I have a question about the automatic climate control in my 1977 XJ-S. I notice that when I first start the car, the fan only works on DEF. After the car warms up, all of the other settings work. The only problem is whether on DEF or any other setting, the heat doesn't get hot and the a/c doesn't get cold. I think the problem may lie in my electronics in the car but I am not sure. Anyone have any thoughts?

The rest of the car is in good shape, no overheating, no slipping belts, the hoses all get nice and warm like they should but the compressor is not engaging. Don't care much about the a/c right now more about the heater.

 

"Def" setting bypasses the automatic controls and defaults to full heat and highest fan speed.

If the temp selector is set to call for heat the system will remain inoperative until the coolant is hot enough to actually provide heat. This is accomplished by a temp sensor attched to the inlet pipe of the heater core. You can jumper the switch to eliminate this function....and see what happens.

"No heat" could be an inoperative heater valve (not allowing hot coolant into the heater core) or a heater valve not being commanded to open....which could have a variety of causes.

Set the mode control to "lo" and slowly move the temp control knob. Do you hear a faint whirring? If so, the system is recognizing to requests for temperature change and the servo motor (which makes the whir noise) is adjusting the flaps accordingly.

No "whir" suggests the control portion of system is inop and not recognizing requests. Check/clean/replace all four system fuses as a first step.

Cheers
DD

 

Thanks for the good starting point. Makes no whirring noise when you change the temp, one of the reasons I thought it was electrical at first. The temp dial seemed to do nothing. It is nice out here today so I will take a stab at fixing it today.

 

My heater control valve is doesn't seem to do anything. I am pretty sure that this is the problem. The car, once properly warmed up, allows all of the settings to function (like you said they would) only it does not get warm. It stays the same temperature. I checked the valve which has great vacuum but is rusted and seems to be stuck. I will go get a fancy plastic one tomorrow and try it out.

 

Beware the "stuck open" thermostat as well. My 95 made for hot hoses and all, but I never got warm air until I replaced the thermostat. It would run about 160Deg F, vice 190 with ambient about 70. I would've thought I'd get some heat from that, but it wasn't so.

 

I was thinking the same thing with the thermostat as I have had that happen in a number of cars. However, the temperature gauge sits where it is supposed to and the car seems to heat up fine. The system seems like it is under pressure as well. The other thing is that the air when the heater is on doesn't blow lukewarm or a anything it is just kinda cool like when vents are open. Since it is a cheap fix and I am new to this particular car, I will probably buy a new thermostat anyway. Thanks for the reminder.

 

You'll need two thermostats and make sure you bleed the cooling system correctly.
Make sure to get the correct, Jaguar-specific thermostats

Cheers
DD

 

Good thinking.

Bear in mind that you'll need a heater valve that closes with vacuum and opens without vacuum.

If you're also gonna replace the thermostats (or anything else) on general principles then you should do it all at the same time so you'll have to bleed the system only once.

Cheers
DD

 

Mine was rusted shut I used a Ford heater valve I got off eBay worked a treat and quite cheap

Heater Valve Tap 003 Falcon EA EB ED EL EF AU 6 cyl | eBay

cheers
Warren

 

Ok, here is my update. After reading some literature and driving the car around a lot, I decided to not just replace the thermostats for no reason. I mean the temperature gauge works properly and starts on C rising to N once the car is warmed. Wether driving or sitting still, the temp stays solidly on the N. I know in previous cars I have owned with no heat due to stuck thermostats, the car would run cold all of the time (obviously) and that the temp would tend to drop at highway speeds.

Actually in my daily driver, the thermostat failed during the summer but I didn't notice the problem until fall and it got below 50 degrees out. On the coldest night before I replaced that car's thermostat, I regularly varied my speed with bursts of full throttle to get some heat out of the system. Today it was 41 out and the Jag warmed up just fine and still did not blow heat.

I did replace the heater control valve with a new one I found a plastic replacement and now my heater hoses are both nice and hot both before and after the valve. I do not smell any antifreeze nor have any leakage or steam inside from the heater core itself. I am not ruling the core out yet but it so far seems unlikely.

