2010 Laramie 1500 5.7 - this happened this morning - temps 32F outside. Coolant temp gauge did not move / checked engine monitor stated -40, rad fans on and blowing, then check 4wdservice light came on. Truck did not overheat, drove fine, all other engine numbers seemed good? Trany temp good.
Any thoughts? The winter front is still on - last week -25C and -35C with windchill! Maybe open 1/2 the fronts?
Thanks: Ed

 

Finally, Someone Else!!!! My 2010 TRX4 1500 5.7 started doing the SAME EXACT THING about a month ago!!!! 4wdservice light, and no coolant temp. Restarting fixes the issue. I live in GA so Winter is fairly mild. It started when it dropped to around 30F. If you turn off the truck then back on, the engine has a major hesitation when starting, probably about 5 seconds of it trying. If I turn it off then wait about 5 mins, it starts fine. It is every morning that it is doing it.

 

Mine does this abolut twice a year - always after it gets cold out ( - 25 C), or if it's gone in a warm garage for say an oil change or service. Usually if I disconnect battery for 20 minutes ( may take a couple of tires) it will clear itself for about 6 months or so! Not sure what's up with it??? Or is it a Ram thing?

 

if the coolant temp sensor circuit is open the sensor will read -40*, the rad fans will come on and a dtc will set.
since the ecm has no input to coolant temp with a open circuit, it turns the fans on as a precaution. have the code read and post it here. its an easy diagnostic.

 

Code reads P2181- Cooling system Performance

 

Will need to get a Code reader!

 

unplugged battery for about 3 hours and replaced the coolant temp sensor, didn't fix the issue. Crap...

 

this dtc sets when the pcm detects the ect reading is not transitioning smoothly or changing at all.
-make sure the coolant level is not low...air pockets in the system can set this dtc.
-after the vehicle has sat overnight and is cool. start the engine and monitor the ect reading with a scan tool. it should increase smoothly. check the upper rad hose and scan tool reading to confirm the thermostat is starting to open at 80*C. When the stat starts to open the scan tool voltage will drop slightly and then continue to rise. the stat should be fully open at 100*C. If not replace the thermostat
-if those are ok check the ect circuit
-unplug the ect sensor connector. a dtc for sensor circuit high should set.
-one of the wires should have reference voltage...5volts...check with a voltmeter
-with a jumper wire connected across the two connector pins; a dtc for sensor low should set
-if you have all this...ect sensor replacement is indicated
-if that still does not fix the problem, you may need a pcm

 

-40 on the ambient temp sensor is default for open circuit. It's located in the left mirror and is replaceable. I can't remember what the default for engine temp is sorry.

 

I replaced the Thermostat Last year, I guess I will flush the system and replace the thermostat... again. As stated above also, I already Replaced the ECT sensor and cleared everything, This morning, Same thing happened, no engine temp showing, Lighting bolt light comes up on the dash, and Service 4wd light comes on. If I turn off the truck then back on, everything comes back to normal. Except the P2181 DTC is there.