[edjack]

I have been giving out incorrect information about charging the A/C system. My understanding was to monitor the evaporator outlet temperature via DIS (there's temp sensor there, readable by DIS), and add/subtract refrigerant until the outlet temp was 4 deg C. I must have picked this up from an old copy of the TIS.

My 2007 TIS says to charge by weight. This requires evacuating the system before recharging. The old technique may have ignored psychrometric issues, and been unreliable.

One needs a very precise and accurate scale; the weights for the M52, M62, up to 12/97, for instance, are 1225 +/-25 g. Tolerances for later models are +/-10g.

I apologize for any trouble I've caused. Hopefully, You've ignored my advice!

 

[bluebee]

We should probably cross reference to the canonical A/C threads:
Air conditioner refrigerant & PAG oil (1) & how to refill your A/C system (1) (2) (3) (4)

BTW, I've never recharged my AC so may I ask others if they'd add anything to this summary?

- Air conditioner: Refrigerant 134a (aka R-134a) with poly alkylene glycol oil, sometimes referred to as poly alkaline glycol oil (aka PAG refrigerant oil). Bentley book II, 640-2 & 640-3 & 640-23. The PAG oil is known by other names such as ND8, PAG46, and BMW PN: 82.11.1.468.042 and travels with the refrigerant as a mist. Best to empty and then refill refrigerant by total weight; second best is the evaporator temperature method (i.e., in the shade, ambient temperature less than about 75°F and relative humidity below about 60% - then add R-134a until the evaporator exit air temperature with A/C set on max at 60°F is 4°C/39°F or colder - the best you can do without MoDIS is probably the center-dash vent at something like 10°C/50°F); third best is the pressure method (i.e., add refrigerant until the low-pressure side is 25 psi to 45 psi ... aim for around 32 psi (assuming shade, ambient, & humidity listed prior); worst is by can weight (e.g., adding a 12oz by weight can or a 16oz can by weight but the AC is a critical-charge system that won't tolerate refrigerant quantities much outside + or - .05 kg). [Volume: E39's built up to 12/97 contain 1,225 grams +/- 25 grams (2.70 lbs +/- 0.05 lbs) of R-134a; E39s built after 12/97 contain 750 grams +/- 10 grams (1.65 lb +/- 0.03lb) of R-134a. The amount of PAG oil misted in the system is said to be about 1 ounce by volume.] Replacement Interval: Lifetime charge (sometimes the system develops a leak though).

EDIT: I had to look this one up!
psychrometric:
Psychrometrics or psychrometry or Hygrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. The term derives from the Greek psuchron meaning "cold"[1] and metron meaning "means of measurement".[2]

 

[rdl]

... stuff deleted ...
EDIT: I had to look this one up!
psychrometric:
Psychrometrics or psychrometry or Hygrometry are terms used to describe the field of engineering concerned with the determination of physical and thermodynamic properties of gas-vapor mixtures. The term derives from the Greek psuchron meaning "cold"[1] and metron meaning "means of measurement".[2]

Edjack raises an excellent point regards ambient air-water vapour properties. Such psychrometric data is required to calculate cooling effect across an A/C evaporator. An especially significant factor is relative humidity.

In even moderate conditions 1/3 or so of cooling BTUs are used to condense water vapour rather than reduce temperature; in extremely humid conditions well over 50% goes to condensing. Therefore A/C system operating parameters are always stated in terms of ambient temperature and relative humidity. Descriptions such as "my A/C is 41F at 90F ambient" is incomplete and useless for comparison or diagnosis.

If you're interested, a simplified explanation follows.

The A/C can be thought of as performing three functions on the air being cooled as it flows through the evaporator:
1) reducing the temperature of the ambient air-water vapour mixture to the dew point
2) condensing water vapour to liquid at constant temperature at whatever the dew point of the ambient air happens to be
3) further reducing the temperature of the air-water vapour mixture while also condensing out more water vapour as the temperature is reduced.

In moderate ambient conditions the A/C actually operates at only a fraction of its design capacity in order to cool air to ~4C. (4C is the regulated minimum temp to avoid icing the evaporator and blocking air flow to the cabin.) As humidity &/or ambient temp increase, the A/C expansion valve in the evaporator automatically increases the R134a flow rate to provide more BTU/hr cooling to condense more water while striving to maintain 4C output.

