Delanaire II climate differential controller testing

Hello everyone,
Hopefully weekend is going well.
I have 1985 XJ6
I have various documents, such as the sense system test, but I can find anything regarding electrical testing the “differential control”. Unless it’s just a lever and cable.
For Delaniar III (1989 XJS), it can be tested electrically, based on the testing documents I have.
Basically, the main issue I’m having is that cool air is coming out of the center vent, even if temp selector is set at 85 degrees.
With temp selector set at warmest setting:
When I turn differential controller left, air is very cold.
When i turn it all the way to the right, the air coming out of the center vent is less cold. I would like to have no air coming out of the center vent when temp selector set at the warmest setting.


To confirm, you’re talking about the slide control under the radio, yes?

Good. That’s what is supposed to happen

The slide control is just a temperature trimmer. Which vents are opened or closed is controlled by the amplifier and servo.

The center dashboard vent should be open only when the system is in cooling mode. If I understand correctly this is not the case on your car, so there’s a fault to be traced. But it won’t be the slide control.

First things first. When you turn the temperature control knob do you hear a slight whirring sound? You should. It tells us that the system is at least fundamentally trying to respond to temperature change requests.

If no whirring is heard then the first step is is bring the control system back to life.


Thanks Doug for the info.
I have replaced the temp selector pot so now that is working. I also replaced the amplifier and it does seem to be working.
When I select 65 degrees, I do get full center vent cold air and no air on the bottom ducts.
I believe the cold air must be coming from stuck open air recirculation from outside since still in the 30’s to 40’s in Massachusetts.
I have currently non working AC compressor.


But the various ducts are controlled by the servo, and 'tis the servo that makes the whir. If the servo isn’t working then the proper opening and closing of the ducts cannot occur.

Of course hearing the whir does not guarantee proper servo operation in all respects. But if there’s no whir then the servo isn’t working at all…and this must be sorted out first.


I am hearing the whir sound as i adjust the temp selector.
Applying the sensing system test, the temp selector pin 7 and pin 6, feedback potentiometer pin 9 and pin 8, then pin 9 and pin 5 all tested fine.
Applying power between pin 1 and pin 2 , and then reverse power and ground, drove the servo motor to cool and heat normally.
The only failed test is pin 7 (blue wire) and pin 3 (brown wire), I got 162,000 ohms no matter which temp selected to any mode selected.
Maybe mode switch is not working correctly or something else in the circuit before the mode switch.

Assuming that the Amplifier and servo work as they should, when the system is on Heating mode the center vent is supposed to be closed - no vacuum at the actuator. I suspect that something is wrong in the corresponding vacuum switch at the servo, or the vacuum line is connected at a wrong port.

Are you sure that the servo is indeed at the Heating mode position?
What happens when you put it on Defrost?

Firstly, jdere; a test regime can only do so much…

The system does not ‘know’ that the compressor etc is not working - it just assumes it is, using available ambient air to cool.

Basically, the amplifier compares ‘in-car’ sensor’s resistance with that of the temp control - then uses the ‘error’ to correct the servo’s position (whirring). The aim of the amplifier’s program is to equalise the two, using flap positions, fan speeds, water valve etc to rapidly acquire, then maintain, set cabin temps. All such manipulations are done by the servo - se sole function of the amplifier is to position the servo…

So a fundamental factor, apart from, as Doug stresses, verifying servo buzzing is to know cabin temp. If cabin temp is above the ‘65’ set, the system goes into cooling mode. Which indeed opens the centre vents, and closes others including open air inlet - recirculation mode.

So indeed, if ‘cold’ air enters; it implies the system is not in recirculation mode - flap open to outside air. This may be a PO modification; since there is no AC cooling - he (or she) may have disabled the high speed recirculation switching. This is easily done by disconnecting the electric connection to the solenoid operating that flaps vacuum switch. Another possibility is that vacuum is not reaching the flaps’ vacuum operator, disconnected or defective, or flaps simply stuck, as you say - take your pick…:slight_smile:

You do not mention the system’s behaviour in ‘heating’ mode - which is important for general diagnosis before detailed fault finding. Set the temp control to ‘75’, heat up the engine and set function switch to ‘auto’. The fans should run at high speed, the centre vents should be closed and all other vents, except defrost, open - the air flow should be ‘hot’. As the cabin temps reaches set cabin temp; the fans should drop to low speed - and the centre vents may open ‘ajar’.

This requires monitoring cabin temps; a thermometer is ‘useful’ - ‘feelings’ are deceptive…:slight_smile:

xj6 85 Sov Europe (UK/NZ)

What are the pins you are referring to? I have a similar Delanair problem and so would appreciate any knowledge you can share.

This is at the amplifier, I presume?

I’m not familiar with the test nor the correct values/results. Is this from the Delanair Mark II training guide?


Doug and Scaphan,
The Sensing System test came from a Jaguar Delanair II file that I downloaded from a past Aristides post.
Here is a copy of just the Sensing System Test

Thanks Aristides. I believe you may be on the right track.
I’m pretty determined to get my climate control system fixed.

Thanks Frank for the information.
I will continue investigating with the latest advice.

The system will not work properly without the AC working, jdere …:slight_smile: But the specs mentioned should still gel. For general testing; measuring temps is important to verify functions…

The 7 to 3 measure resistance in the temp selector - and such high resistance indicates a break. Which doesn’t gel with the servo responding to temp set alterations…?

Generally, ohming must be done with wires disconnected at the measuring points, of course - ohming on connected circuits is unreliable/ meaningless…

xj6 85 Sov Europe (UK/NZ)

Amplifier disconnected. Ignition off.
Ohm readings taken from the servo side of the electrical socket.

And measured between 3 and 7, jdere?

With the temp pot replaced (why?), you should read spec values as described - or you have to revisit the area.

Incidentally, when you replace the pot, did you connect up the hose to it? Connected to the motor housing it draws cabin air through the pot to give cabin air temps…

xj6 85 Sov Europe (UK/NZ)

Cansel that, jdere; senior moment - can’t see how
I brought in the irrelevant in-car temp sensor! :slight_smile:

xj6 85 Sov Europe (UK/NZ)

Frank, according to the Delanair II schematic, the blue wire (pin 7), goes through the temp control and multiple sensors. The brown wire (pin 3) goes through the mode switch.

You’re absolutely right, jdere - which also clarifies the different test resistance readings with ‘def’ selected. Don’know whether it was my brain or eyes that failed - they are both getting old…:slight_smile:

The high blue resistance readings still implies a failed sensor - to be tested individually…?

xj6 85 Sov Europe (UK/NZ)