AWR-Fix: AC problem diagnostics

[andrewwynn]

Had some very crazy things going on with wife's AC yesterday.

Road trip just 3-4 hour drive one way and no problem until halfway home and driving into the sun suddenly AC not very cold. And getting more and more uncomfortable in the car especially since aiming into the sun.

I had the foxwell with it was actually plugged in at the time I was monitoring O2 due to the weak cat on that car that occasionally sets MIL/SES duebroow cat efficiency.

I dig though and find HVAC real-time data (note: very happy I did the 20-pin short out under the hood so this works from the cabin)



Evaporator 20° warmer than the heater cores. That's literally not possible so I'm confused.



1.58bar is way too low so I'm sure freon is low, froze evaporator solid ice blocking the air flow.

I turned off the AC and left the vent on with temp set to 68 or so. In four minutes enough ice melted to get air flow and for the last 10-15 minutes driving home had ice conditioned air.

I got some R134a (with dye and sealant since of course it must be leaking albeit slowly it's been 5 years since I refilled)

Go to hook up the gauges and when I start to remove the high/hot side valve cap it starts venting like mad.

I had the car off so I hook up the cold/low side to get a sense of how much freon lost

It was over 70-80 psi equalized (ish) with high side so I pull off the cap and as quickly as possible put on the gauge to hold in the leak.

I tried without success to reseat the shraeder valve but I didn't have a spare nor know where my vaccum pump is at the moment so I figured I can. Rely on the o-ring seal on the cap for now.

It only took maybe 5-6oz R134a to get it back to normal but the best part was the sealant in the R134a can helped: I exercised the high side valve a few times and when I took off the gauge it held!

Now for the weird part:

The pressure on the gauge read 45 psi. Which is 3.1 bar but only registered like 1.85 bar on the foxwell so either the sensor is way off or is measuring maybe the output of the evaporator into the compressor vs the same line where the cold port is.

In addition, the evaporator temperature never came back to normal. Now it's possible it was still insulated by a block of ice that didn't melt yet so I'll have to do another test today.

One time after hitting a bank of snow the air duct where the ambient air sensor is was insulated with snow pack and read the same temperature for about 30 hours I'll report back what I find out.

Anyhow the symptoms were: fan was going faster and faster as the cabin temp rose up to 85 but you could barely feel the air flow just lots of noise.

In addition evaporator temp hotter than heater core temp.

Then when I figured out evaporator froze just turned off the AC and after five minutes of driving plenty of cool damp air from the vents from the air blowing over the block of ice.

It came very quickly, we'd been using AC every day for the for day trip and it worked fine up until the last day when we used it for the 4 hour drive home and the last 90 minutes the AC just stopped working.

From experience with this car when I first bought it, freon was low and the evaporator would freeze up within ten minutes but If i turned off the AC for 5-10 minutes it would melt the ice and work again for another ten minutes. Was a very warm/cold ride home the first five hours of ownership.

When freon gets a little low, it actually increases the efficiency of the system and you'll get a frozen evaporator.

I'm wondering if my EVAP thermistor is defective because I thought the compressor won't kick on when evaporator is below a certain temp but maybe DME uses the outdoor temp to make that call.

I'm curious enough I'm going to go measure mine and wife's now to see the temp and pressure.

[upallnight]

Low refrigerant pressure is required for a freezer, but not for automotive HVAC. A freezer is not being used to removed moisture from the air so moist cold air is not being cooled by the evaporator. Also defrost free refrigerators have thermistor that control a heating coil that defrost a refrigerator to prevent ice up. Lowside pressure should be around 35-40 psi. Any lower pressure is a guarantee that the evaporator will start to ice up.

Not sure if a thermistor is in the BMW since the BMW does not have heating coil to defrost the evaporator core.

[80stech]

The evaporator temp sensor is in the airflow coming out of the evaporator not stuck into the evaporator itself.

[andrewwynn]

I did a test drive today and it first I was confused because the evaporator temperature read 67 but after I drove a couple miles It registered 40.

And heat Exchange is both red numbers warmer than the evaporator so all that started to make sense.

i’m still confused about where the pressure sensor is because it red much lower than the inlet port on the low side. but I’m thinking the inlet port is before the expansion valve and evaporator and the pressure port is actually between the evaporator and the compressor.

Also oddly on my wife’s car the pressure sensor reports the pressure in bar and on my car which is also a 2001 but has the pressure sensor reports in volts.

I was very glad to see the temperature on the evaporator drop to 40.

Thanks Don for filling in a bunch ofdetails I left out.

[andrewwynn]

The point I was trying to make is that when freon gets low a primary symptom is that the dash air will get much colder than it had been because the Δ T is directly proportional to the Δ psi. I aim for 45/250 which is 5.5:1 ratio. When freon was low just now it was closer to 35/225 or 6.6:1 ratio. I find that when the ratio gets over about 6 you'll be at risk of evaporator icing up. If you get to 8:1 you'll get 10 minutes max of cooling on a humid day.