AC Ranco Override

XJ
1

Dear all,

while flipping through the pages on the AC system in the workshop manual as well as through the wiring diagrams, I noticed that the AC system includes an override switch for the ranco thermostat. The workshop manual says that the Ranco will be overridden if max. cooling is requested and there is no danger of frost for the evaporator. Unluckily, neither the workshop manual nor the wiring diagrams give sufficient information on why this override switch, which is part of the servo unit, is installed at all, and under what conditions it will be activated, that is to say, where it draws its “information” from.

Any clues on that? I am planning to install a manual switch for declutching the AC compressor, and I am yet not sure wheter I should give the servo the chance to override my manual switch as well, or not. It would be much easier to speculate on that topic if I understood the idea behind this override mechanism.

Regards,
Florian

XJ12 Ser. 3 H.E. '83

2

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To me, Florian - the ‘information’ is pure hogwash…

It is not mentioned in my more elaborate manuals on the Del 2 or 3 - and would in any case be pointless. The Ranco is there to prevent icing in the evaporator, which is absolutely essential - and the system runs max cold until cabin temp reaches ‘set’ temp. Overriding the Ranco means re-engaging compressor, which is only relevant if it is dis-engaging - which it does if the evaporator temp is below +2C.

Which means that there is(!) already danger of frost in the evaporator! Ie, the system will run as cold as it can while the compressor is engaged and controlled by the Ranco - and overriding the Ranco invites freezing…

To remove any lingering doubt; cutting power at(!) the compressor clutch will unavoidably disengage the clutch - whatever happens ‘upstream’. So your proposal is safe; you’ll lose cooling and desiccation of vent air - and the system reverts to using ambient air for cooling…

Frank
xj6 85 Sov Europe (UK/NZ)

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Dear Frank,

I absolutely agree with you – I see no real point in overriding the Ranco, and I am still puzzled by the statement of the manual. However, the wiring at the Ranco in my car does have those two extra lines which lead to the servo and might be connected to an override switch there (didn’t open the device).

After doing some more research, I just found some information on the topic in the Delanair Mark II Service Manual for 74–87 XJ and 76–87 XJS. There it says on p. 33, regarding the Ranco:

“The thermostat is conductive in all modes and functions except when evaporator temperature falls below 2°C (36°F) or when overridden by Ranco override switch in high ambient situations.” (p. 33)

and with respect to the servo unit’s components:

“Ranco override microswitch: To override Ranco thermostat in full cooling mode to allow minimum cool down time.” (p. 38)

Does this mean that this microswitch only opens for a small amount of time? Is it supposed to avoid the clutch from going off an on and off and on in critical situations?

Anyway, I gess, there is not much we can do about this. One might cut the wires leading from the Ranco to this microswitch, but I think I will just leave it the way it is an set my manual switch into the line feeding the compressor clutch, that is to say after all these mechanisms.

Regards,
Florian

4

As I’ve related before, the A/C in my FF’s 1968 AMX had a temp knob that rolled all the way around to “Desert Only”. Living in Florida, that’s pretty much where we left it. So one day we’re on a long drive, and after an hour it seemed like the A/C wasn’t working very well. I peered through a vent and saw a lot of white up in there! HUGE block of ice! We turned the A/C off for a while, and by the time all the white disappeared it was cooling like it should.

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Are you sure you have the override switch?

At some point it was deleted… although it would take quite a bit of digging thru notes to tell you exactly when.

Cheers
DD

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Hi Doug!

No, I’m not sure. I just came across the switch in the manual and in the diagrams, and I can see that the corresponding wiring is installed in my car (what basically implies nothing at all: there are also wires for cold start injectors and all sorts of stuff no longer needed in 1983). I see no easy way in checking for the existence of the switch, be it not by digging into the servo unit, which I would not want to do if not absolutely necessary. Or is there an easier method? As we have winter here in Europe, max cooling is rarely needed at the moment, so the switch won’t come on naturally.

I am actually thinking about installing my manual switch before the override mechanism, so that it integrates into the system just like the Ranco does. During summer I should then be able to manually declutch the compressor via the modified Ranco wiring and see if it comes back on by means of the override microswitch. If yes, the switch is there, if not, it’s gone.

Regards,
Florian

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What good does it do to override it? I thought it increased cooling capability since in only turns off below 2°, and the air would either be cool by then or icing up and blocking the flow?

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In principle the colder the evap gets, the more thermal gradient there is for removing heat from the cabin. Condensing/freezing water from the air obviously blocks the air flow, negating the advantage of below freezing evap temperature. But, with a fixed setting, the Ranco is designed to avoid freezing in the most humid air. Completely dry air could never freeze the evap OTOH. So setting the Ranco lower for less humid ambient, or eliminating it altogether for desserts seems realistic. In the Series 1, the Ranco is continuously adjustable. IMHO.

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I see, but if the matrix goes below 0°, the condensation that will invariably form (unless in a desert maybe) will begin to freeze and reduce capacity, so if it’s not cooling enough the matrix is too small or the air is not hot enough or too slow. It’s probably set to 2° as a safety margin so it could be set a little lower.

Of course if it was designed to be bypassed it makes more sense than I think.

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This doesn’t even make logical sense, Florian…:slight_smile:

Ranco ‘conductive’ presumably means power is free to flow to the clutch, engaging it. And ‘override’ in the above context must mean that the clutch power is cut - same as the Ranco does. Are you sure you are quoting the manual correctly…? :slight_smile:

The cabin temp is regulated by flap settings; after passing through the evaporator, the air is directed by flaps past (in ‘full cold’) or, partially or entirely, through the heater core for suitable reheating. Ie; the running compressor always runs the evaporator at maximum cold - the ambient air heats the evaporator. If ambient air is not hot enough to keep the evaporator above +2C - the Ranco disconnects the clutch. Which, with an efficient set-up, cycles compressor on and off as required at ‘medium’ air temps…

As the system presumably runs ‘full cold’ until set cabin temps are reached, the only way to speed up cabin cooling is to lower the evaporator temp - the result is vividly described by Kirbert, whose AMX obviously has an extra setting. He describes external evaporator ice - but any water in the refrigrant would also freeze. Which is not good either.

