I have a 2003 Ranger with a 2.3L engine. While running I got a check engine light with temp gauge in the instrument cluster going full hot. I checked and got a P1299 code. This indicated the CHT sensor was in protection mode. The actual coolant temperature was not overheated when I checked and later when I started the engine in a cold condition the code threw again with the temperature gauge going full Hot (when it was absolutely cold). OK, so I will replace the CHT, I believe since it was in protection mode it was giving the computer the wrong fuel mix causing my engine to run rough also. I unplugged the CHT (until I install the new CHT) and the engine runs OK with a new P1280 code (expected since the CHT is unplugged) and the temp gauge inside is dead with no input and needle below the C position.

There are two sensors, one is the mentioned CHT sensor and the other is the ECT (Engine Coolant Temperature) sensor. The CHT signal goes to the computer and the ECT is supposed to supply the signal to the temp gauge on the instrument cluster.

Question 1 (Important): Does anyone know the correct torque setting for installing the CHT? I know it is a low setting because you can break off the threaded portion due to it being thin brass.

Question 2: Why was the temp gauge reading full hot as if the signal to it was coming from the CHT instead of the ECT sensor? The temp gauge signal is supposed to be coming from the ECT.

Question 3: Does the CHT override the ECT input to the temp gauge when it thinks the coolant is too hot?

Question 4: After I unplugged the CHT I got no temp gauge reading. Is this a condition of the possible override (question 3) or do I have a bad ECT also? Note: I got not codes relating to ECT.

I would appreciate any comments that help clarify this.
Thanks,

JIm

 

The torque spec for the CHT sensor is 9 lb-ft.

Under normal operating conditions, the temp gauge in your 2.3L is controlled by the ECT sender. When the PCM goes into Fail Safe cooling mode, it pulls the gauge to full scale.

 

Thank you for the reply, it helps tremendously. I guessed the torque was 5-8 lb-ft so I was a little low. The old sensor is about 2 inches long and the new one is only about a quarter of an inch beyond the threads. I think sensing units are better now than 9 years ago but should I be worried that it won't sense correctly or consistently because it is short and may be enveloped by air bubbles?

Your comments about the operating condition when the PCM goes into Fail Safe cooling mode is helpful. I saw the gauge go to full scale Hot when the CHT was connected. However, when the CHT is disconnected it is no scale - no input (below Cold). Is this normal? My assumption is since the CHT is disconnected and no signal is obviously being sent to the PCM that it is in some other mode and that mode also overrides the ECT signal?

Thanks again, Jim

 

I don't claim to know exactly how it's set up but it appears to be wired with two parallel paths to ground. The first path to ground is through the variable resistance ECT sender like many conventional temp gauge circuits. The twist is that there is a parallel path through a resistance and into the PCM. I haven't investigated it but my guess is that the PCM grounds that wire in FSC mode and the gauge goes full scale.

I couldn't say for sure what the expected behavior would be with the CHT unplugged.

As to the difference in the appearance of the old and new sensors:
Are you sure they sold you the right part?
Are you sure that you have correctly ID'd the CHT?

 

I checked behind them on their website part search and it is the right CHT for my '03 Ranger. However, it is aftermarket (Duralast from AutoZone), which has a fair reputation and seems to be made for and fits quite a few fords. This multifit is possibly why it is different in length. I have not been able to see a new Ford dealer replacement to compare to my original or the Duralast. I may comment about that to them and see if I can take it back if I experience problems even though it is electrical.

Thank you for your further expanding comments on the system's logic. I think I have a good handle on it now and you answered my most needed questions. I think the visual logic the engineers were shooting for is the CHT to override the ECT to let you know another way (beside the Check Engine light) there is a problem and if it is dead or unplugged they don't want you to see a normal temperature. So many people ignore Check Engine alerts these days and with that logic the engineers would most likely get owners to look into the problem and fix the CHT.

 

I need to replace the CHT in my 08 Ranger 2.3lt. and all of the deep sockets I have don't even come close to being long enough to remove it. special tool needed?