1994 e320 ac evap freezing up?

[J. M. van Swaay]

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

[Matt L]

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.

[david s poole]

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.

[Arthur Dalton]

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

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

[Matt L]

Originally Posted by Arthur Dalton

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.

[david s poole]

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

[J. M. van Swaay]

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

[david s poole]

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.

[J. M. van Swaay]

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

[J. M. van Swaay]

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

J. M. van Swaay

[babymog]

I have had two '87 300Ds fail in a similar manner: the compressor would decide to come on during EC mode and even OFF. In both cases I replaced the CCU to rectify, these were both the non-bosch OE CCUs replaced with the Bosch OE CCU.