Evaporator temperature.

Post your Cit/Peu/Ren air conditioning queries or advice.

Moderator: RichardW

User avatar
fastandfurryous
Posts: 1383
Joined: 07 Jul 2004, 17:57
x 3

Evaporator temperature.

Post by fastandfurryous » 21 Jun 2005, 04:17

I've read on a number of occasions about the BX aircon that can put frost on the outside of the windscreen. This means that the evaprotator must be well below freezing (about -10 to -15 at a guess)
Now, the aircon components on a BX must be similar to that used in a 405 (same engine, similar size car etc.) but the 405 has a frost sensor on the evaporator, meaning that it never gets below about 6ish degrees. This means that even on a hot day, the compressor cycles in and out, maintaining the evaporator at 6degrees, but failing to get the car particularly cold. (It certainly gets cooler than outside, but never what could be described as cold)
I'm wondering if this is correct, or if it should be possible to get the car colder by having the compressor cut in all the time, and hence the evaporator much colder.
I'm fairly sure the 'fridge system is in good order, as it had a new condenser and reciever/drier last year, along with a decent vac out, and full charge (800gm) of R413a. It takes just 10-15 seconds for the evaporator to get down to it's cut out temperature if the interior circulator is off. WIth the fan on fullbelt, it takes about a minute to get down to temperature, cuts out for about 20-30 seconds, and then back in for another minute.
I could of course bypass the evaporator sensor so that the system "thinks" the evaporator never gets down to temperature, thus leaving the compressor engaged continuously, but this is a bit of a bodge, and I'd rather know if the system is supposed to be as it is (a tad feeble) or if the system is cutting out too early, and should be colder.
Any suggestions / comments? Thanks in advance.

alan s
RIP 2010
Posts: 2542
Joined: 26 Jan 2001, 16:53
x 5

Post by alan s » 21 Jun 2005, 04:57

I've never seen inside a 405 system so I can't really comment apart from saying that the system in a BX isn't common even throughout its range. A 16V has a different condensor than say a TZi; the TZi has a much larger surface area and the 8 vlave systems seem more efficient than the 16V one because of it.
The frost sensor I would imagine to be the thermister which is I suppose a fancy temp sensor as a simple explanation. If this was by passed, you could end up with an extremely low temp for a short period until the fins on the evaporator became choked with ice whereupon this would cut off the air circulation and you would then totally lose all cool temperature and possibly end up ankle deep in water when it defrosted.
Added to the BX system is a slide control that also has a bearing on the set temp and I understand works on a variale resistor principle; I must be honest here and say that I am more conversant with the mechanical & physical side to air/con than the electric/electronics side of things, however, a guy over here did do a write up on this side of it a couple of years back and swears to this day it is a big improvement on the original system; this is what I copied from his write up on another forum:
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">A few months ago I put a post up on a few Cit forums asking if anyone knew the characteristics of these sensors (when they were working) and I got no replies. So I decided to reverse-engineer a solution.
Symptoms: When it is working properly, the BX aircon slide is actually a variable temp control with off on far left then cool on the left graduating to cold on the right (yes I had one that worked once upon as time). Most controls now just seem to engage the compressor as you start to slide from the left and the compressor never disengages and the aircon gets very cold. Water condensate freezes onto and clogs the evaporator until no more air can flow through and constant compressor engagement chews up lots of petrol. The resistance of the temp sensor seems to only measure at about 500 to 800 ohms.
Reverse-Engineering: The control module is electronic and the sensor wire come from the module which is behind the dorr open display panel and goes up the right side of the air box disappearing into the air box beside the coolant pipes going to the heater element. I cut the sensor wires, connected a variable resistor and found that the sensor resistance needs to be about 5000 ohms at the just on position and 12000 ohms with the aircon hard on ie the sensor has a negative temperature coefficient with resistance rising as tenperature drops. This is the same characteristic as a thermistor and on examining spec sheets, I found that a device that was 4700 ohm at 25 degress C that rises to about 12000 ohms at 0 degress C - perfect.
Fix: Unfortunately, Dick Smith only stocks a 100000 ohm one and Jaycar don't stock any. So I contacted Prime Electronics in Bowen Hills and they had a couple of hundred of the 4K7 thermistors at a cost of less than $2 each. I bought one on the way home from work, soldered it onto a piece of thin speaker wire, shrink sleeved the joins and lightly coated the unit in silastic to water proof it. Then I poked it about 10cm into the air box through the same hole that the other sensor wire use for entry and hooked it onto the sensor wires going to the control module.
The system now controls the air temp at about 16 degress C when control is just to the right of off down to 0 degrees when far left and compressor cycles on and off quite frequently. It is now working better than original as far as I can remember.
Good Luck with your unit. Email or post of you have any queries.
Ken W<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
I have seen performance enhanced by moving the tem sensor a further distance away from the source of refrigeration as air con temperature sensing seems always to be done based on air temp rather than coil temp, but be warned, if it is set too far away, it will cause over freezing and become counterproductive by allowing the coil to ice over and therefore lose tep just as I suggested by passing the temp sensor would, so it has to be moved in very small increments.
Alan S

