Dodge Truck Heater



I just purchased a 98 Dodge Ram 1500 4x4. 151000 miles. Mechanic confirms truck is in really good shape, dry underneath and functional.

My problem is this, the heater heats fine on low and medium, but when on medium high and high (full fan w/hottest setting) it tends to blow luke warm air. As if there is too much air going through the heater for it mainain a hot temperature. I have yet to check out the A/C portion of the truck since it has not been above 35F. I have the Haynes book but it does not give any troubleshooting for the hvac system. Any suggestions?


does it have a temp gauge or an idiot light? is the temp in the middle of the range? does it stay there? does it vary?

have you checked the radiator hoses? are they hot after you have the truck out on a long run?

if the thermostat is bad, it will let the water stay too cool, so you would get less heat than normal inside.

BUT, the heater core may be clogged too. but you first eliminate the thermostat as the simplest problem.


also, look in the engine cooling portion of the manual, since that is where the info on the t stat is.


Guage operational, registers below lowest temp when first started, after a few minutes it rises to 190 and stays, engine operates at this tempurature at all times there after. Hoses are hot but are not swollen or cracked.
Radiator and hoses alll secure.
Coolant is bright and semi clear. Level is normal when cold and cap off. The engine draws coolant from resevoir/overflow when needed.
Have not checked on the core flow yet…will need outside tempurature to warm up to at least 45F to work on the truck out side.


I did look there, no trouble shooting chart or proceedure listing. I will check the flow on the core when it is possible.


well, with that info, it sounds like the thermostat is ok, BUT, the thermostat is so easy and cheap to change, i would change it anyway.

next is the heater core.

by any chance is there evidence of a radiator leak, or other water damage to the radiator? this could indicate a radiator stop leak may have been used, which may clog up the heater core.

on a different note; does the temp get hot on full blast, until you change the temp dial? (try shutting down the truck, turn off the key, put the temp dial all the way to HOT and then restart, notice any difference? if this changes the heat level, turn the dial down and then back up. does it come back hot? if it doesn’t , the diverter valve needs to be changed. this valve is in the heater hose loop, where the hoses go through the firewall. this valve would also effect the AC.

if you aren’t up to diagnosing this, take it to a radiator AC shop. they should get this diagnosed, and fixed in an hour or so, unless the heater core needs replacing, then it’s more.