Charge by weight or pressure? Seeing hi pressure with a lot LESS than recommended R134 oz

[r-mm]

I did a complete R134a conversion on my 89 BMW and saw recommendations for approx 32oz of R134a. The system was fully vacuumed then I put two 12oz cans in, and I know you never get all of the refridgerant, so it has less than 24oz in there. Yet I see consistent 100psi hi/lo static, 50/225 lo/hi at idle. This is in approx 85deg/70%humidity. I am seeing about a 20deg drop from ambient to vent.

Any ideas? My instinct is its overcharged but I have no idea how that can be. If anything I think there is closer to 20oz in the system.

Could it be excessive air in the system? As I said I pulled 30hg vac for an hour, and tested it was tight, but I didn't purge the yellow fill hose prior to filling the system with each of 2 cans - is that enough to cause this symptom?

[bohica2xo]

134a conversions on R12 systems are never the first choice. Euro R12 systems were not very robust to begin with.

Did you flush all of the system components to remove the mineral oil?

Was a new, 134a compatible dryer installed?

How much & what type of oil did you install for conversion?

Depending on model BMW used a few different compressors. Some are better than others when it comes to conversions.

[r-mm]

Hello -

I replaced the receiver, pressure switch, exp valve, condenser (parallel flow) and compressor (reman nippondenso 10P) and some of the lines, flushed the others.

The reman comp came with 3oz PAG46 and the mfgr told me to add 3.75oz elsewhere so I did so to the receiver and one of the lines.

I do have the attached document from BMW published in 1992 which actually lists very similar numbers to the ones that I am seeing, but they seem very low ambition. Most of the cooling numbers show ~23-35 degree drop. At 80% rh, 80degf (conditions last night) it says to expect 47psi/218psi lo/hi and 57deg f at the vent. That is similar to what I had but I can't help feel this is incorrect.

Mostly I am confused about the low charge. I did a lot of reading last night and believe that a TXV system (which I have) may show confusing pressures if undercharged on account of liquid/gas phase changes not being correct. Can anyone help me understand this or how to use the sight glass (which the dryer has) to diagnose?

[bohica2xo]

Well that is all good news. I have seen too many Wing Cell systems with a double charge of oil from a quickie conversion.

So you have the correct oil charge, fresh parts and an upgraded condenser. All good.

The charge level in your system may not match any factory spec with a PF condenser. The good news is you have a sight glass.

With 134a the glass may not look the same as it does with R12, but the function is the same. You want to see liquid, not foam or a turbulent mess. By the time the refrigerant passes the sight glass it should be homogeneous.

To arrive at an acceptable charge on an unknown TXV system takes a little patience. You need to add small amounts of refrigerant, then wait for a minute and observe pressures. What you are looking for is the flat spot in the curve, where additional refrigerant is just sitting in the receiver as liquid. This is the space between 1/3 & 2/3 full on the receiver. When you are in this area, adding an ounce of refrigerant has little or no effect on pressures.

Run the car at idle for a few minutes with the gauges attached & the A/C on to let the system stabilize. Note & record the vent temperature. Set the cabin fan to low speed.

Now add an ounce of refrigerant. Wait for 90 seconds, and record the pressures. Add another ounce, or less. Wait. Record. If you see +2 psi, +2 psi +2psi as you do this you are in the flat spot. when you see +10psi at idle from an ounce of refrigerant you are probably slightly overcharged. Recover the last ounce.

Once you arrive at a charge level you think is acceptable, do a full load test & check the pressures. Doors open cabin fan on highest speed. Run the engine at 2000+ rpm for 90 seconds. While holding that speed check & record the pressures. This simulates drive away after a few hours in the sun.

Pressures run higher on 134a conversions. Your current high side pressure is very conservative. We use a P:T ratio to describe condenser performance, with P being the high side pressure, and T being ambient temperature. It is not uncommon to see a P:T over 3:1 on a 134a conversion at full load. In fact your posted charts show ratios of as much as 3.18:1 Here in Las Vegas at 115f ambient & 375 psi a system is still working ok. The HPCO will take care of anything like a blocked condenser before the pressures do any damage.


Since you are in a swampy location at 70% RH use the 40% RH table you posted, but add 15f to your ambient. That is the performance I would expect in your conditions.

[r-mm]

Thanks this description is much appreciated. I will try adding small amounts of refrigerant as you suggest and take a closer look at the sight glass.

What would be suggested to you by seeing a slight to modest decline to the low side pressure when I go from idle (which is low, maybe 750rpm) to ~1500-2000 rpm?

[bohica2xo]

What does the high side do in relation to the low side @ 2000 engine rpm?

[r-mm]

I just repeated this test at 86deg f / 65% RH

I'm not seeing that great a difference between hi side pressure at idle and 2000 rpm.

Idle:
40psi low
200psi hi

2000rpm
20-25 psi low
200psi hi

[bohica2xo]

Looks like the TXV is doing it's job. And you are slightly undercharged.

With the cabin fan on the highest speed, and the doors open the high side should climb some.

The TXV is trying to maintain a 40f discharge line by modulating the flow. At idle there is not enough compressor volume to pump down the suction line.


There is a substantial slope in any compressor output curve from 500 to 3500 RPM at the compressor shaft. This is not your specific compressor, but you can see what I mean:

[r-mm]

Thanks for your help. I see the relationship that the graph implies but I wonder if you could be a bit more explicit about what I should see as far as pressures are concerned? Do I want to see little to no drop on the low side when I rev to 2000? Or am I expecting to see a drop (and if so about how much) but one that is coupled with an increase in the high side?

I added ‘a little’ charge and am seeing somewhat greater drop.

[bohica2xo]

The operating envelope of MVAC is huge compared to stationary stuff. That is one reason we never talk about superheat.

The airflow & heat load on the evaporator vs the airflow & ambient temperature for the condenser, and the compressor capacity - think about an EFI map with all of that on it. That is why a single set of gauge readings don't tell the whole story.

We ask for things like a full load test with the doors open, the cabin fan on highest speed & 2000+ engine rpm. That is close to what happens on a summer day after work when you start the car & drove off. Evaporator load high, road speed low, compressor volume high, Condenser load high.

At Idle the compressor is not moving a lot of refrigerant mass. The evaporator & TXV can manage a medium fan speed on a car that has cooled down. So the low side pressure may look a little high, because the TXV is wide open - and the air over the evaporator is recovering the heat from the refrigerant. Increase the engine speed a little with the fan speed on medium - and the low side drops. The TXV closes a little, and some of the charge stacks up in the receiver. The high side stays flat, or may even drop a little as the engine fan improves condenser performance.

So we normally shoot for the corners of the box if a system is acting up. Cabin fan on low, Idle then 2000+ rpm. Cabin fan on high doors open idle then 2000+ rpm. The rest of the points in that box are usually not where the trouble is - 65 mph on medium fan & 80f ambient for instance.

The low side should drop when you increase engine speed IF you have a significant heat load on the evaporator.