I have a 98 Jeep Cherokee Sport. Normally the heater is so hot I can barely stand it. The other day it isnt getting cold. Luke warm maybe. Today I also noticed a burning plastic smell after a little while. It dont smell it on A/C though.

About a week ago I put in a new switch for the off-road lights. Could I have done something to the heat? I didnt move anything. Just pulled in wire and put in the switch.

Where should I start debugging?

 

on my 94 the switch for the fan on dash melted the plug smell around there whrn on

 

As long as your blower is still blowing the lights have nothing to do with it. I would do a check the coolant level first. If it dosn't have hot coolant reaching the heater core you wont have heat. If you still don't have heat do a coolant flush to try to get what ever is blocking the passageway out. Oh forgot make sure your Jeep is up to temp around 200 or so depends if your fan is on. Next step is is new heater core and you don't want to do that if you don't have to.

 

Just so you know, the temp rides right at 200 all the time (after warm-up) and all the fans and fan speeds work fine.

I only smell it when it is on heat. If I turn the temp dial to cold, the smell goes away.

Or if the temp is on heat and the blower is off, of course then I do not smell it either.

It's getting to cold in arkansas to have this happen. lol

 

maybe some thing in the vents around the resistor area that sucker does get hot

 

Latest: if I don't turn the blower higher than 2 then it has warm air. No smell. So I think it might be two parts. 1. Core isn't getting hot... Maybe needs flushed? 2 blower motor going out? That might account for the burning smell if I turn the blower too high.

All guesses here.

 

I had this problem and have done a back flush on 2 different ocassions and I have GREAT heat now. If you search heater core back flush, you should find something. Pretty easy to do.

 

My heater was blowing cold air too. I just pulled off the heater control valve and srayed it out with a garden hose and pressure nozzle. I forced water in each port of the heater core as well. Some brown junk came out. Connected everything back and the heat blows HOT!!!

 

Unfortunately, I don't have the heater control valve under the hood, it's all incased inside with the heater core, tucked in the dash.