I was jsut wondering if someone could give me some advice on the following problem. I can start my car, and when it get's up to temp the heater blows normal hot air. But if I have had it on for awhile, turn it off, then try to turn it back on I only get cold air. I also noticed that I have strong airflow out of the side vents, and the rear seat vent, but the front middle vents blow almost no air. I don't really know how the heat can work one minute and not the next??? I also am somewhat stumped on the middle vent problem, could it be a vaccuum problem or a flapper problem? Thanks in advance for the help. Cole

 

The center facia vent opens for A/C, and stays closed for heat. Often times the vacuum servo fails, so it never opens.

Why I brought this up is to know if yours in fact will open with full A/C selected. I had a '91 in last week, the climate control was cycled to full cooling all the time because of a climate control ECU failure. Yes, it happens all the time on Series III and earlier, but this was the first microprocessor type used in XJ40 that I had a problem with. If your center vent never works, perhaps you have the same fault.

Try selecting Defrost with the rotary switch next time the fault occurs, and see if the defrost flaps open and whether you get warm air there or not.

1990 doesn't have a heater water valve, earlier and later do. It would help to know what year your car is.

 

Jagtech,
Sorry for the lack of info, my car is a 94 VDP. I will check the A/C tomorrow to make sure the center vents open up. As far as the heat going off when I adjust the temp, it has only happened a few times. I did try the defrost setting when it started blowing cold yesterday, and it blew air but it was cold as well.

Today I drove the car for a few hours, stopped a couple times, and the heat worked flawlessly. The only thing is, I did not try to adjust the temp or change any settings (for fear if I did that it would not work and I would freeze). How can the heat work until I try and change the temp, and then it won't go back to heat again???

 

The question about the center vent is just a shortcut to trouble shooting everything.

On the '91, the girl drives in freezing and the center vent is open. Nothing changed in response to the temp control. Defrost, cold air only too. So the system is acting just likethe driveris asking for full cold, and it isn't cycling to provide heat through the defrost vents,it just opens the flaps. Her center vent flap servo worked, so her vent was open all the time and the system was "frozen" in A/C mode.

IF your center vent servo is inoperative at all times, those symptoms would match yours. That would suggest to me that yours is going to full A/C mode by itself, and yourcenter vent would be open too if it worked.The upper and lowercooler and heater rotary flaps are positioned by electric servo motors, they are getting a bad signal and cycling to full cold. I have not seen a bad control panel module, perhaps an intermittant fault in the panel would produce the same symptoms. The '91 was perfect with a good climate control ECU plugged in.

So... does the system give you A/C on demand (full cold, MAN) and is the center vent open at that time? If the center vent is opening and closing appropriately, the troubleshootingpath changes.

 

sorry for the delayed reply.
Yes the center vents work with AC like they are supposed to. The last few days the car has acted normally in regards to the heating system. I hate it when cars do this (act up one day and normal the next). I am not sure where to even begin troubleshooting.

 

Agreed, intermittant problems are a nightmare.

I guess one point to mention, now that we know the center vent opens with A/C, is to note whether it opens when you have your loss of heat. If you are suddenly getting only cold air and the center vent opens, the system has cycled by itself to full cooling.

Normally, problems like a thermostat sticking open, heater core blockage, or aheater water valve fault will not cause such a drastic change from good heat to none at all. Loss of vacuum to the heater water valve would result in it staying open all the time, so that is not on the hit list.

It would be interesting to know, when the fault recurrs, if one or both of the heater hoses is hot. Normal thermostat operation would show a good temperature gauge reading and the inlet hose to the heater core would be hot to the touch. Verify all of that if you get the chance, and then see if the outlet hose from the heater core feels close to the same temperature...or not. The location of the heater water valve on a '94 makes it virtually impossible to see if it is open or not, so feeling the hoses is the best indication. If all of this sounds like I'm rambling because I'm grasping for straws, good call.