How Colds your Aircon

Vanny · Post by **Vanny** » Tue Dec 29, 2009 10:15 pm

To be honest i've never had cause to access one with the unit installed, the control unit is mounted almost to the underside of the plastic aircon box (HVAC unit). Its off to one side as well, to the right of the unit. I suspect if you remove the radio and centre tunnel trim then you should have access to it quite easily. There are two screws holding it in,

The thermistor its self is inside the unit and you have to open up the casing to get to it. Many years ago AlanS suggested that you should always install a couple of extra thermistors when retro fitting AC, i can see why!

I could possibly get you a picture if it helps?

Vanny · Post by **Vanny** » Tue Dec 29, 2009 10:19 pm

Have a look at the pic above, this is the HVAC unit with the recirc fans on. You can see bottom right under the HVAC the wires for the AC control, the multiblock connector controls the compressor. Gives you an idea of where the thing fits.

Post by **mat_fenwick** » Tue Dec 29, 2009 11:46 pm

Vanny wrote:on the BX you can't have the AC on 'half'. The evaporator (cold bit), in theory, maintains a constant temperature and you add heat to the conditioned air (moisture removed, air chilled) by opening the tap to the heater matrix.

But you can vary this 'constant' temperature by moving the slider to a different position - the compressor will then cut out earlier when the evaporator is less chilled. (However, on the aftermarket kits with the switch by the handbrake I guess this would just be the equivalent of the slider set to max, or off).
In practice (on mine at least) it's not a constant temperature, especially when set to less than max, you can detect when the evaporator is warming back up, and then feel the sudden rush of cold air when the compressor kicks back in.

Vanny · Post by **Vanny** » Wed Dec 30, 2009 4:16 pm

you sure you dont have a fault mat? Every AC BX i've been in the sliding pot does nothing

Modern cars simply hold the evap and heater matrix at constant temperatures and an additional door controls the mixture of warm and cold air. Much smarter way of doing things, means no dodgy heater tap for a start!

Post by **mat_fenwick** » Thu Dec 31, 2009 12:33 am

Vanny wrote:you sure you dont have a fault mat?

Errr, no. Every possible system is working correctly on my BX! (Then I woke up).
The heater tap seems like a bit of old hat design - I've worked on cars older than the BX and they've had air mixing heating systems. Can't see any benefits really, you have a slower heater response time and the matrix is more likely to get silted up in summer if there's any crud in the system.

Vanny · Post by **Vanny** » Thu Dec 31, 2009 11:09 am

Cost, its all down to cost. Tooling up for an extra door inside the main HVAC unit will run into hundreds of thousands of pounds (or millions of francs back then), using an off the shelf sliding gate valve is cheap. It probably reduces the build time of the main HVAC assembly which are usually 90% hand built. Valeo have always been a little behind with the time, you only have to see a BMW x5 HVAC unit to know that.

I've been looking at trying to make the BX HVAC unit automatic (that is with climate control). Its relatively easy to fit the steppers, but splitting the doors and adding an air mix door would be rather tricky. I suspect the Xantia unit will be largely carry over and might give the answer. Just need to find one that is breaking!

Post by **docchevron** » Thu Dec 31, 2009 1:38 pm

I'll find one!

I really rather fancy climate control in mine, so you can practice on mine if you like!*

*Plis...thank you!

kiwi · Post by **kiwi** » Thu Dec 31, 2009 6:41 pm

Vanny wrote: Valeo have always been a little behind with the time,

Given we are talking about a car that stopped production nearly 20 years ago. Things have advanced a fair bit since!

The design used in the BX for Aircon is one of the most basic user designs for its simplicity.

One the reasons for asking the original question was to establish how simple and effective others have found in thier experiances with this system as a user (occupant), I never thought it would hhave created some heated debate although the technical information for fault findiing has been very useful.

Post by **mat_fenwick** » Thu Dec 31, 2009 7:42 pm

kiwi wrote:heated debate

What? Where? Did I miss it?

Vanny · Post by **Vanny** » Thu Dec 31, 2009 7:48 pm

You reading another thread? I thought this was turning into an interesting discussion about HVAC systems, not sure where you got the heated debate from?

On the Valeo front, they are still using principally the same system in current production vehicles that they where when the BX was designed! If that's not behind the time then i dont know what is.

I wasn't criticising the BX unit, it was ok for its age, though other manufacturers where doing better (ie LEXUS). But wouldn't it be nice to have modern control systems on the BX? I mean creating a 1x1x1 auto climate control system can't be that hard given the collective knowledge here?

kiwi · Post by **kiwi** » Thu Dec 31, 2009 8:39 pm

I know as I write this your a year behind the times

As I sit here with the warm summer sun of the first day of 2010 pondering how efficiant the Aircon will be this year and how much I will need it.

So please note the humour in the referance to Heat and cooling in this debate.

Other than that until this point was very useful informative thread so lets get back to it.

Jaba · Post by **Jaba** » Fri Jan 01, 2010 7:53 am

Back on track Kiwi.
Check this post out from way back...... http://bxclub.co.uk/forum/viewtopic.php ... light=cool

You say yours cuts out and wont restart for some time. This is more likely to be insufficient gas rather than too much.

There have been a few inaccuracies so far about how the BX system works. The slider is effectively an aircon temperature control and works well changing the vent temps on my cars between 15 and 1 or 2.

If your system goes overpressure due to too much gas or more usually very high temperatures then there is a two stage response from the pressure switch. The first response is to put the rad fans on high speed with the compressor still working.
If the pressure continues to rise then the compressor will cut out leaving just the fans on at high speed to bring the temperature and with it the pressure down again.
Vent temperatures will obviously rise while this is going on. Do you get this Kiwi (high speed fans)??? If not then you are probably low on gas. But get a decent accurate temperature probe to find out the true vent temps you are getting.

I have fitted a warning light using the orange over temperature light on the temperature gauge as per PRM's idea. This is an essential tool to see how your aircon compressor is behaving.

kiwi · Post by **kiwi** » Fri Jan 01, 2010 8:25 am

Not sure about the High speed fans kicking in and the only clue that the aircon has gone off is the fact warm air is circulating. The problem I have in verfying this at present is tha fact my drive to work is 15-20 minutes (20kms) so the aircon is not really used that much. Drving around the local town aka speeds of 50-70kph I switch off the aircon going for the older metihod of air cooling (open the windows

)

One thing that is noticable is the drag on the engine when the aircon is switched on. On a hill or overtaking manauver I switch off the aircon to give a little bit more horse power.

There is also a bit of drama with the Aircon idle adjustment screw, as I remembered back a few months ago that was causing idling problems that most put down to a fault in the carb.

Could well be low on Gas I checked the service history the Gas was redone back in December 2000 and I know in the 3 years I have had it I have not checked the gas. So guess that may well be a problem!

The price was $280 back then so its one the reasons I have not had this done.