230TE Aircon freezing up- Fixed

[bahnstormer109]

Gday

My 1986 230TE has an aircon issue where the evaporator seems to be freezing up and blocking the airflow. It also seems that the compressor and radiator fans are running constantly instead of cycling on and off.

A bit of history- When I bought the car nearly 4 years ago the aircon didn't work and the cabin fan only worked on high. Fiddling with the plug on the compressor got it to work again and taking out the cabin fan and oiling the bearing got it working on all speeds. For the next three years I had good aircon, but it sometimes on a long drive would freeze up and block the airflow. There was always a drop of dye on the bottom of the compressor so I knew it was leaking and on borrowed time and I would fix it when it ran out of gas.

I have now got to replacing the compressor, condenser, receiver dryer, tx valve and the two switches/sensors next to the receiver dryer as well as a re-gas. The aircon is now icy cold, but the evaporator is freezing up and the compressor and radiator fans are running constantly.

Can anyone tell me what needs to be done to stop the evaporator freezing up? Is there a sensor on it somewhere? And should the compressor run constantly or cut in and out? I have the temp wheels inside the car set on white- 22* but whether on MIN or 22 it still seems to be running at full cold and freezing up. I have also changed the Klima relay with a couple of others and there has been no change in behaviour.

Any advice is appreciated.

cheers,
Paolo.

[Tony From West Oz]

Sounds like the thermostat is shot. IIRC, the W124 used the Air conditioner to give a constant cold air supply and adds heat to give the desired temperature. If the Thermostat is short circuit, it will have the compressor running flat out without shutting off at all, to allow any frost on the evap to melt and drain away.
That is the starting point I would choose.
What happens if you use the EConomy switch on the HVAC panel? Does it defrost and allow airflow again?
If so, that supports my theory.

[MMWA]

Klima relay hey? Is this the early version as used in the w126 with a separate control unit box under the passenger side of the dash?

My 1990 uses the later style with everything inside the operation/control panel and the MAS relay switches the compressor on as well as fuel pump and some other consumers.

On mine the compressor cycles on briefly and about double the time cycled off with the heater solenoids clicking on and off to maintain temperature. It should not remain constantly on unless it is a very hot day and the system is failing to keep up with the heat load in the cabin.

I would suggest your control unit / operation panel is failed. Depending on the version (mine is dual zone) you can test the temperature sensors at the evap coil, armed with the wiring diagram too.

Also sounds like some wiring butchery going on potentially, the aux fan(s) should come on when either coolant temp exceeds a certain amount or refrigerant pressure is too high (on a hot day) if someone has modded the aux fan to come on with the compressor, common with R134a conversions, id be looking at this modification too.

[bahnstormer109]

Hi Tony,
Thanks. The compressor turns off when I select EC and back on when I select AC. Where is the thermostat located?

Hi MercmadWA thanks. Mine is the earlier type with the buttons that clunk in and out, not the later type where you touch the buttons and a light comes on to tell you it's on and the car can turn the AC on and off itself.

The radiator fans running constantly only started happening yesterday, I suspect it may be a faulty new switch or just because the compressor is running constantly?

[Christo C]

My '86 230E is probably fairly similar to your '86 230TE in this regard.

Apart from the KLIMA relay (N6), which basically controls the Compressor Electromagnetic Clutch (A9) there's also an Automatic Temp Control Unit module (N20) behind the glovebox which controls a line to the Refrigerant Pressure Switch (S31) (vertical switch in the pipe next to the Receiver/Dryer).

The Automatic Temp Control Unit module (N20) considers all sorts of readings from a series of Temp Sensors around the Heat Exchanger, inside Air and Evaporator (B10/3, B10/2, B10/4, etc) .... as well as controlling the Duo Valve (Y21) (in heater hoses, probably near the Brake Booster) and the Heater Circulation Pump (M13) (in engine bay, in heater hose before the Duo Valve).

There's also a (always running when ign is on) small air blower (M9) behind the glovebox constantly drawing air down a 1cm tube from the Inside Air Sensor (B10/4) (sensor is located in the Interior Cabin roof Light).