That said; there is a vague possibility that the Del 2 or 3 has a more gradual transfer from ‘full cold’ to ‘maintain’ flap settings - and an override on flap settings in hot weather. But that would not involve the Ranco - but using the air temp sensor as a trigger for AC amplifier reaction. However, avoiding evaporator icing is essential - and the Ranco is the only item to do it…

Frank
xj6 85 Sov Europe (UK/NZ)
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I have a manual switch in series with the Ranco switch, I call it A/C On - Off, otherwise known as the Napoli mod, and I never had the A/C kick in by it’s own volition…
Overriding the Ranco makes no sense at all.

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Dear all,

I totally agree with you, Frank, that this mechanism is somewhat strange, and I still can’t see why it is needed at all. However, I did’t misquote the manual (which is an old manual from the 80’s). In the meantime, I had the opportunity to double check the information by flipping through the Jaguar Dealer Training Environmental Control System Manual, which is just a newer version of the manual for the Delanair AC system.

This manual describes a “Ranco Override” microswitch as well, it is part of the servo unit. An illustration of the camshaft sequence shows that this microswitch is activated only within the last 25 degrees of the camshaft movement towards the full cooling limit (p. 7). The circuit diagram for the compressor clutch (p. 19) shows the Ranco and this arcane switch as two parallel options for activating the clutch: either by a closed circle in the Ranco (typical case, above 2°C) or by microswitch closing – or both. This microswitch indeed overrides the Ranco in the sense that the microswitch can engage the clutch when the Ranco refuses to do so.

What I found most intriguing is a short passage on p. 18. There it is stated in a quite laconic manner:

“The magnetic clutch will energise through the Ranco thermostat except when the temperature of the evaporator matrix falls below 2°C. The Ranco thermostat opens at this temperature to prevent excessive temperature drop. The Ranco switch is overridden, however, when the camshaft travels the last 25 degrees of rotation in the full cooling mode. Cooling should not be selected if the exterior (ambient) weather conditions are cold enough to cause evaporator matrix icing.”

There we have it: Seems they intentionally installed a desert mechanism which effectively endangers the evaporator unit – relying on us Jag drivers being reasonable human beings and turning off the AC when the time has come to do so? A small hint in the driver’s manual would have been a kind gesture. :wink:

Anyway, I installed my switch in the meantime in the same manner as you, Aristides, did. My switch cuts off the Ranco circuit, but it doesn’t interfere with the override mechanism. I also installed a small control lamp which will give me a hint when the clutch engages. So, I will know exactly what the servo is up to! :slight_smile:

All in all, I have to admit that this has become a purely academic issue, but it still puzzles me. After all, the manuals I could obtain refuse to explain which ambient conditions allow the camshaft to enter those famous 25 degrees befor max. cooling. Maybe I’ll find out next summer. :slight_smile:

Best,
Florian

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The logic escapes me, Florian…

The driver has no influence on whether the system heats or cools; driver selects desired cabin temp with temp selector - and the system heats or cools as required. Which is done by resetting flaps with the servo - turned by AC amp orders according to its program. And what prevents the cam for turning the full travel under normal ‘full cold’ operation?

And the Ranco’s function is to prevent evaporator icing, so why bypass it? Adding that in some subzero temps; the evaporator temp will be below +2C anyway - and you need the AC ‘on’ for heating. Adding that the cold evaporator acts to dry the ambient air - which is not necessary a low temps as the air is dry.

I do agree that the topic is somewhat academic, but somewhere in the manual you refer to there must be a further explanation of the logic? I sorely need it! Anyone out there in a cooler climate - what’s your experiences on evaporator icing…?

Frank
xj6 85 Sov Europe (UK/NZ)
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I always run the AC in subzero temperatures, so the cabins don’t fog up with drying clothes and cold windows…
Big difference so it does cool and hasn’t frozen up yet. But isn’t the air warmed first, then dried? Not sure.

15

Pretty sure it’s the other way around David.

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All air is first passed through the evaporator, David, intake or recirculated. To be passed proportionally through the heater core to be tempered to set temps.

Water in the air condenses on the cold evaporator - hence the drain hoses. With suitable cold weather the air is dry, and in fact; the compressor is tripped by the Ranco and basically does not run. Ie, the evaporator is always tempered by ambient air…

Frank
xj6 85 Sov Europe (UK/NZ)
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Frank: I went through the two Delanair manuals again and couldn’t find more information on the topic yet. They seem to deny us a straight answer to the simple question as to what this mechanism is made for, and under what circumstances (ambient situation) it is active. If you want, it would very much like to share these docs with you (and whoever else on this list is interested), maybe you can make more sense out of it and solve this riddle. Just write me a pm.

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If you can’t find anything, Florian - I’m unlikely to as well…:slight_smile:

As an aside; there is a cryptic reference to the override within the Pectron test unit procedure, but no details are given, Ranco is not mentioned - and I don’t have the unit anyway.

Since the override information/use is contradictory, and override does not seem to cause any problems; I suggest we ‘chew the cud’ until something revives the topic…?

Frank
xj6 85 Sov Europe (UK/NZ)
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Yep, that’s a good idea. A little bit of mystery is not too bad, is it? :slight_smile:

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It irks me not knowing why the Ranco override was installed in the first place, Florian; no gain versus icing risk - that the Ranco is there to prevent. Makes no sense…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)
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