Ford Ranger Heater Temp Control Not Working?

I have a 1998 ford ranger that had the same problem. The truck ran cold and the heater was cold. It started blowing luke warm a while back then would only blow cold. After checking everything and finding no solution I removed the water pump and found the pump blades worn completely down. the water pump wasn't pumping enough water thru the heater core to provide heat. It also wasn't pumping water thru the thermostat to the heat sensors so I was getting a low temp reading on the gauge. I replaced the water pump and also put in a new thermostat and temp sensors and that solved my problem.

My heat is now good and warm and my engine temp reads a good steady temp. Logic would tell you that a faulty water pump would cause overheating problems but that wasn't the case on my truck. The water pump seals were good and the shaft bearings were good so there was no indication of a problem with the water pump. Over time I guess the coolant had eroded the pump blades down to nothing and I wasn't aware until I removed the pump. Good luck with your issue, I hope this helps.