Technical Mirrors, External Gauges and so on...

[Bonefish Blues]

Rather than carry on piggy-backing on another thread, I thought I'd start a new one.

As suspected, my car has had a new LH mirror at some point, but the sensor wasn't replaced. One or several queries, if I may?

Replacement sensors are as a cheap as chips on eBay. I assume that to get access to fit I just need to pop the glass out, I don't need to take the coloured housing off do I? (Oh and hope the ends of the connector wires are still in place, of course!)

On a manual AC version with the knobs, not the digital display, is the external sensor used by the AC in any way to help regulate temperature? That is, will it improve operation in any way if/when re-fitted?

 

[The Panda Nut]

You should be correct. Mirror is a push fit ball and sockety fitting. Mirror cover change is not difficult either once the mirror is out.

 

[Panda IIs]

Rather than carry on piggy-backing on another thread, I thought I'd start a new one.

As suspected, my car has had a new LH mirror at some point, but the sensor wasn't replaced. One or several queries, if I may?

Replacement sensors are as a cheap as chips on eBay. I assume that to get access to fit I just need to pop the glass out, I don't need to take the coloured housing off do I? (Oh and hope the ends of the connector wires are still in place, of course!)

On a manual AC version with the knobs, not the digital display, is the external sensor used by the AC in any way to help regulate temperature? That is, will it improve operation in any way if/when re-fitted?

As indicated in my previous reply in the other thread, I think the connection is in the car behind the door panel or trim. I don't see how it can influence the manual air con which is essentially open loop (on or off). What it does do is trigger the "possible ice on road" warning I believe.

 

[Bonefish Blues]

I've seen a few articles saying it (the external gauge) does interact with the AC, even on manual systems like mine/ours, so was interested to know more from anyone about the Panda's system.

 

[Panda IIs]

I've seen a few articles saying it (the external gauge) does interact with the AC, even on manual systems like mine/ours, so was interested to know more from anyone about the Panda's system.

Well I'm happy to be corrected but I cannot see the rationale nor requirement for any interaction. There is a pressure sensor and possibly temperature within AC systems which will operate reflecting external conditions (over pressure etc) but I don't see the mirror sensor being a contributor. My Volvo 940 (1998) has AC but definitely no external temperature sensing capability.

Obviously the higher spec climate control has cabin temperature sensors.

 

[Bonefish Blues]

Well I'm happy to be corrected but I cannot see the rationale nor requirement for any interaction. There is a pressure sensor and possibly temperature within AC systems which will operate reflecting external conditions (over pressure etc) but I don't see the mirror sensor being a contributor. My Volvo 940 (1998) has AC but definitely no external temperature sensing capability.

Obviously the higher spec climate control has cabin temperature sensors.

Just for clarity, I'm not seeking to correct anyone, but appreciate the additional info

 

[Panda IIs]

Just for clarity, I'm not seeking to correct anyone, but appreciate the additional info

Sorry I understood this I think (not appearing to be oversensitive I hope!). Generally forums are great in that they gather information from which a consensus forms giving some sense of the actual reality (and this is one of the better ones believe me! ). On others I find you do get some (sometimes) well meaning folk that perpetuate what can most politely be described as myths!

For me I think its best and most useful to share actual experience.

 

[Bonefish Blues]

Just by way of update, I realised I have an acquaintance (son in daughter's class) who is a senior climate guy at JLR - gets gigs in Central Spain in summer, and the like.

I have sent him a message and asked the naive question, so will report back anon.

 

[Panda IIs]

Just by way of update, I realised I have an acquaintance (son in daughter's class) who is a senior climate guy at JLR - gets gigs in Central Spain in summer, and the like.

I have sent him a message and asked the naive question, so will report back anon.

Great, that will be good to get some professional insight

 

[Bonefish Blues]

So, man he say, verbatim:

The controller will likely use this signal to determine when it's safe to use the compressor for AC (not used in freezing temps)

 

[Panda IIs]

So, man he say, verbatim:

The controller will likely use this signal to determine when it's safe to use the compressor for AC (not used in freezing temps)

OK I'm at risk of contradicting my own rule on forums and speculating here!
I'm a little surprised at the statement about not using in freezing temperatures since if you hit the demist button on the heater controls this turns on the AC since it provides "dry air". I think the manual reflects this too? You could test this by trying this out if its still cold (I'm away from the UK at the moment and also can't check the manual). With an engineer's hat on the AC system should not have any water in it (has line dryers in place in fact) so low temperatures should not be an issue? There I go speculating!

