300D AC diagnosis help request - PeachParts Mercedes-Benz Forum

brilliantblue · #1 07-08-2014, 01:25 PM

I've been searching and will continue to . . . but please offer advice if you have it.

Here's the situation - 1984 300D "California", 194000+

My AC does not blow cold. This seems to have happened during a seasonal break several years ago, and I cannot remember if it coincided with an accident (deer) or after.

The compressor is not receiving voltage. The Monovalve appears to have 13.4 volts continuously.

All fuses are good and verified voltage on both sides (save for headlights, etc that were not in use). Working blower motor.

I removed ACC and checked & reflowed solder joints, tensioned connecting sockets, cleaned connecting pins and switches, verified continuity of traces and functioning of relays on ACC board.

I've swapped places of both 'ice cube' relays, but do not have a known working one to put in.

Just figured out where the aux water pump is and haven't yet checked voltage there.

Temp wheel changes temperature from cool to hot - its a little slower going back to cool again, but the range is there - just no AC, no Cold.

What is the next thing I should check and how? I've wondered about the temperature sensor. Turning the wheel does not consistently result in a "click" as I remember from the past - though it seems to make the sound when coming off of the Max Heat detent.

At one point a few years ago, I asked my brother (mechanic) to charge the AC while I had the car at his shop. He couldn't get the compressor to kick in and at the time thought it might be related to the ACC unit. I needed the car (have only one) so did not leave it for more in depth investigation. However, after going over the unit, I believe something else is suspect.

Thanks for your time and help!

Jeff

brilliantblue · #2 07-08-2014, 01:28 PM

A few years ago, a vacuum pod failed in the cabin causing all the vents to close unless set on cold AC - this was in the winter. No wanting to tear into the dash (without a spare car) I opted to disconnect the corresponding vacuum line - now all vents are "open" except for the floor vent. (Center vents - hooray!)

vstech · #3 07-08-2014, 01:40 PM

first step is to see if any refrigerant pressure is in the car.

Phillytwotank · #4 07-08-2014, 01:41 PM

simplest thing could be that it is low on refrigerant. If you car has been "converted" to r134 this is a very likely scenario as r134 cannot be contained properly by the original hoses.

Check the static pressure of the refrigerant. this will tell you if there is any refrigerant in the system.

Short the wire leads to the pressure switch on the dryer. if everything else is good this should cause the compressor to kick on regardless of refrigerant status.

If the pressure is too low the pressure switch will prevent the compressor from engaging.

vstech · #5 07-08-2014, 01:42 PM

next I'd get a leak check done, if it was empty, and fix the leak

then I'd recharge the system with a pound of refrigerant, and see if the compressor will click in.

if not, I'd trace out the circuit and find out why. the pressure switch may be bad.

brilliantblue · #6 07-08-2014, 06:38 PM

I'll update after checking the switch and the charge.

brilliantblue · #7 09-06-2014, 10:34 AM

Sorry for leaving a stub . . .

I confirmed the compressor would engage by bypassing the low side switch, key on, engine off.

My shop mechanic brother had gauges and freon and was able to do a leak test - no leak, determine there was no charge, and put a small charge into it. Compressor cycled on but made a someowhat uncomfortable sound. Receiver drier had a restriction - it was getting cold there instead of in the cabin. I initially purchased a new RD and Expansion valve. Discharged/reclaimed system freon. When replacing the expansion valve, some green-blackish gunk came out. This and the labored sound of the compressor led me to purchase a new compressor. I've got the drier and compressor waiting for another opportunity to get to his shop, where we'll flush the system before installing the new components.

leathermang · #8 09-06-2014, 10:51 AM

" no leak, determined there was no charge".....
' DEER' is a large question..
but why no charge if ' no leak ' ?
does not compute..

which compressor do you have ?

that greenish blackish..

given the cost of the parts you are going to install...

all things considered... I THINK YOU NEED TO TAKE APART WHAT YOU CAN OF THE SYSTEM AND FLUSH IT PROPERLY...
otherwise you may ruin the new parts and once again have to start over...

lots of threads in the archives... and some in my signature about AC stuff..

Would like to know more about the ' receiver restriction' ....

This sounds like the container of dessicant gave up the ghost... some are small bags of it...and through time and moisture and vibration can disintegrate....
but I do not think that would cause it to be cold... two different issues...

leathermang · #9 09-06-2014, 10:55 AM

You have not said which refrigerant you have... you said ' freon' which technically refers only to R12.... but is often used instead of ' refrigerant'...
we need to know if you have R12, R134a , or something else...