I found the attached diagram but can't figure out how it's supposed to work. My heat is always on full. I understand that applying vacuum to the water control valve should close it blocking water flow to the heater core. What I can't seem to find is the source of the vacuum and an explanation of how the system works. Specifically the function of the vacuum reservoir, the non-return valve and the vacuum valves (nos 4 in the diagram). Any help pointing me in the right direction would be appreciated.

 

What year is your car? Two different HVAC systems were used, depending on model year. Delanaire MkII units were used until roughly 1987, and MKIII units thereafter. I know the MKIII uses electrical solenoids to control vacuum to the valve. The MKII might also, but I'm not sure.
The heater valve is almost always open unless the temperature is commanded near full cold. Flaps in the HVAC box regulate how much air goes thru the heater core and/or evaporator, depending on the commanded temperature vs the actual interior temperature. It may be that your flaps aren't operating properly.
The stock metal heater valve is also notorious for sticking in the on position. They corrode in that position. There are much cheaper and more reliable generic plastic valves available that work without modification.

 

Thanks for the reply,
The car is a 1996 4.0L with the MkIII system (as noted in my signature)
I understand the theory of the valve and flap operation. What I don't understand is what controls the vacuum routing to the valves and flaps. Like, where is the source of the vacuum and how is it controlled. The diagram I attached shows a vacuum reservoir and a number of vacuum valves but doesn't seem to show the source of the vacuum. or what controls the valves. I have ordered the Four Seasons 74803 plastic valve but I'd like to determine exactly what's happening before replacing parts.

 

Go to post #19 of this thread. You'll find a Delanair Mk III manual which should be helpful

https://www.jaguarforums.com/forum/x...-manual-83961/

The Mk III system in your '94 probably has some minor changes that are not mentioned in this 1987 manual but I think the fundamentals are the same

Cheers
DD

 

There should be a vacuum line on the engine. I know where it's at on the V12, but not the 4 liter. It eventually goes to four solenoid valves that control the heater water valve, the blower fresh air intake flaps, the center vent flap, and the lower flap in the box. You can access the solenoids removing the heater outlets/console plate on each side of the forward console. There are two solenoids on each side. With the engine running, there should be vacuum on the inlet ports of each solenoid. The solenoids are controlled by the controller on the right side of the HVAC unit.

 

On pre-facelifts, and I expect on facelifts too, the vacuum reservoir is under a hatch that is accessed through the engine bay. The hatch us on the other side of the engine bay in the position that the brake servo/master cylinder would be in, if the car had the steering on the other side.

 

Thanks guys.
I downloaded the manuals now I need some time to digest the info.

jal1234 said ". . . You can access the solenoids removing the heater outlets/console plate on each side of the forward console"
Me: I hate working under the dash ;-))

 

Me too! My neck aches just thinking about it.
Jon

 

I just found, in a 1994 supplement, that 94 and up cars used a 4 solenoid block instead of individual solenoids. It controls the recirculation, defrost, coolant valve, and center vent. There is an additional separate solenoid that controls "main recirculation"
I haven't found anything that shows exactly where the solenoid block is located.

 

Here are couple of pages that describe the changes.

 

They say "revised to simplify assembly". Not serviceability!
I'm guessing they stuck it deep down under the dash. I'll take a look tomorrow and see if I can find it.

 

Found a vacuum diagram. This helps quite a bit,

 

Remember that the heater valve is either fully open or fully closed. There are no intermediate positions. If it's getting vacuum with the temperature turned to full cold, your problem lies elsewhere. The servos that drive the rotary flaps in the HVAC box may not be moving, allowing air to come thru the heater core at all times.

 

While we're on the subject of repairing Delanair Mk III systems......

Keep an eye open for corroded connections. Many of the wires and connectors are quite small and just a but of corrosion can really bolix things up. I've fixed a couple of misbehaving systems by cleaning the connections. They'll be obvious when you remove the trim panels and can see the wiring....except a couple that are up near the top of the dash and hard to reach




This pic illustrates what I mean. It's the connector for a Del III servo motor

Cheers
DD

 

I agree. I actually do think the problem is with the flap controls. The problem there is that sorting that out will not be a fun job. I thought I'd swap the water valve out anyway as it is quite old and the replacement is an easy and inexpensive swap.