 

[jrkitching]

With an engineer's hat on the AC system should not have any water in it (has line dryers in place in fact) so low temperatures should not be an issue?

I think you may be getting confused between the refrigerant circuit and the airflow through the unit.

There is indeed a dryer in the refrigerant circuit, as any moisture in the refrigerant would lead to internal corrosion in the compressor & A/C lines.

The airside part of the system is generally always wet, as moisture present in the air flowing through the unit condenses on the outside of the evaporator matrix when the air is cooled. A system of pipes and drains channels this condensed water and discharges it outside the car, which is why you'll sometimes find a puddle of water under the drains when the car is parked after using the A/C. Sometimes the pipes become blocked; when that happens, the water usually ends up in the footwell, a problem not infrequently reported here.

The freezing problem is that if the surface temperature of the evaporator falls below 0C, any moisture in the incoming air freezes and blocks the airflow. Remember the surface temperature of the evaporator will be significantly less than that of the air coming into the heater box (otherwise it wouldn't cool the air!)

Note that it's the temperature of the air hitting the evaporator which is the important figure, which will be very different if the air if being recirculated within the car (as opposed to being drawn from outside). Speculating now, but logically any temperature sensor used by the A/C logic needs to located directly in the incoming airstream just before the evaporator (and therefore be integral to the heater box). Measuring the outside air temperature won't be much use if recirculation is being used, so I'd doubt that the wing mirror sensor information is used by the A/C logic; it likely only drives the temperature readout and icy road warning message on the MFD.

Most car A/C systems inhibit compressor operation when the air coming into the unit falls below about 3C, though I've no specific information about the Fiat system.

Domestic dehumidifiers (basically an A/C unit with the condenser and evaporator placed back-to-back) generally monitor the evaporator temperature and periodically interrupt operation to run a defrost cycle whenever conditions are likely to lead to frost formation.

 

[Panda IIs]

I think you may be getting confused between the refrigerant circuit and the airflow through the unit.

There is indeed a dryer in the refrigerant circuit, as any moisture in the refrigerant would lead to internal corrosion in the compressor & A/C lines.

The airside part of the system is generally always wet, as moisture present in the air flowing through the unit condenses on the outside of the evaporator matrix when the air is cooled. A system of pipes and drains channels this condensed water and discharges it outside the car, which is why you'll sometimes find a puddle of water under the drains when the car is parked after using the A/C. Sometimes the pipes become blocked; when that happens, the water usually ends up in the footwell, a problem not infrequently reported here.

The freezing problem is that if the surface temperature of the evaporator falls below 0C, any moisture in the incoming air freezes and blocks the airflow. Remember the surface temperature of the evaporator will be significantly less than that of the air coming into the heater box (otherwise it wouldn't cool the air!)

Note that it's the temperature of the air hitting the evaporator which is the important figure, which will be very different if the air if being recirculated within the car (as opposed to being drawn from outside). Speculating now, but logically any temperature sensor used by the A/C logic needs to located directly in the incoming airstream just before the evaporator (and therefore be integral to the heater box). Measuring the outside air temperature won't be much use if recirculation is being used, so I'd doubt that the wing mirror sensor information is used by the A/C logic; it likely only drives the temperature readout and icy road warning message on the MFD.

Most car A/C systems inhibit compressor operation when the air coming into the unit falls below about 3C, though I've no specific information about the Fiat system.

Domestic dehumidifiers (basically an A/C unit with the condenser and evaporator placed back-to-back) generally monitor the evaporator temperature and periodically interrupt operation to run a defrost cycle whenever conditions are likely to lead to frost formation.

Ok, I understand all the control parameters and thanks for walking us through those. The point was that the outside air temperature sensor, in the wing mirror, probably does not figure in the AC control (loop or otherwise). For the instance where AC is on and sub-zero condition exists then almost certainly the heater will be maxed out too so cabin air temp is probably going to prevent the icing up you described. The AC is definitely recommended for demisting in low temperature conditions.

I have encountered this issue (icing) with two stage cooling systems on environmental control systems but not in a vehicle.