But then as the evaporator increases the R134a flow rate to generate more BTU/hr of cooling, the condenser has less less time to cool a unit of R134a. Temperature & therefore pressures (which move in lockstep) increase.
Further, an A/C system's maximum cooling capacity (BTU/hr) is controlled by:
1) RPM, i.e. R134a flow rate varies with RPM
2) ambient air flow rate and temp over the condenser, i.e. removing heat from the compressed R134a. Air flow at testing conditions should be fixed by fans. Higher ambient reduces heat removed from condenser => higher R134a pressure
3) air flow rate through the evaporator. Slower means more time for the evaporator to condense and cool a unit of incoming air-water vapour.

So, at some point the cooling draw at the evaporator equals the heat rejection in the condenser; both are affected by ambient temp and humidity. Net result is that ambient temp and humidity determine A/C R134a pressures (both high & low sides) and output air temperature. RPM also affects cooling capacity, of course. In extreme conditions maximum cooling capacity of a perfectly good A/C can be reached and the air coming off the evaporator rises well above 4C &/or R134a pressures can reach a maximum cut-off point.

Therefore a complete table or graph of A/C performance parameters will always specify: ambient temp, ambient RH, RPM, HVAC fan speed and A/C outlet air temp. Function of the auxillary fan in front of the condenser should be verified too. All measurements are required to determine whether a particular system is OK; one, two, three or four are not enough.

 

[Fudman]

An excellent writeup on the operating characteristics of the e39 AC! RDL, you know your thermodynamics and your HVAC! This is why I do NOT recommend using those DIY charging systems available at auto parts stores. You are more likely to put in too much rather than randomly put in the correct amount of refrigerant. I have a really slow leak that disables my AC every two years. This is the one thing I have my indy do every time as I do not have a vacuum pump nor a way to measure the weight of refrgirerant.

 

[rdl]

Fudman
I couldn't agree more regards DIY charging kits. One may be lucky and not do any damage, but I wouldn't bet the farm on it. The insidious thing is that one may recharge the system on a hot, humid day when much of the R134a is in gaseous state without any problems. Then days or weeks later in cooler temperatures, one has more of the the refrigerant in liquid state and slugs the compressor to destruction.

It's too bad that A/C recovery & charging sets are so expensive. I really do prefer DIY, but A/C just doesn't come close to making economic sense.

I sometimes wonder if those DIY kits just might be subsidized by an association of A/C repair shops. Just joking of course, absolutely no reason to believe this is true.

 

[rdl]

An excellent writeup on the operating characteristics of the e39 AC! RDL, you know your thermodynamics and your HVAC! This is why I do NOT recommend using those DIY charging systems available at auto parts stores. You are more likely to put in too much rather than randomly put in the correct amount of refrigerant. I have a really slow leak that disables my AC every two years. This is the one thing I have my indy do every time as I do not have a vacuum pump nor a way to measure the weight of refrgirerant.

Fudman

You're lucky to live in a juristiction that permits recharging. Here in Ontario every A/C tech must be licenced & account for every oz of refrigerant on pain of fines &/or loss of licence. Top-ups even on old systems are not permitted without finding and repairing the leak, which must be supported by an invoice for the repair. The single tiny exception is addition of 1 or 2 oz with dye to a very low system in order to help find a leak.

On the surface this sounds reasonable. Except the rule of thumb is that a brand new, out of the factory R134a system will lose ~15 g per year to molecular seepage past seals, O-rings and through rubber hoses. I've seen a UK based reference that mobile A/C (automotive, trucks, ect.) typically lose 7.5% of initial charge per year. On this basis, an E39 with a 650g charge would lose ~50g / year.

So my car is now 10 years out of the factory => using the lower estimate ~150g probably lost, ~20% of original charge. In a year or two or three when I lose cooling capacity I'll be forced to pay for a repair to a non-defective part with a non-existant real leak. BTW, industry data indicates that the compressor accounts for 50 to 70% of this normal seepage loss. Guess which (expensive) A/C component is going to turn up needing "repair" when the A/C tech goes looking for a "leak"?

More likely I'll take a quick trip across the Canada-US border for an A/C evac and recharge to spec and carry on for another 10+ years.

BTW, I do support the notion of controlling greenhouse gas emissions, including refrigerants. Unfortunately the Ontario law & regulations are poorly written.

 

[Topaz540i]

Maybe this is a dumb question but why would you need a/c in canada? Isnt it snow 10 months of the year?

 

[doru]

Maybe this is a dumb question but why would you need a/c in canada? Isnt it snow 10 months of the year?

This question for real?

