I have a 1999 Dodge Ram 1500, as of a few weeks ago its has had a weird issue. The cars temp gauge will go to normal temp and drop down to cold. I figured that it may be the temp sensor so I replaced it with a new one, and re burped the system and replaced the coolant. Warmed it up and it still did it.
So today the next thing I tried was the thermostat, changed it out for a new one with stock temp (195) burped the system and let the truck run for 20-25 min, and it is still doing it. I have checked the temp at the hoses and the thermostat. The lower Rad hose is reading 110F and the upper hose is reading 155F and about 165 at the thermostat housing. Seems like the car can not warm up to get the thermostat open, I'm confused on this one. One thing I thought may be causing this is when the heater is on my AC compressor is on and maybe the blend door is stuck and the AC is cooling the heater core down as im getting no cool air when set to cool. I disconnected the AC clutch and its still doing it.

Here are some videos of the situation, one showing the factory gauge and the other showing live data on how the temp jumps around. Hoping someone may know what the problem is. Thanks

https://www.youtube.com/watch?v=fGI1...ature=youtu.be

 

That's a gauge, sensor, or electrical issue. No way it's happening that fast in real time.

 

There's no way the temp is fluctuating that quickly, I think it's gotta be electrical. I'd look at wiring to the ECT sensor and all grounds. For giggles run an aux ground from battery to somewhere near the sensor and see what happens

The worst case scary scenario: PCM issue. But I'd consider that after everything else has been exhausted.

 

Agree on it likely being an electrical problem. As background, the ECT sensor is a negative temperature coefficient (NTC) sensor meaning its resistance becomes lower as temperature increases. If the PCM registers a sudden drop in temperature that would equate to a sudden increase in resistance. Think broken/corroded wiring or similar.

 

How is the heat? Is it steady when it's up to operating temp? It looks like bad connection somewhere in the temp circuit.

 

What's outside temp as well..... What thermostat did you install? That looks like it could also be a thermostat that is stuck open.

 

I live in CA, Temp outside was 75 yesterday, I changed the temp sensor out for a new one and it still did this, and as far as the heat in the cabin it hot.
I will go ahead and look at the wiring to the sensor and see If I can see anything that may be the culprit, just wanted to see if someone else may have run into this before, I though it was totally weird that it fluctuated as fast as it did.

 

Have a scanner that can read live data? See if the computer is seeing the same temp fluctuation..... that will narrow the problem a bit.

 

Already did that, its the second link on my original post

 

That's just it - it goes from 138 to 77 and 168 to 91 (*F according to the scanner) in the span of 2 seconds.

IF we ASSume the readings are accurate..........

.......well they can't be because even a t-stat opening suddenly wouldn't cause an ~80 degree drop at the sensor in under 2 seconds. Nor should a t-stat open at 138 one minute and then 168 a few seconds later..

An air pocket is going to hang out at the highest point and won't "circulate" (not that anyone is suggesting this - I'm just thinking out loud) so it's not like the sensor is seeing hot coolant/air/hot coolant/air and on and on

I'm not imagining anything other than erroneous data here.....which reminds me if you put an N95 on the intake it'll probably solve everything wrong with the truck......?