Yes it makes no sense, but apparently it is what is happening…
This is a part of the A/C electrical diagram that I have, and it’s clear that the servo (6) can override the Ranco switch (4). It’s even stated as “Thermo Override”.
If by “override” it meant to open it when it would normally be closed, it would have to be wired in series with the Ranco. It is not, it is wired in parallel with it. That means its only purpose is to close that circuit when the Ranco would have it open. As I understand it, the Ranco would only have the circuit open when the evap is so cold that it’s at risk of icing up. Am I right so far?
It’d be helpful to know when the servo decides to close that override. I’m having trouble thinking of any situation where you’d want it overridden. It’d be nice to override it in a desert when it’s too arid to freeze up, but how would the servo know you’re in a desert? There are no humidity sensors that I know of.
Typically, the reason the Ranco would need to open the circuit is A) the fan is running so slowly that it’s not moving enough air to keep the evap’s temp up, or B) the ambient temp is so cold that the evap easily cools it enough to approach freezing. But in neither case would it seem a good idea to override it.
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That is indeed the function of the Ranco, Kirbert - cutting power to the clutch to stop the compressor if the evaporator gets to cold.
Like you, I have trouble understanding the logic behind the override - which clearly is shown in the diagram, also provided by Aristides, that the override switch is indeed in parallel with the Ranco. Providing power to the clutch when the Ranco power is cut.
Nominally, this allows the evaporator to cool below +2C - which indeed gives cooler air. There was a cryptic remark in the manuals’ excerpts provided by Florian; overide is induced by the last 25 degrees of servo travel - or words to that effect. However, system is initially in ‘full cold’ to quickly acquire set temp - ie, the servo is in full deflection. Which then means that the Ranco is always bypassed in full cold - which makes no sense…
The compressor is going ‘full bore’ whenever engaged - though it’s cooling effect may increase with rpms. As the compressor cools the evaporator; the air temp heats it. So either the cooling capacity is adequate to drop temp below +2, and the Ranco disconnects as intended - there is adequate cooling for maintaining set cabin temp. Or the cooling capacity is inadequate to get the evaporator to +2; to high for tripping the Ranco - and the bypass is pointless.
Certainly; in dry desert air, there is little danger of icing, but there is no sensor for air humidity, as you say - nor any other control to induce override, manual or otherwise. It is taken as given that the ‘65’ and ‘85’ on the temp control is meant to acquire those temps - not going on cooling or heating in these extreme settings. However, this may be a misinterpretation - in which case a simple statement in the manuals would clarify it; which they don’t. Which they should, to avoid misuse of override with unintended results; it’s a common misunderstanding of thermostats that setting the temp higher or lower than the intended temp to get there quicker…
Consolation, there are no reported incidences that the override has caused any problems…?
Frank
xj6 85 Sov Europe (UK/NZ)
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As I wrote the other day, I installed my switch in line with the Ranco. Hence, this infamous override mechanism still should be able to do its job. In addition, I installed a small control lamp which tells me if the compressor clutch is engaged, so I will see if the Ranco and/or my manual switch are overridden by the Servo.
I did some test driving, and no results yet. Even simulating high cabin temperatures (by tricking the sensor mounted at the passenger side) did nothing. Of course, there were no high ambient situations at all, hardly 10°C, at most. Let’s see what spring/summer brings – or maybe I’ll seek for some way to trick the sensor for the ambient temperature as well. The question is, when does the servo enter the last 25 degrees towards max. cold, and how long will it stay there.
I mean, if there was to be any logic in this system, it could only be that at very, very, super hot ambient *and in-car temperatures they wanted the compressor to run, no matter what. I can’t think of any other explanation. But I willingly admit that this very theory falls short due to the fact that it risks the health of the evaporator unit, which is not what anyone could seriously want to happen.
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The best idea is to observe the servo operation, Florian - at ‘full cold’…
Set temp switch to ‘75’ and the function switch to ‘auto’. Applying heat at the in-cabin temp aperture will simulate a hotter than 75F cabin. The fans should run at high speed and the center vents should open - verifying correct system reaction…which is important.
The servo should travel to full cold position - to be observed at the servo. Revealing if there is room for more (25 deg?) travel - and if the override cam has activated the microswitch. Then turn the temp selector fully to endstop ‘65’ to see if the servo turns further - clarifying if such setting induces override…
At present, with 10C(?) the servo is set to heating - as the cabin temp is also at ambient temp? The test will not simulate extremely hot outside temps, of course - but the ambient air temp sensor is assumed to be just a bias sensor for setting the flaps. Ie, depending on outside temps the AC amp sets the servo to slightly hotter/colder to reduce system cycling to regain set cabin temp…
As an aside, setting the system ‘auto’ and leaving the temp control at your preferred cabin temp - the system will automatically acquire the set temp as quickly as possible. And further; if heating is called for, and engine temp is below 42C - the fans will not run…
Frank
xj6 85 Sov Europe (UK/NZ)
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It can override the Ranco and your switch?