C'mon Topaz, you know we live in igloos together with our pet white bears.......

The e39 is just for decoration. the sleighs & dogs are doing the HP.

 

[CSMBlack-540i]

...you know we live in igloos together with our pet white bears.......

.

Do you rub noses like eskimos too?

 

[doru]

Do you rub noses like eskimos too?

Yessir - that's the fun factor :bigpimp:

 

[bigugly fab]

so big deal, replace the drier. the system will thank you for it anyways when you need to recharge it.

 

[CSMBlack-540i]

Yessir - that's the fun factor :bigpimp:

I thought dressing up as a cowboy and going to Stampede was your fun factor?! :thumbup:

 

[doru]

You did your research....

 

[bluebee]

psychrometric data is required to calculate cooling effect

I added this post to the referenced threads so that others will more easily benefit from the information in the future.

 

[rdl]

This is why I do NOT recommend using those DIY charging systems available at auto parts stores. You are more likely to put in too much rather than randomly put in the correct amount of refrigerant. I have a really slow leak that disables my AC every two years. This is the one thing I have my indy do every time as I do not have a vacuum pump nor a way to measure the weight of refrgirerant.

To reinforce the point that one cannot charge the A/C refrigerant with pressure gauges, have a look through attached PDF. It's data from a GM Cadillac Catera factory service manual. Note how much low side, high side pressures and vent temperatures vary with ambient air conditions.

A heads up: these data are not for universal application to other cars, so don't use them to diagnose an E39. The pressures and temperatures at various condtions are determined by the specific operating characteristics of the heat exhangers, compressor and expansion device of the particular car. Further, GM does not state the data is optimum, rather maximums as which point repair should be made.

Then consider the image of a graph showing system pressures vs charge weight in identical condtions. Note how little the low (suction) pressure varies with significant under and overcharge. Consider that E39 charge spec is +/- 1/3 oz. or 1.5%.

I hope this lays to rest the notion that a DIY kits with a pressure gauge on the low pressure side only is able to indicate refrigerant charge.

 

[rdl]

I posted this information in a B-forums thread in response to a request for ideas. It then ocurred that the info would be useful here.

Here are some points worth considering as you have your R134a charge checked. The information is intended to help defend against a shop that will wish to replace your compressor when it is unnecessary.

At the time the E39 was designed, typical, expected R134a loss rate on brand new systems was 15 to 20 grams per year. Some 70% of this was around the compressor shaft seal: 10 to 14 g/yr.

Now for the leak test. The shop will check the system and inform you that your compressor shaft seal is leaking. They will recommend compressor replacement. However, dye checks do not indicate leak rate. Any visible dye is taken to be a leak needing repair and a few g/yr leak over several days will leave a dye track. The electronic sniffers are able to detect leaks in the 3 - 5 g/yr. Again the shaft seal comes up as defective. For that matter a brand new, out of the BMW assembly plant compressor would have turned up as defective.

E39s are now 10 to 15 years old, so the expected loss is ~250g vs a factory spec of 650 +/- 10g, i.e. low by ~40%. Enough to compromise cooling efficiency. Most will simply need a recharge to spec unless there is a real leak.

Find a shop with a sniffer that has selectable sensitivity and an indicator for leak rate. These instruments are available and not very expensive, ~$250, which is not terribly much for a shop specializing in auto A/C. Ask what the indicated leak rate at the shaft seal is. Insist on a simple recharge to factory specs, unless the shaft seal is leaking at well over 15 g/yr, or there is a large leak somewhere else.

 

[cn90]

I agree with "rdl" that R134a slowly leaks out of the hose over the years. The "natural loss" is probably very slow.

I am lucky because knock on wood, 14 years later, the AC still runs fine.

PS: Re issue of charging using the pressure gauge, rdl posted great info.
An analogy that makes people understand better:
- If you cook with a pressure cooker, whether it is 1/4 full or 1/2 full of water, the pressure cooker generates the same pressure for cooking.
- However, when it is empty....no pressure.
- Too full....no worky.

- So, the idea is to have the right amount (not using pressure gauge).
- The only problem is: in order to do this, you need to: a) evacuate R134a, b) apply vacuum, c) recharge with the right amount of R134a.
A process that most DIYers won't be able to do.

- So, I usually use the "best guess" approach:
* If system runs fine, don't mess around with it.
* If pressure is low, chances are the R134a amount is low.
Now if you have no leak, then it is safe to add a can of R134a.
This may not sound scientific but it is an acceptable practice for most DIYers.