I just bought an XJ and am trying to get the heat working. The heat valve broke the other day when replacing the valve cover gasket so that was replaced 3 days ago. I hadn't tried the heat, but I was told it wasn't working when I bought it. Today I was flushing out the heater core because I was doing a coolant flush anyways. I was running some coolant system cleaner through the system and turned the heat on full blast. I let the car run while adding water and then went to check the air temp coming from the vents. Lo and behold there was no air coming from the vents. I turned it on the "max" setting and air started coming out at 136° F. When I switch it back to the "heat" setting all air turns off again. I had previously noticed that there was a plug that had the other end broken off, seen in the attachment, not sure if that has anything to do with it, but I would also appreciate if anybody knows what sensor that is for.

 

Really not sure what is going on with your heat.
But my guess is that connector is for the under the hood light.
So has nothing to do with your problem if that is the case.
Most guys just delete the heater valve and run the '97 and up hoses, which don't have the valve, on their '96 and down.
I do have to wonder about the heater control itself though.

 

Does it blow out the defrost vents when you switch to defrost? What about the floor vents when HEAT is selected? When on Vent, is it blowing out the defrost?

When on MAX, it closes the outside air inlet so cabin air is recirculated and should also engage the AC compressor (assuming your AC works).

 

Thanks for the response. I won't worry about that plug for now. Looks like you were right.

 

I'll have to get back to you on this. I spent the weekend redoing the drum brakes and temporarily forgot about the heat thing.