005 2.3l Duratec could use a different kind of IAC valve and control, never tested newer one.

When you put an automatic into gear computer should bump up RPMs by 50

Air Conditioning on also causes 50RPM bump

If testing IAC voltage with warmed up engine running, you can create a vacuum leak, which will cause higher RPMs, then you should see the computer respond by closing the IAC valve a bit to bring RPMs down to "target level", warm engine in Park level



But most automotive electrical system controls use the Ground side for that control.
It started over 80 years ago, lol, an ignition coil is given 12volts with key on, the Points in distributor was the Coils Ground, cutting the ground cause coil to spark.

Fuel injectors all get 12v with key on, computer Grounds each one to open it

Much safer to run multiple Ground wires from engine to computer than 12volt wires