User avatar
Kowalski
Posts: 2557
Joined: 15 Oct 2003, 17:41

Post by Kowalski » 21 Jun 2005, 05:01

The air-con trade seem to think the system is healthy if the air coming out of the vents is 10 C, its probably a matter of taste, on a hot day if the system isn't doing enough to keep the whole cabin cool you might want it colder.
The problems you get with getting your evapourator too cold are twofold. Firstly, you end up with all of your refrigerant and oil in it which isn't so good for the compressor, if you've got any water in the system it can end up there too and freeze blocking things up even more. Secondly and more importantly, you get ice building up on the evapourator which will block its fins up so that it loses surface area and because of the reduced surfac area even though it is colder it works less effectively. If you want to get the car cooler you'll have to make sure that you move enough air over the evapourator to make sure it can't ice up, only then can you run it colder.

User avatar
fastandfurryous
Posts: 1383
Joined: 07 Jul 2004, 17:57
x 3

Post by fastandfurryous » 21 Jun 2005, 06:15

The "air-con trade" get nowhere near my car! I trust very few people to do decent quality work. As soon as I buy a manifold & guages for myself, no-one else will have to go near it.
Getting the evaporator too cold is no concern at all, as most R12/R134a systems will max out at about -35degrees. Frosting the evaporator externally isn't too much of an issue either, as it is of course possible to run the system in recirculation, thus reducing the amount of moisture available to be condensed out. After all, if a BX can freeze it's windscreen, the evap must be well below freezing.
If I could be bothered, I'd use some system reversing valves to run the evaporator at -20 most of the time, and then reverse it to defrost it every now and then. It'll probably never happen.
On the BX (and CX) The temperature slider for the AC controlls the air temperature by cycling the compressor according to the temperature sensed in close proximity to the evaporator (and thus roughly the evaporator temperature). Whilst variable on a BX/CX, it appears to be a fixed temperature on the 405. From what I can gather the temperature sensor does go a little bananas with old age, and that may be the problem with my system.
I wonder if anyone else has experienced the other problem I have had with the A/C:
Having had the A/C on for a while while it's warm, the evaporator obviously becomes wet. I then drive into rain, it's cooler, and so the A/C gets switched off. Problem now is that the windscreen and other glass on the car gets cold. The A/C evaporator now warms back up to the air inlet temperature, the moisture on it warms up too, and blows onto the heater matrix (they're only 3 inches apart in the 405). This now means that there is warm moist air blowing into the car, which condenses on the inside of the windscreen. Not good at all.
This situation continues until the A/C evaporator dries out, which can take a surprisingly long time. I've had to pull over on a couple of occasions now to let it all dry out (which isn't easy, since it's usually raining!) Bit of a design fault as far as I can see, as I've had other cars (most noticeably a range rover) which have remote A/C evaporators, and hence don't suffer from this problem. I've tried everything to get rid of this problem, and even putting the A/C back on again doesn't clear it.
Before anyone suggests it, yes, the condensate drain is clear (there's often condensate dripping from under the car) and the heater matrix is brand spankers, so no leaks there.
I'm fairly sure it's just a design fault, but has anyone else experienced this?

User avatar
Kowalski
Posts: 2557
Joined: 15 Oct 2003, 17:41

Post by Kowalski » 21 Jun 2005, 15:52

On the Xantia there are some drains in front of the windscreen, I've had these full of leaves on occasion which rot and block the drain that meant the rain could get into the heater matrix. Vacuum cleaner with a long thin crevice tool is really handy for getting leaves out.
It might be worth having a look at their equivilent on the 405 (if you haven't already done so), you'll probably need a torch (and a flexible endoscope if they're anything like the Xantia).