Yes, that’s the current layout. I want to observe things a little. My main purpose is of course to switch off the compressor when AC is not needed, so the override mechanism (high ambient temps.) should not interfere. When I fully understand what’s going on, I might just change the installation to be able to totally cut off the clutch. It’s just about relocating the wires a few inches.
Florian,
You mentioned “you want to cut off the compressor manually”. I think you may be “over thinking” this objective. I likewise do not want my compressor to run all the time either. Simply, I installed a switch under the steering wheel to flip the compressor on/off. I simply cut the power wire to the compressor running it to the switch (with additional wire of course). From the switch, I ran another wire to the the compressor (where I had cut it). I put a fuse inline for protection. I did not mess with anything such as the servo, etc. I simply flip the switch to allow power to go to the compressor via the switch. It has worked perfectly for me for 15 years with 5 XJ6s!
Richard
Sorry, Richard, that might be just a misunderstanding. English isn’t my mother tongue, and I should habe been more precise. What I meant is this: Right at the moment I control the clutch via the Ranco wiring. That means that the override mechanism can still engage the clutch even if I turn my switch off. A small lamp will tell me if this happens. The alternative would be to install the switch at a slightly different position in the circuit, then being able to cut off the override mechanism as well. I might do that some time later, don’t know yet. As I already said, this whole topic has become purely academical. 
PS: My switch for the compressor clutch is located in the ashtray on the driver’s side, where it is accompanied by a switch for the fresh air mod. Control lamp sits in the other ashtray. Looks quite good, and can easily be hidden by closing the ashtrays. I’m clearly not the first one who’s done that, but let me say, it’s a very convenient solution.
Which colored wire have you spliced the switch into, Florian…?
Frank
xj6 85 Sov Europe (UK/NZ)
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I didn’t cut a wire, I just disconnected the green/brown line from the Ranco and set my connections for the switch there. Therefore, setting the switch to “off” is exactly equivalent to the Ranco cutting the current to the compressor clutch.
Fair enough, Florian - the diagram shows the override switch connects to the clutch further downstream…
I assume your lamp is connected to the ‘clutch side’ of the switch. I would be surprised if the lamp ever comes on…
Frank
xj6 85 Sov Europe (UK/NZ)
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Dear all,
The academic riddle is solved. Today I checked my evaporator unit for dirt and possible leaks by means of an endoscope cam (good news from there: everything seems fine, just a little rust on the housing of the Delanair, nothing to worry about). And, as I already had the side panel removed, I took a closer look at the servo. It was a bit fiddly, but finally I was able to get a glimpse on the Ranco override microswitch (see pics below).
The first picture (unluckily quite out of focus) shows a servo element (white) pressing against the switch (that is, activating it!). I ran the car the other day and set it to full cooling (65° F) before parking. Neither the in-car nor the ambient temperature was very high. But obviously turning the temp selector to full cooling was sufficient to get the override mechanism going.
So, you might ask, did the compressor come on? Well, it didn’t. The reason for that is easily seen on the second picture: There is no current source connected to the microswitch, and as far as I can see, there never has been. Everything in there looks as if nobody never ever had done any work down there, so I assume Jaguar left the microswitch in place, but decided – no later than 1983 – not to use the mechanism any more. Did they want to protect drivers from going into desert mode and thereby unintentionally damaging the evap unit? Curiously enough, the wire leading to the compressor clutch circuit (green/brown) is connected to the microswitch. Why they didn’t fully disconnect it? A little bit of mistery remains.
Here are better pictures of the microswitch and the servo, just for orientation:
See especially the last pic of the series. There, the override switch is fully connected, and it’s activated as well.
Best,
Florian
1 Like
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Since ‘desert mode’ was not an option on ‘our’ Jaguars, Florian - just turning the temp selector to ‘65’ would obviously engage the override. Which inevitably, with the cooling system working properly, would freeze up the evaporator, unless really torrid. And before or later cause damage as any water in the refrigerant would also freeze. And as gullible drivers may turn the selector to ‘65’ for a quicker cool-down, oblivious to the fact that the system, set to ‘75’ would cool just as quickly - disabling the override was a safer option…?
As Jaguar bought the servos wholesale, the override switch was easiest left in place and power just left unconnected. Leaving the option available, with the compressor connection present and correct, for systems with clearly marked ‘desert mode’ or for connections in Jaguar markets where the desert conditions do exist - and with better explanations for the driver? And Jaguar was never famous for keeping wiring diagrams updated or individual car specific…
You have gone above and beyond duty on this one, Florian - thank you…!
Frank
xj6 85 Sov Europe (UK/NZ)
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Um………. No.
But I can’t really see the advantage of overriding the switch. But, the system does have a full cooling mode.
There are two independent factors at work, David. The Ranco controls the evaporator temp - without the override the Ranco is in full control. And I certainly agree; the override seems no advantage compared to the downsize. And water, anywhere, freezes below its freezing point…
The second factor; as the set cabin temp is reached, the serve turns into ‘cabin temp maintain’ position - nominally taking the servo out of the ‘full cooling’ mode?
Frank
xj6 85 Sov Europe (UK/NZ)
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