Both the heat and the AC are on what feels like maximum hot and cold at the same time. I get hot from the footwells and cold from the side vents. The fans will blow low and high, but turning the temp dial has no effect, and nothing blows out of the center vent. When I switch to defrost, it will move to the defroster.

The vacuum hoses are connected under the battery, but there is no vacuum to the heater valve on the engine side of the firewall. I manually closed the valve, so I have no heat, which is fine for now, but doesn't really fix the problem.

It just seems from looking at the threads on the heater that it could be one of many many things, some of which I can't even find, so I'm sort of overwhelmed to the point of inaction staring at the bundles of wires.

Can someone suggest a good place to start running through testing things?

thank you very much, bryan.

If my signature doesn't say, which it should,--it's a 1987 xj6

 

I would replace the Heater Valve, seems like it is stuck open. You will need a new one sooner or later, so start there. Try to replace it with the same part but new.

 

Do you have any vacuum from the vacuum resevoir?

You need vacuum to make these things work. Check the hoses ...one way valve, ... vacuum reservoir,.. Vacuum comes from intake manifold.

Get a vacuum diagram and see how your car compares

Do you have vacuum at the large hose near the battery tray ?

One supplies vacuum from manifold to A/C and the smaller one operates the water valve, again using A/C controlled vacuum to turn heater (water valve)on using vacuum

 

thank you. i will check the vacuum tonight. can i ask two questions: 1) where is the reservoir in the engine bay? (I couldn't find what i was supposed to be looking for), and 2) i presume i can just pull off one of the hoses and feel the vacuum on my finger? (I tried that at the heater valve and could feel nothing so plugged a syringe in to see if the plunger would move and also nothing, so assumed no vacuum was reaching that point)

(I promise i'm not lazy, just a little dense)

thank you

 

I assume you have an American model , and the vac reservoir is the little round tin that lives behind the power brake booster has a vacuum hose stuck on it. Take care of the hose sizes from firewall connecitions...smaller goes to water valve.

Vacuum comes from under the manifold , I am trying to remember what things were like when I had the Jag 6 in my car . My car is now 350 powered. So guessing a bit. Pay attention to the Ts and one way valve orientation.

Also you may have a vac line off inside the climate control but I would start by seeing if vacuum is present on the line connected to the "intake manifold connection nipple" when engine is running.

 

thank you so much. I have vacuum from the manifold to the reservoir (boy is that thing tucked away) and then all the way inside the passenger cabin, but none of the vacuum is going to any of the colored wires or to the heater valve line. yet the defroster works and I can disconnect it and feel a vacuum.

What should I check next? I know i'm bothersome, but thank you guys so much.

 

So to be clear,...the car is running and the climate control is turned on and a temperature selected?
If so what do you hear ,..what is idle speed of engine ? normal ?

 

Yes the car is on and idling normally. Turning the temperature gauge inside seems to have no effect at all. changing the fan setting will change from low to high and redirect to the defroster, but nothing else. I can hear a whistle from the front of the engine that I can't decide if it is a vacuum leak or just a whistle on a belt.

 

driving down the highway at 70 and the a/c switches to the center console vents. i am reluctant to change anything, because it's working, but i'll turn the temp gauge on the way home tonight and see what happens. (I still have the heater valve clamped closed, so no heat, but maybe the a/c will turn off)

i'm excited.

 

ok, moved the temperature gauge and it changed the direction of the airflow (once the airflow finally moved to the center vents), but the compressor never kicked off. thus, even at 85, the ac stayed on. (the heat can't kick on, because i pinned the heater valve in the closed position)

any thoughts on where to go next. i obviously need to drive around long enough to warm up the car or hit whatever sweet point occurs that causes the center vent to get vacuum and open, but i'm stuck as to what to poke around with next.

 

The big outlet in the center of the dash, right?

Which way did you turn the temp control?

This should open only when max cooling is needed...so if you selected a temp that is way colder than existing cabin temp, the system did the right thing


It isn't supposed to kick off. The system is designed to have the compressor engaged *in all modes*. All incoming air is refrigerated first and then heated as required. This dehumidifies the air.



Let's go back a bit.

When you slowly turn the temperature knob from one extreme to the other do you hear a faint whirring sound? You should.

Cheers
DD

 

I didn't realize the compressor stays on all the time. I know I don't hear a whirring when I'm turning the temperature gauge while parked in the driveway I will have to take it on a drive and wait to see when the vents starts working and then turn the temperature gauge and see if I hear whirring. Thank you for the continued help

 

No whirring = the control servo isn't working. The servo is what executes all the changes in the heating/cooling/defroster flaps, the airflow direction flaps, and fan speeds.

Driving or parked really shouldn't make a difference as there's nothing in the system that 'knows' if the car is moving or not.

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

Amplifier failures are common. Servo failures not common.

But, first, have you found and checked all 4 system fuses? Two in the fuse box, one clipped to the RH side of the heater case behind the console RH cheek panel, and one on the amplifier ground wire behind the console LH cheek panel

Cheers
DD

 

I will check the fuses again also. I've checked them before, but I do tend to blow them any time I poke around.

 

I finally got back around to this and replaced all four fuses--they looked fine, but 2 were old lucas ones, so all new now. when i set the controls to full cold, I get heat and cold at the same time until, randomly, the car will change to max cooling by switching to the center vents and turning off the heat from the lower and rear vents. just prior to that, I can hear whirring from the center dash. it will stay in the correct position for awhile, and I can turn the temp gauge and everything works fine until it stops working, and I get max cool from the side vents and max heat from the lower and rear vents again.

it seems like I may need a new amplifier, if I understand the last post. that seems like a hard thing to replace, so I might be happier if it's something else.

thanks so much for your help