User avatar
fastandfurryous
Posts: 1383
Joined: 07 Jul 2004, 17:57
x 3

Post by fastandfurryous » 21 Jun 2005, 17:04

This is not the problem. The vehicle has a brand new pollen filter, and leaves do not build up on 405 models.

User avatar
demag
Donor 2016
Posts: 1436
Joined: 18 Oct 2004, 05:03
x 77

Post by demag » 21 Jun 2005, 18:28

fastandfurryous my system works the same as yours, cycling in and out. I haven't checked the temp yet but a few degrees lower in hot weather wouldn't go amiss.

User avatar
fastandfurryous
Posts: 1383
Joined: 07 Jul 2004, 17:57
x 3

Post by fastandfurryous » 21 Jun 2005, 20:45

Interesting. I'm guessing in that case that the 405's system just isn't as sophisticated as the Citroen systems. I think I might have the sensor out and see if it's getting a bit ill.
demag, do you also have issues with chronic misting if the A/C is switched off mid-journey?
David.

User avatar
demag
Donor 2016
Posts: 1436
Joined: 18 Oct 2004, 05:03
x 77

Post by demag » 22 Jun 2005, 18:04

Yes, but only in damp or rainy weather.
In this hot weather, no probs at all. (apart from the heat! lol)

User avatar
Kowalski
Posts: 2557
Joined: 15 Oct 2003, 17:41

Post by Kowalski » 22 Jun 2005, 18:53

I don't think the Xantia system has a temperature sensor in it anywhere, at least not according to the Autodata stuff I've looked at, and I can't find one either. What it does have is a pressure cut out switch, it cuts out if the pressure gets either too high or too low. If the pressure gets high, first it sets the radiator fans going flat out instead of low speed, then at a higher pressure it turns the compressor off.
I would have thought that the systems on the 405 and Xantia would be shared and identical, but they might not be, the Xantia didn't go into production until around 1993, where as the 406 came along in about 1995 so there 405 was an old design when the Xantia started.

User avatar
demag
Donor 2016
Posts: 1436
Joined: 18 Oct 2004, 05:03
x 77

Post by demag » 23 Jun 2005, 04:16

Ok then,
Checked the temp at the dash outlets this evening whilst at work (well might as well get paid for doing it eh?) With fast fan and fresh air it was about 8c. On recirc with fast fan about 7c.
Recirc with slow fan best I got was 4c. Fresh air with slow fan about 6c. Outside temp was about 27c at the time.
So it would appear I am ok. Surprising really, I thought it was higher than that. The system was cutting in and out like mad. So on the way home I set the temp at about 22c with medium fan and the system didn't work so hard but it was still cool in the car.

alan s
RIP 2010
Posts: 2542
Joined: 26 Jan 2001, 16:53
x 5

Post by alan s » 23 Jun 2005, 07:06

On any system as I've mentioned on other posts, 4 - 6 is accepted, 8 is marginal, 10 you've got problems.
Any repairer who is satisfied with 10 should be doing something else; pastrycook sounds a good idea.
Alan S

User avatar
fastandfurryous
Posts: 1383
Joined: 07 Jul 2004, 17:57
x 3

Post by fastandfurryous » 23 Jun 2005, 15:37

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by Kowalski</i>

I don't think the Xantia system has a temperature sensor in it anywhere, at least not according to the Autodata stuff I've looked at, and I can't find one either.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
You won't find it. It's buried in the aircon/heater system and measures the air temperature in close proximity to the evaporator. If your system didn't have one of these it would frost the evaporator within minutes of startup.
<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">What it does have is a pressure cut out switch, it cuts out if the pressure gets either too high or too low. If the pressure gets high, first it sets the radiator fans going flat out instead of low speed, then at a higher pressure it turns the compressor off.<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
All systems have these.
About the only similarity between 405 A/C and Xantia A/C is the compressor. The Condensor, Evaporator and all associated pipework will be unique to the model. Even my Tx valve is different, as it's an old R12 system which has had a drop in R413a conversion.
It would appear that not only is BX A/C superior in power, but also more sophisticated in the way it works and controlls.