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I have a 2000 DODGE Ram 1500 v8. My heat only work while its blowing low and when I switch speeds it starts to get cold. I almost checked everything that could be check other than the wires.

  • Do you have a temp gauge on your dash? I'm thinking your thermostat is stuck open. Either that or you have a blocked heater core. – Pᴀᴜʟsᴛᴇʀ2 Jan 13 '16 at 1:31
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I'm pretty sure the thermostat is stuck open. The coolant/engine won't heat up fully and you'll get cool coolant through your heater core. In most vehicles (and I'm pretty sure this is how the Dodge Ram's work) the coolant goes from the engine to the heater core directly. If the engine is running cold due to an open thermostat, only cooler coolant will be making it to the core. Your description of the heater putting out warm air when sitting still, then going cool after rolling fits right into this scenario.

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Is it a gradual change? Like when you turn the speed selector dial, does it slowly get cold? Or does it just literally instantly change? Are there any noises you hear when you change from low to different speeds?

This is important. I believe they use HVAC control units in those and that may be causing the problem. There are two motors, one for vent control and the other for your blend door. If there is an issue with the control unit, it can act up. I've seen similar issues electrically in many Dodge vehicles. Dodge (Chrysler) is known for having issues with their HVAC (Heating, Ventilation, Air Conditioning) systems.

  • If I let it set and heat up then switch speeds it want change right away but it does eventually. I check my blend doors and they open and close, I just flush my heater core today and it went clear almost instantly. It doesn't run hot at all. – Virgil Jan 13 '16 at 3:37
  • For those circumstances I would deduce that it would relate to your thermostat. It could possibly be the water pump beginning to fail; Doubtful though. Your thermostat is probably the issue here. @Paulster2 is most likely correct. – cloudnyn3 Jan 13 '16 at 3:46
  • Quick question if my truck isn't running hot how could it be a thermostat. Just asking... @Paulster2 – Virgil Jan 13 '16 at 4:32
  • One of the few purposes of the thermostat is to help the vehicle heat up quicker. When closed it blocks the flow of coolant circulated within the engine. Once the engine reaches operating temperature it will open the thermostat allowing it to circulate coolant throughout the entire system. When the thermostat is stuck open a vehicle will not heat up properly because it is flowing throughout the entire system constantly rather than allowing the engine to get up to operating temperature – cloudnyn3 Jan 13 '16 at 11:34
  • @cloudnyn3 Did you mean to mention "vacuum operated" HVAC controls. This would be correct, but not likely to cause this problem. – Fred Wilson Jan 14 '16 at 5:59
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Some cars have a small electric pump for creating coolant flow around the heater matrix. I don't know if your car has one of these, but if it has, it could be the problem.

  • No extra pumps on these. – Fred Wilson Jan 14 '16 at 5:57
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I have a 1999 dodge ram did the same thing. It was the heater core. Mine would blow heat on low but no other speed. Once I replaced the heater core everything worked great.

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