I have ano 02 grand cherokee. I'm having heater issues and not sure exactly what it is. The heat kind of comes and goes a little. It will get hot for a little bit then cool right off. If you turn the control to cold it gets cold then crank it back up as hot as it goes it will get good and hot for a few seconds then cool back off. Thermostat seems to be working fine engine temp gets up and stays where it should. Thought maybe the doors in the dash but not sure and someone said maybe the heater core. But if it was the heater core it shouldn't get hot at all I wouldn't think right?? Not sure what I should check or do here. My local shop isn't sure either without tearing it all apart.

 

I would first suspect the thermostat, sometime opening partially, fully, and not at all.

 

. But the engine temp is where it needs to be. If it's stuck open it should typically stay cool, if it's shut which really never happens then it would overheat. If you turn the temp down all the way to cold and then crank the heat all the way back up it has warm air for a few seconds then cools off again. Not real sure.

 

Then you need to test out the A/C and Heater Control Assembly, the vacuum lines attached in the rear all the way back to the manifold, and the electrical connections. There's a test using a vacuum gauge that is explained in the Haynes Manual. But if you don't have a vacuum gauge you can first try finding it by listening, looking, and felling for any leak.

 

If you have the automatic climate control, there are temperature sensors around inside the cabin that the unit reads to adjust the climate. There is also a self-diagnostic procedure you can activate and it will give you trouble codes, although I don't know how to get it started.

With the manual heater control, the temperature is controlled by the blend door that is activated by an electric stepper motor. That's the center **** that is colored red to blue. The mode control, the one on the right, is vacuum controlled and moves the heat or air to be discharged in different places.

Just for grins, try this. Turn the temp all the way up, put the mode control in recirculate, and the fan on high. You should get heat only thru the dash outlets and it should get real hot and stay hot. This is because you're reheating inside air and outside air may be too cold to get hot at high blower speeds. My heater works fine when the outside air is above 30*, but sucks at lower temps.

One other thing it could be is partially plugged heater core. This may be my problem. When the blower is on low, the airflow is low enough for whatever heat is getting thru the core to warm up the air, but on high, the core isn't reheating fast enough to do the job.

I hope this helps. You can start by getting the heater core power flushed.

 

thanks for the info. It goes in nxt Fri to get diagnosed I will show them these helpful hints and hopefully they can figure it out.

 

thanks for the help. It goes in nxt Fri and I will relay all this info and hopefully it helps.