Air-con and radiator cooling fan operation

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white exec
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Air-con and radiator cooling fan operation

Post by white exec »

Wonder if anyone can help with this one...

I've been looking at the Citroen wiring diagram for the factory-fitted AC system, in preparation for getting the system on to R134a in the new year.

Apart from needing gas, the system appears to be operational, with both radiator fans firing up as soon as the dash slide control is moved away from Off. There's also very faint warmth at the top pipes of the condenser, so it looks as if the compressor is attempting something, when switched on very briefly; good signs.

My problem is fully understanding the Citroen circuit diagram:
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click to enlarge
BX aircon EWD___1.jpg
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↑↑ The radiator cooling fan part is straightforward enough - two-stage thermo-switch, operating the usual 3 relays, to put the fans into series (slow) or parallel (fast) running.

It's the action of the dash slide control I'm not clear on:
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BX aircon EWD___2.jpg
↑↑ EDIT: Diagram labelling now revised - see post below ↓↓
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The diagram shows the switch with 4 positions, and a 2-pole ganged arrangement (A, B).
On the diagram, the right-hand position I take as Off, for both A and B. Have called this Position 1.

Then it gets interesting:

In Position 2, A also connects to nowhere, but B runs to Gnd, so switches on Relay 742, which activates the compressor. It also switches on Relay 733, which puts the fans into Slow speed (similar to the low-temp thermo-switch operating). OK, as expected so far.

In Position 3, B remains grounded (compressor continues to run), but A switches to Gnd, thus putting the radiator fans into High speed. Is this correct?

For Position 4, A keeps the fans in High speed, but B switches to nowhere, so seemingly cutting the compressor (and the Slow speed connection, over-ridden by the High speed running).

What is the function of Position 4 ?

Also, the action of the dash AC slide control is (on the one here anyway) without detents, just a continuous travel across. I'm not clear exactly how it might cleverly operate #-o.
Last edited by white exec on Sat Dec 05, 2020 8:41 am, edited 2 times in total.
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Re: Air-con and radiator cooling fan operation

Post by Jaba »

Briefly what you describe is correct. In position 3 fans are full speed because the pressure in the system has risen to a preset high level of around 19 bar and the fans runs fast to bring the pressure down again. Position 4 keeps the fans on fast and cuts the compressor at 26 bar because that is regarded as the maximum operating pressure when the system should be shut down until the pressure drops back down. Normal running of the fans at slow speed is while the pressure is below 19.
BTW the slide control is not connected to the pressure switch positions as your schematic seems to suggest. It effectively alters the cutoff temperature of the evaporator sensor.
The dash slider control, a variable resistor, is a cooling temperature control with max cooling happening at the RH end.
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Re: Air-con and radiator cooling fan operation

Post by xantia_v6 »

I think that the confusion has arisen by the red overlay on the diagram wrongly labelling the switch as the dash slide switch. It is the refrigerant pressure switch, and the compressor control and fan control sections are mechanically linked.

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Re: Air-con and radiator cooling fan operation

Post by white exec »

Thanks, Jaba and Mike.

That all makes sense, so thanks for putting that right. I was struggling with the circuit diagram, as I didn't have a list of electrical component numbers which covered AC (Haynes doesn't). Was using the components location drawing to identify the parts, but slipped up on #700.

That also accounts for #277, which has to be the dash slide switch. The electronic relay (#60) presumably responds to the slide in the Off position by cutting the supply electrical supply to the AC system.

Also confirms the action of the slide control in only bringing on the radiator fans at Slow speed, which is what would be expected.

Thanks both!
.
↓ Re-labelled diagram
BX aircon EWD___3.jpg
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Re: Air-con and radiator cooling fan operation

Post by Jaba »

I toyed at one time with modifying the circuit to change the slow speed fan operation from being on continuously with AC on to running only when the compressor was running. My logic being that there was sufficient condenser cooling when the car was moving without the fans having to run at all and when the compressor was off and no cooling was needed until compressor kicked in again then the fans were effectively redundant except to remove any heat already in the system.
Also the slow fans run even when there is no refrigerant gas left in the system which does seem unnecessary.
But I never got round to it although I did do this mod to a non factory aircon system fitted to one of my BXs as this had enough relays, 7 as I recall, to enable this to be done easily. This did work perfectly in our UK summer temperatures but may not be the case with higher southern Europe summer temperatures where the extra efficiency of removing as much condenser heat as possible might be desirable.
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Re: Air-con and radiator cooling fan operation

Post by white exec »

Interesting. Have thought on similar lines, too.
I suppose you're not likely to turn the AC system on if there is no gas in the system, so the fans wouldn't run unnecessarily.
(When looking at BXs-with-AC to buy, was surprised to find all three that I looked at had factory systems, but no gas in them. Guess the owners had had to give up on R12, and just decided not to use the system any more. :? )

On the XM here (which does lots of very long journeys), the fans also run at slow as soon as the AC is switched on. This is just plain daft for long motorway/clear road journeys, and I've wondered about building in some sort of speed-sensing arrangement, so that the fans only operate with AC at slow road speeds. Problem is the possible unhelpful effect of a strong tail-wind, which could reduce the air flow through the radiator.

There is this simple device, which could be nose-cone mounted, and simply interrupt the 'fans-on' command from the AC, over a certain air speed. The price has so far put me off experimenting.
https://www.google.com/search?q=air+flo ... GnNXu4kp4M

You're right about southern European temperatures, though. 30-35°C pretty normal mid-summer coastal, and 40-45°C possible inland.

We live up some pretty steep mountain roads - 490m above coastal level, where we shop, 9km away.
Temp gauge to be fitted to the BX shortly - a digital one, until I can find a GTi instrument section.
Will also fit a couple of discreet LEDs, so that slow and fast fan speeds can be seen. The same two wires from the thermo-switch will also be used to add a dash switch, so the fans can be manually turned on. Our previous 19D did a lot of caravan towing and got similar mods, including a bulkhead header tank.
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Re: Air-con and radiator cooling fan operation

Post by Jaba »

Good ideas.
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