kricoperry

Hi,

When I turned on the A/C the other day all I got was hot air. A couple of weeks earlier when I tried it for the first time this spring, it seemed to be working. Now the compressor isn't engaging. I'm going to do some trouble shooting. Does anyone have:
-a (digital file) wiring diagram for the 1994 W124 climate control
- the location of the N6 relay
- most probable causes
- any suggestions on where to start the process?

Your help is appreciated.

Regards,

James

Eliot

Any fault could put it in failsafe mode... which is full on defrost. Its worth checking the vacuum pods.

Chappy

Hopefully, you won't need a heater core replacement (notorious on the W124) and one of the most expensive things to fix on the car. I would suggest picking up a copy of the W124 Bible, if you haven't already.

kricoperry

Thanks for the comments on the AC. I've ordered a copy of the "Bible".

James

Chappy

James,

I checked when I got home....I've got on original Mercedes-Benz Electrical Troubleshooting Manual (USA) for the W124, but it only covers Model Years 1986-1992. Sorry....

kricoperry

Chappy,

Thanks for checking. Hopefully with the Bible and by doing some fault code checking and circuit checking I can identify if it's an electrical problem or a mechanical issue.

James

vanwiek

Here are w124 wiring diagrams.

http://mercedes.vanpeters.com/124wiring.pdf

and climate control testing

http://mercedes.vanpeters.com/climatecontroltesting.pdf

Kestas

To add to Chappy's advice... check the pressure of your a/c system. If all the refrigerant is gone, then you've narrowed down the problem.

kricoperry

Thanks Vanwiek,

My car is a 1994 E320 with the 16 pin diagnostic block, so I'm not sure if the wiring diagrams and test procedure strictly apply, however I will look to see if I can use them.

Thanks again.

James

kricoperry

Thanks Kestas. I guess I'm hoping it's an electrical fault, and I could eliminate that before taking it to the shop, but you are right, the pressure check is basic. I'll have to look into getting a PG or take it to the garage after doing a bit more checking.

Regards,

James

vanwiek

James, your welcome. I also have a '94 E320. It will work, I've check for codes in the climate control system in the past just for the fun of it. Here's some more info from http://www.alldatadiy.com. It is a great site for learning how the car works. Between Alldata, the E-Class Owner's Bible and the Factory CD's you should have all the info you need, they all complement each other. I think all three resources are a must have. Alldata is also cheap, it is $24.95 for the first car for the first year. Renewals and additional cars are only $14.95 per year. I have alldata subscriptions for all my cars.

Anyway below is some info from alldata.

Testing with the impulse counter should be carried out for rapid fault diagnosis. The impulse counter output shows only existing faults, since the ACC system does not have memory capabilities. If one or more faults are indicated, repair as necessary and repeat the test. Be sure that fuse #7 is good, the battery voltage is between 11-14 volts and the temperature selector wheel is set to 22°C.



Connect the impulse counter to the on-board diagnostic connector.
Black lead of the impulse counter to socket 1
Yellow lead of the impulse counter to socket 7
Red lead of the impulse counter to battery positive terminal.
Turn the ignition to the "ON" position.
The LED "U Batt" will be illuminated. If the LED "U BATT" does not illuminate, check for voltage between socket 1 of the on-board diagnostic connector and the positive terminal of the battery. If the voltage is not between 11-14 volts, check for an open circuit or loose ground connection.
Test for voltage between socket 7 and socket 1 of the diagnostic connector. The voltage should be 6 - 12 volts.
Depress the start button for 2 - 4 seconds.
Fault codes will be indicated by the number in the impulse counter display window. Take note of the impulse display number.
Faults are displayed in ascending order.
The number 1 indicates that no faults are stored in memory.
During the impulse readout, The LED indicator in the fresh air/recirculation switch will blink.
Repeat step 3 until all faults are indicated. After all fault codes have been displayed, pushing the start button again for 2 - 4 seconds will repeat the fault code readout sequence

IMPULSE (FAULT) DISPLAY RESULTS




Refer to the IMPULSE DISPLAY RESULTS, then proceed to DIAGNOSTIC CHARTS for specific diagnosis.
Repair the indicated faults. Be sure to turn the ignition switch to the "OFF" position before disconnecting connectors or components unless otherwise directed by testing procedures.

vanwiek

James, you can do a visual check yourself, see below.



Let me know if you need help finding the inspection window on the drier.

-Ken

kricoperry

Ken,

Thanks for the help! I'm not sure where the window is.

Regards,

James

vanwiek

James,

I'll take a photo and post it as soon as the sun comes up.

-Ken

vanwiek

James, the window is in the yellow circle in the attached picture. It's the drivers side between the headlight and ABS pump.

Ken

bicylindrico

The best advise on this thread yet. It is typically best to start with the basics and perform the easiest tests first. You guys are working backwards. I personally would start at the low pressure switch and short the harness with a blade fuse. If the compressor engages, you are out of freon or the switch is faulty. If the compressor engages at startup but only momentarily, I would suspect the speed sensor on he back of the compressor. Start with these and get back to us. In my over ten years "tinkering" with these cars, I NEVER read fault codes from the ACC unit. My opinion is it will just confuse and drag out the diagnosis.

The ACC units were pretty dependable by the mid 90's and rarely fail. I am not saying it never happens and you don't have a strange fault with your car, but at least confirm there is adequate freon pressure to turn on the compressor clutch. Maybe I missed something.

kricoperry

Thanks everyone for your help. All the information was helpful. I did check the AC fault codes (after building a code tester) and everything was OK. I took the car to an independent local MB specialist who checked the system pressure and found the refrigerant charge low. It's been evacuated and recharged (with dye added). So far it's working OK and I haven't seen any obvious leaks. Thanks again everyone. It's great to have such a wealth of freely offered information out there!

James