I have an 2004 ford ranger fx4 level 2 and i bought the truck 2 years ago and have been trying to fix this ever since. When the truck is cold it acts normal (if c to h is a 100 point scale) until about 15-20% then it jumps to H and stays there. While starting after hot it always stays on H.

What I have done:

new thermostat
both sensors replaced
new pigtails for the sensors (others were poorly done and could have been corroded)
replaced the instrument cluster with a new(used) one.

none of this ever fixed the issue. don't know what else to do.
Thanks for the help.

 

so after i had the new wires put on a check engine light came on. the tech said it was from pulling out the sensors so he tried to reset it and said that didnt work and may be bad sensors. so i will be replacing them one more time today

 

The ECT sender for the dash board gauge will have a red/white stripe wire going to gauge, and a Black/white stripe wire for the ground.

The ECT sensor, only used by computer, has green/red stripe and gray/red stripe.

I believe these have different resistance curves so MAKE SURE parts guys sold you a SENDER not a sensor

You, of course, want to test the ECT SENDER and its wiring.
I would test Black wire for good ground first, use volt meter to test battery voltage, then test voltage from battery positive to Black sender wire(battery negative), should be exactly the same.
Temp sender is a ground, the needle on the gauge measures the resistance to ground(the black wire)
As the sender warms up the resistance gets higher and needle gets higher.
That's what the Red wire is for, so if you unplug the wires from the sender, and then turn on the key, there is no Ground for the gauge, so super high resistance, no connection, gauge needle should go to MAX(hot).
If you ground that red wire, no resistance, then needle should go to MIN(cold).
If it does this then wire and gauge are "most likely" OK
If it does the opposite of this then it is a Reverse resistance gauge, which is fine, just different tests.

The in between MIN-MAX(cold-hot) is done in the sender, and the sender having a good ground.
If you have the old sender you can use an ohm meter to test its resistance.
Use 20k ohms or higher
After testing it write down results
Then drop sender in a bowl of hot water and wait a few minutes and test again.
Ohms should be higher now that it has warmed up a bit

If you use an old style analog ohm meter that's the same as the dash board gauge.
As the sender warms up ohms(resistance) goes up and so the needle on the analog ohm meter goes up.

ECT sender ground could be the problem, it could be corroded, causing higher resistance after warm up, which will show higher needle on gauge.

Here is the wiring diagram for '04 instrument cluster
B6 is the ECT sender connection, ground is at G204(don't know where that is though)