I have 98 E-150 and when I bought it the temp gauge after starting was buried in the red but not overheated. I also felt the heater was not blowing hot enough so I thought perhaps the tstat was stuck open.
So I replaced the tstat and the ECT. Well now the temp gauge won’t move off of cold and the heater still isn’t blowing hot, only warm.
online research is showing a 2nd sensor that actually send a signal to the gauge but for the life of me I can’t find where it is located. The sensor I replaced was directly behind the alternator and to the left of tstat.
Would that also be causing my heater to not be hot? It La worth mentioning that it also has a rear HVAC system. It also is only warm and not hot. Both are pushing plenty of air but not hot enough

 

Everytime you open the cooling system to do something , you allow the entrance of air into it.

Something you might consider, When the engine is warm,not hot, temp turned all the way up, feel the heater hoses coming from the engine to the heater at the entrance to the heater , in the engine compartment. They should feel pretty much the same temp as the one hose is bringing warm coolant from the engine to go thru the heater core.
If air is trapped in the heater core, the only way to get the trapped air out is to carefully remove the ,upper heater hose from the heater core while the engine is idling just far enough for the air trapped in the heater , to exit thru the opening. Once coolant comes out, reinstall the hose and the heat problem should improve. If the heater core is restricted/plugged, you may need to replace it and do the same bleeding procedure . Hope you installed a 195 degree thermostat not a 180.
Let us know how you make out ,thanks.

 

Yes I did replace with a 195°. Any idea on why the gauge isn’t working and where the 2nd sending unit is located?

 

What engine is in your vehicle?


If all the other gauges are OK, we can go after the sending unit .

 

It has the 5.4 Triton. All the other gauges work fine. What has me scratching my head is it was in the red and now with new ECT it’s not moving at all. Also from what I’ve read it sounds like the sending unit for the gauge is located somewhere else.

 

The engine temp sensor is usually located in the intake manifold near the front of the engine,

It should be easy to spot because a CTS,coolant temp sensor, that sends it's signal to the PCM has 2 wires while the engine temp sender only has 1 wire.

If that one wire gets a direct path to ground , the gauge will usually go all the way to hot, if it loses the connection all together it will not move from the cold spot.
Some engines had a CHT , cyl head temp sensor located at the rear of one of the heads.