Attached is a PDF circus -

83-0506 - Wiring diagrams Air conditioning automatic temperature control Model 124.pdf

My '86 230E is "Model 124.023 from August 1985 to August 1987" on Pages 8 & 9 - yours I'm guessing is similar....

Here's some other helpful PDF -

83-0020 - Function description automatic heater control system A. General, B. Function of temperature control, C.pdf

83-0500 - Function description air conditioner automatic temperature control Aut. temp. ctl.-unit, Components, Compressor, Evaporator, Aux.pdf

Now this last document is quite interesting in light of your problem: In Section E it talks about the Temperature Sensors, in particular it suggests a correctly operating Evaporator Temperature Sensor is critical to prevent compressor running continuously and evaporator icing up.

[bahnstormer109]

Thanks Christo, they are very useful manuals.

I did a bit of poking around this morning and found that the cabin temp sensor in the roof light had broken clips and was sitting up in the roof instead of clicked into the vent. I swapped the whole light unit over with one that had a sensor properly clicked into it.
I then found the little air pump under the glovebox and it was not running. I took it apart and cleaned it and sprayed WD40 into it and it is now running smoothly. (I wonder how many parts have been thrown out and replaced or even whole cars scrapped for things that a bit of WD40 could have solved?).

It seems that the aircon is now not excessively cold when set at 22* and when I move the temp wheels to MAX hot the compressor switched off, and back on again when I moved the wheels back to cold. I haven't had a chance to take the car for a long drive to see if the evaporator still freezes up, but having the cabin temp sensor working properly again hopefully will make a difference.

The radiator fans still run constantly when the compressor is on though, which I suspect may be a separate issue, and maybe a faulty new switch that was just replaced.

After a short drive just now the pipe in the engine bay is forming ice which I suspect it shouldn't? I have a suspicion that if ice forms here, it will also be forming on the evaporator? (I tried to post a photo of it here but the forum tells me the file is too large no matter how much I reduce it).



I think I have come to the end of what I am able to do, and it will have to go back to the mechanic, who I will show this thread and attached manual references to.

[Christo C]

Yeah in these old cars with complex systems faults are hard to diagnose, esp when there can be multiple faults all contributing ... I too found the little air pump behind glovebox was stalled; WD40 fixed it for a time but it was not happy; the bearings and brushes were cactus after 30 years ... eventually I replaced it with two tiny 12V 30mm computer fans wired in parallel but mechanically in series - they fitted into the impeller cage in the original holder quite well, and are cheap as chips.

If you measure the resistance of the cabin air sensor (at "room" temp) all the other sensors should read about the same.

Ice on pipe in engine bay seems excessive, though I wish my AirCon could get as cold as yours, well nearly....

[MMWA]

The early version with remote electronics modules under the glove box as per w126 develop bad solder joints. Do you have a spare module to try ?

Suction line should never be getting iced, evap coil should never get iced even the 70's bezes i've owned before have methods to prevent the coil icing. This issue is electronic.

[bahnstormer109]

Hi MercmadWA

I haven't examined that box yet but I think I know which one it is, I saw it when I was taking out the air pump. I'll try and take it out and see if I can open it up.

I don't have a spare one yet. I think things are pointing to the sensor on the evaporator which I need to find as well.

Cheers,
Paolo.

[MMWA]

i doubt the issue is with the evap sensor it's probably the dodgy module just not seeing it.

[bahnstormer109]

I have resolved this issue- in this case it was the evaporator sensor.

I took one from a 1994 E220 at a wreckers and I thought it would be a good one since that car is a lot newer than my car.

I drove for over two hours non-stop today after changing the sensor and the aircon is all working correctly. The compressor and fans cycle on and off and the air temperature is responsive to what I set the temp wheels to.

The sensor on a W124 is located on the left side of the centre console up beside the glovebox, it is the same as the sensor in series two W126 but on these it is located in the right side footwell up the back above the driver's foot rest.

I'm happy this was an easy fix and not the ECU box or anything more complex.

Thanks to everyone who gave advice.

[MMWA]

well that's good news.

something to remember for the future, it's almost certain to happen to someone else too.