I've just had a problem with my 2001 Ram 1500, 5.9. Two days ago I wrapped up my project of changing the valve cover gaskets. Next morning I tried starting it up and it took several tries and ran really rough and wanted to die, and would hardly idle at first, then after about 10 minutes of struggling it seemed fine but I noticed that not only was the "CHECK ENGINE" light on but also the "CHECK GUAGES" light was also on and the Temp. gauge was pegged at 260. The temp. gauge maxes out at 260 even if I just turn the key on without starting it. All the other gauges seem to be fine. Now after it finally was warmed up a bit it drove fine.
I'm sure I pulled on a wire too hard in the process of the valve cover gasket change. I checked the heat sensor behind the alternator and it looks like it's plugged in and fine, and yes I was able to push down on it to make sure it was plugged in.

 

You only have one temp sensor on your truck, and it is used by both the PCM, and the gauge. If it is reading REALLY hot, that IS going to make it run funky, and turn on the check gauges light. Highly likely the check engine lite is on because of that little problem as well. I suspect you have a short in the wiring somewhere. The harness runs along the valve cover... so, make sure you don't have some wires pinched somewhere.

 

Thanks so much for the input. On enlarging some images I took during the project I see that the blue and green wires going down to the temp. sensor look like I've practically pulled them out from the top of the sensor. I'll check them closer tomorrow and let ya know.

 

I realize that my previous picture was the green & blue wires to the alternator.
But here is the sensor wires and connector, they look fine but with the key on and using my test light sticking the needle into either connector with a good ground on my test light I'm not getting any response from the light at all on either connector.
And the temperature gauge is still pegged at 260 with the temp. sensor connector unplugged.

 

Most sensors are only 5 volts. May not be enough for your test light.

 

You have a short somewhere in the temp sensor wiring. Disconnected the PCM should read very high resistance (=very cold) but it does see very low resistance (=very hot). The check engine light is likely a P011x temp sensor circuit code. Do the "key dance" to retrieve the code or use a code reader.

 

I just thought of something. I remember there being a black ground wire that I reattached to a rear valve cover stud that had another nut on it for that ground wire. Maybe I missed the black ground wire for the temp. sensor and it's back there just hanging around, unattached?

 

I don't know what "the key dance" is. I'm guessing that it has something to do with turning the key on & off a few times and a code will pop up on in the instrument cluster?

 

I don't know what "the key dance" is. I'm guessing that it has something to do with turning the key on & off a few times and a code will pop up in the instrument cluster?

 

This 2nd Gen Ram FAQs, DIYs & How To's sticky lays out the "key dance" steps at the top of the first post: Diagnostic Trouble Code List