1994 e320 ac evap freezing up?
 

1994 E320 Wagon
223,000 miles
new evap and expansion valve 3 years ago
blower motor good, possible fault with vac pods on fresh/recirc flap

Symptoms:

AC cools very well when car first started. Sometimes, AC will slowly loose cooling efficiency after 20 to 30 minutes. Under hood inspection sometimes reveals white frost/ice on low side plumbing between expansion valve and fuel cooler. Also, when switching to EC mode, some cooling can still be felt for up to 10-15 minutes after switch to EC. Climate control module gives 1 blink when checked for codes. (however, home brew code reader led stays on dim after giving one bright flash.......not sure what that means)

I suspect evap freezing caused by a faulty evap temp sensor.

Question for the experts: Is it possible that a faulty expansion valve could cause this problem, or is it more likely that the evap temp sensor is bad?

My plan is to check (with Ohmmeter) the temp sensor and compare to spec values from the service CD.

Any other suggestions or comments?


Thanks,

J. M. van Swaay

It does appear that you need a new sensor.

But it shouldn't freeze even without the working temperature sensor. That sensor is a back-up safety device. The pressure in the evaporator should be regulated by the control valve in the compressor, keeping it above freezing.

It could be that the expansion valve is not letting enough refrigerant through. It's more likely that you are a bit low on refrigerant.

it would be a good idea to read the pressures while all this is going on.i suspect that you could be overcharged by the symptoms that you describe.it is not normal to have enough extra refrigerant to freeze the suction line back to the fuel cooler.

Check your intake air filters.....................

Frost on suction line is never low refrigerant..just the opposite..over-charge or evap temp sensor.

Of course. I don't know what I was thinking.

But still, I do not believe that this system is supposed to cycle. Am I correct in assuming that it uses a variable-displacement compressor?

Certainly the system pressures are important, especially when it is getting close to icing up.

it's supposed to cycle--standard clutch.two ways this happens.the thermostat is satisfied or the evap temp sensor opens the circuit.

Thanks for the responses.

Matt, I believe this is a cycle system.

Mr. Dalton, thanks for the post. I doubt the system is overcharged. When I replaced the evap a few years ago, correct refrigerant quantity was added based on scale weight--nothing has been added since, and the ac has been problem free for two years.

Evap temp sensor is fairly cheap and easy--think I'll try that first.

Thanks again.

J. M. van Swaay

based on your last post,charge not changed for two years,the only thing it could be is restricted air flow over the evaporator.either the evap coil is very dirty or the blades on the fan are full of dirt and not moving enough air.

Forgot---I'll check the cabin air filters as well. I replaced them when I did the evap, maybe they are due for replacement again.

J. M. van Sway

Air flow from vents seems normal. Blower motor replaced about 1 1/2 years ago.........squirrel (sp?) cages relatively clean.

J. M. van Swaay

David, if poor air flow, wouldn't compressor shut off because of low evap temp? Seems this would indicate bad evap temp sensor................

J. M. van Swaay

Update:

Removed evap temp sensor--Ohms check good, values in line with chart provided in service information.

Reinstalled sensor.

Connected gauges, after 10 minutes and at 2000 rpm windows down doors closed max cold and max blower high side about 19 bar (280 psi), low side about 2 bar (29 psi). OAT about 29c (85F)

Vent air felt cold but I didn't have a thermometer....... Pressue values are in line with performance chart in service information. (high pressure maybe very slightly low, but compressor is original--223,000 miles...) No ice forming on low side plumbing.

Is there something that could cause an intermittant icing condition?

Thanks for reading.

J. M. van Swaay

<windows down doors closed max cold and max blower >

I would try windows UP, max cold, and low blower/recirculate when checking for suction line freezing............

Thank you Arthur--I'll try that to see if I can get the thing to ice up again.

I wanted to check system pressures with windows down, doors closed, max cool and max blower because those are the conditions on which the chart in the service manual is based. From this test, I think I've ruled out over serviced refrigerant

I have since bought a vent temp gauge, I'll leave it stuck in the vent over the next few days. If I notice temps below 40F just prior to the loss of cooling, I'll know the system is icing up. If and when that happens, I'll jumpout and see if the compressor is still engaged. Maybe I can figure out what's happening.......

Stay tuned

Update:

I've been driving around with a thermometer stuck in the vent since my last post. Finally today, an observable fault occured. Ambient temp in the mid 70s so AC not really needed. Selected EC mode but noticed air coming from vents about 40F. Checked under hood, sure enough, compressor engaged. Tried turning system to O--compressor still engaged. Maybe magnetic clutch not disengaging properly. Disconnect three pin connector from compressor and compressor disengages. Check schematic--three pin connector leads to N6 (A/C compressor control) Find N6 location from service CD, swap N6 with unit from my other car and now operation normal. Other car compressor operation faulty with suspect N6.

N6 part number is 003 545 56 05, referred to as compressor relay on peachparts and allparts express.

A non disengaging compressor would explain evap icing. Anybody else experienced failure of this part?

J. M. van Swaay