My fan control is working properly as the system functions on lo, hi, auto and def without a problem. I am however suspecting an issue with the temperature control knob itself. No matter where it is set, the air coming out is the same temperature. I tried turning it up and down both with the car running and with the engine off (so I could hear) and I did not hear anything. I pulled the fuse under the drivers side and it was ok. There are four more fuses for this system? Where?

I am suspecting that there is a problem with the automatic climate control be it the temperature sensor or the control knob itself. Anyone have any experience with this stuff? I would like to repair it properly but would have no problem wiring in a simple switch (probably hidden) and bypassing the original equipment if that was necessary. Any suggestions on where to go next?

 

The way the system works in auto to control the temperature of the air is to turn the heater valve on allowing hot water into the heater element, it then uses blend flaps to control the the amount of hot air passing through the evaporator hence controlling the sir temp.

My guess would be these blend flaps which are controlled by servo motors are not operating correctly. A quick check is to pull the blower motor fuses and then turn the temp control from cold to hot and you will hear the motors running.

cheers
Warren

 

Update:

Sorry for the delay, I have been busy lately and had no time to post or work on the car. Warren, I did not get your post until just now and at my next availability, I will try your test. I am thinking the same thing as there is little left to be wrong. I noticed a small coolant leak near one of my thermostat housings and I had a leaky gasket. Since I was taking it apart anyway, I put new thermostats and gaskets on both sides. I notice that the car seems to warm up much quicker than before so I am wondering if they were bad. Since I had to to that, I tried the heater again just in case I misdiagnosed something. It still doesn't work but I paid more attention to the system and heard no noise as I turned the temperature control knob. I never thought to pull the blower motor fuse, I will happily try that. If there is a problem with the servo's or the flaps themselves, where the hell are they? Behind the controls or up higher under the dash?

 

No gentle whirring noise when you turn the temp control?

That means the servo motor isn't operating, as the "whir" is the servo motor.

The servo motor takes orders, so to speak, from the amplifier.

The amplifier compares info from the temp control and cabin sensors and turns it into 12volts to operate the servo motor.

The servo motor operates the linkages and rods which rotate the internal flaps.

The position of the flaps dictates if/how much air goes through the heater core and where the air is directed---upper vents, lower vents, or both.

If all of your fuses are good you *probably* have a dead amplifier. It's the most common culprit. But it'll run ya $200-$300 for a new one so it's a pretty expensive guess. A good guess, but an expensive one. Problem is, if there are other issues with the system it's hard ironing them out *unless* you have a known good amplifier. Catch 22

Cheers
DD

 

There is definitely a problem with the amplifier. I read on another more recent post about bypassing it by applying power and ground from another source directly to the servo. I did so and my servo works fine as does all of the little flaps and linkages.

I set it to full heat for now (as is it cold out) and left it. I am wondering if I can just connect a hidden rudimentary switch and just manually operate the servo between hot and cold as I feel necessary. Any thoughts on that?

Also, I detected another issue. I replaced my heater control valve with a plastic one as the heater hose going into the car was not getting hot. With the valve replaced that hose gets hot and allows the heat to come on. I have notice that it doesn't seem to stay hot for long and soon starts to cool giving lukewarm heat in the car. I find that when I disconnect the vacuum and then reconnect it, it seems to get really hot for a while and then cool off. I am wondering if the valve I picked up works in reverse like the vacuum causes the valve to close and not open. If this is the case, I will have to change it again. What would be the harm in eliminating it all together?

 

Ignore my last paragraph please, I am just apparently an idiot. The heater control valve shuts off the flow when vacuum is applied hence the reason why the heat was not staying warm after I reconnected the line. I feel like an absolute moron for missing that.

At least I figured out pretty certainly that the amplifier is malfunctioning. Just to assure I am correct, when I adjust the temperature control my servo should move and adjust the linkages right? Because no matter where I turn the knob, nothing happens whatsoever. No noises or anything moving (it is currently all apart from the console forward). Is there anything else that I should check before settling on the amplifier? I checked and replaced all four fuses already and I am able to work the servo by applying power and ground directly to it. I have heat and I believe A/C but it is too cold to be sure.