can not make a/c to work - PeachParts Mercedes-Benz Forum

zxdent · #1 04-20-2003, 02:10 AM

Hi everybody,
My a/c on 87 300 d t converted to 134a before I bought it
worked fine last summer, but can not make it to work this year.
Checked freon. It had plenty. I was unable to find anywhere
more info on this models A/C and need schematic diagram and
componnent location diagram so I can trace the problem.
All electrical componnents I was able to find are 2 switches (pressure?) located on the filter dryer.
If anybody knows where to find the info I need, please let me know.
Thank you,
[email]zxdent@email.com

#2 04-20-2003, 07:53 AM

A Haynes manual will have enough info for this. You can't get one just anywhere though, it is only sold at the counter in the UK. There is somewhere in the US from which you can order one. Probably do a google search.

That said, will the compressor kick in? How did you determine that it has enough refrigerant? Is the belt in good shape and tight?

There is a sensor that will shut down the compressor if the belt slips.

Give us some more information and we can probably help you here.

Good luck,

zxdent · #3 04-21-2003, 01:14 AM

Hi,
Thank you for your reply.
I checked all that you suggested. Belt is good and tight and know that there is enough r134a because I charged with 20 oz.
and that is 80% of required amount for r12.
I also have Heines book on 87 124 body MB. but there is almost
nothing on A/C. Beside there is nothing in diagram section on
A/c. I was unable to find any source of information. Seems this
informations is a big secret. Without it I can not troubleshoot
the problem. There has to be a componnent that does not
function. It worked good last summer.
I am sure that somebody on this forum had similar problem and
would appreciate their help.
Thank you,
zxdent

#4 04-21-2003, 07:53 AM

It sounds like you have a leak. Put in some UV dye and use a black light to find the leak. If you don't find a leak at the compressor or any of the connections, look at the water that drains from the evaporator. If there is UV dye in that water, then the evaporator is leaking.

Good luck,

#5 05-11-2003, 11:07 AM

Man you are so right !! For the '87 300E 124 type, there are no manuals !! At least I couldn't find any. It must be the biggest kept secret in MB.

leathermang · #6 05-11-2003, 04:17 PM

Maybe you could switch to a 123... and use this book... about an inch thick...

leathermang · #7 05-11-2003, 04:18 PM

covers these years and models...more people should have this gem...

#8 05-11-2003, 08:34 PM

Yeah, that would be good... IF I HAD A 125 MODEL !!!

LOL

psfred · #9 05-11-2003, 11:06 PM

Does the compressor kick in? You will hear and feel it, and you can see the clutch plate on the front of the clutch spinning when it is engaged -- it will be stationary when not.

Check the following:

fuse for climate control (don't remember which one) -- if it is bad or corroded, you won't get AC.

Make sure you don't have the button marked "EC" pushed (next to to the off button) -- this is climate control without the AC compressor.

If fuse is good and the compressor doesn't run, unplug the two connectors to the low pressure switch and short together. If compressor runs, it's low on freon. You can also use a gauge set to check pressure.

If the compressor comes on once, runs for a few second, then shuts off and won't run again until you shut the car off and start it again, the speed sensor in the compressor is bad OR you have a bad serpentine belt, the belt tensioner is broken, or there is something else making it slip.

From here, you will need more expertise -- the operation of the compressor is controlled by a complex relay (usually a Klima), and you need to know what signals should be where before you start replacing things.

VeggiePup · #10 05-12-2003, 12:45 AM

I have an '85 300TD, the first year (as far as I know) MB put the Klima relay in the A/C system. Although yours is a different chassis number, I think our A/C systems are very similar. As suggested earlier, check the low-pressure switch at the receiver-drier. Three blue wires total; two connected to one side, one to the other terminal on the switch. The other switch is for the aux fan, and should have short pigtail connections instead of terminals. Using a test light (cheaper and quicker that a VOM) check to see if you have power at each terminal with the A/C on. Your test light should light up when touched to either terminal. If it does not light up, check fuses. Use the same test light. If fuses are OK, your climate control switch unit may be bad. You need a wiring diagram to check it.

If you show power at one of the low-pressure switch terminals, but not the other, the switch itself is bad. IF you in fact have plenty of refrigerant in the system.

If you show power at both terminals of the switch, but the compressor clutch doesn't engage, your problem could be:
a. bad speed sensor at the compressor
b. bad tach sensor (does the tach work?)
c. shorted engine temp sensor (cylinder head near air cleaner) or shorted sensor wiring. Check to see if terminal at temp sensor is grounded. It shouldn't be.
d. Bad Klima relay. As I understand, behind the battery, behind a plastic cover on your car. This was the problem with mine; I took the cover off the Klima relay and found a transistor flopping back & forth because of loose solder joints. Resoldering fixed it. Sometimes, if the compressor clutch has shorted windings, the Klima will burn out because of the excessive current. Taking the cover off will uncover the damage. Probably best to buy a new one, in that case.
e. bad compressor clutch. Find the wire (blue with green stripe, I think) going to the clutch. The one that isn't brown (MB always uses brown for ground wires). Using a jumper to connect 12V to the blue/green wire, the clutch should engage. If it doesn' t, the the clutch or its connector has failed.

No list can be exhaustive, so this one isn't. But it should give you a start.

To summarize, power comes through the fuse box, to the switch unit on the dash, to the low pressure switch at the receiver/drier, to the Klima relay, to the clutch. The Klima relay senses engine speed (cuts out compressor at wide open throttle, for example), compares it to compressor speed (to save the serpentine belt), and looks at engine temp. If all are OK, it lets the compressor clutch engage.

Good luck.
VeggiePup