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Old 09-19-2002, 11:47 PM
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Join Date: Jul 2002
Location: Stony Brook, NY
Posts: 160
Question for M.B. DOC or anyone else who might know

Earlier in the week I posted a question about my continuing problem in diagnosing a a/c compressor that suddenly won't engage. To summarize quickly: I removed the Klima relay, opened it and cleaned it with circuit board cleaner and the relay contacts as well. Re-installed it, still nothing. I removed it again and jumped #5&7 on the receptacle in the car and the clutch engaged fine and a/c worked.

So, my question was how to diagnose whether this was a Klima relay problem or something else. Basically, how do you tell if the Klima is bad. M.B. DOC, one of our Moderators was kind enough to post a reply to help me. Someone had told me to check the "105/115" sensor. M.B. DOC explained my 1988 300SE doesn't have this "sensor". Here is what he said,

"To determine whether the problem is inside OR under hood, you must see if the pin number 10 on the "klima" relay gets a GROUND command signal from the pushbutton assembly. WITH the car running measure for battery voltage between pins # 5(power) & pin # 10, IF you have battery voltage then the problem is in the "klima" relay.
Your car doesn't have a 105/115 switch like the 1987 & earlier cars have. Your car has a blue 2 pin BLUE sensor that talks to the pushbutton & if the engine is too hot(or bad switch) the aux fans run & the GROUND command signal to pin 10 is shut off!!!"

Here's what I did: I started the car with the Klima removed. With the climate control system OFF I took a 12 volt test light and put the positive to #5 and the neg to #10. Result= no light.

I then turned on the climate control system to full minimum temp and pushed the button for the system to activate. Hooked pos back to #5 and neg to #10. Result= no light.

So, my first question is did I test what MB DOC was telling me to or was he telling me to use one of those hand held electronic analyzers that I would connect to #10 to test for a ground?

My second question is, if I did test it right and no light means the Klima is OK, what is the problem? Is it the switch in the car? I'd find this hard to believe as all three buttons that would turn the compressor on failed to make the light go on while testing (defrost button and the 2 buttons with the up & down arrows on them). It seems odd all 3 buttons would fail at the same time.

Other than this the car runs great. I should mention that many people have said a bad Klima results in poor tach gauge performance. I have not had this symptom at all. The a/c simply worked fine for the first week I owned the car and literally overnight, the clutch won't engage.


Ron B.
Ron Brooks
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Old 09-20-2002, 07:31 AM
Posts: n/a
I don't know anything about the Kilma relay because I have never had to mess with one, but I can tell you something about your compressor not kicking in.

The most likely reason that the compressor will not kick in would be insufficient refrigerant in the system. There is a pressure sensor that prevents the compressor from kicking in if there is insufficient refrigerant. This serves to prevent compressor damage.

Hook up gauges and see if you have static pressure, which should be around 80 PSI. If you do not, then the system is low on refrigerant and that's the road you need to follow.

Good luck,
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Old 09-20-2002, 08:08 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
The test supports that the problem is inside of the car, EXCEPT if the aux fan switch(blue two pole) is telling the A/C pushbutton that the engine is overheating.
Again the A/C pushbutton sends GROUND command signal to the pin # 10 on the KLIMA relay, that ground signal GOES thru the low pressure switch on the drier. SO if you run the car & have the air on there should be a ground signal at that switch as well. IF not then the A/C pushbutton isn't sending any signal. IF that is the case then you need volt/ohm meter tests on SENSORS to determine why not!
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Old 09-20-2002, 08:59 AM
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Join Date: Jul 2002
Location: Stony Brook, NY
Posts: 160
Thank you both for your reply. Since I don't have the proper gauges for the static pressure test or the voltmeter, I brought it to my local shop since it was time to get the temp inspection off the car and get the car officially state inspected. My mehcnic said he'd give the car a once over (he was not available to do the pre-purchase inspection so another fellow did it) and he said he'd check the a/c system out. the first thing he mentioned after I told him what I did to the Klima was either the pressure being low or the buttons in the car. So, I feel comfortable with his level of expertise since he seems right on.

Thanks again for your input. I'll post the results when he lets me know later today.
Ron Brooks
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Old 09-20-2002, 09:14 AM
Registered User
Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,784

Jumping 5/7 with relay out is simply bringing 12 v to the clutch, it does NOT mean the klima is bad..
You problem is the klima primary coil circuit [ that activates the contactor for the clutch] is not seeing a ground.
So, as Doc says, it can be in the controller..
However, your next step is to jumper the S31/1 switch at the
drier/reciever as the sig has to go there Before it goes to the controller. [ As Larry said]
**[ look at the schematic I sent you yesterday - see -s31/1]

If jumping that sw runs the clutch, it is open/or low on freon and is the prob..,
if it doesn't , is in the control/wiring..
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Old 09-20-2002, 09:43 AM
Registered User
Join Date: Jul 2002
Location: Stony Brook, NY
Posts: 160
Unfortunately, my abilty to read a schematic is quite limited. But I do have until spring to figure it out. If the mechanic can diagnose it today I'll see what the cost is. He has always been very fair so I expect it will be fine. If he can't get to it today, I'll have to spend more time figuring it out. Always good to have a challenging project to keep you on your toes. He is definitely checking the pressure which is good since I don't have the gauges. A good voltmeter is not too expensive so I can get one and learn to use it which would be a good skill to acquire. Fix a problem and get an education to boot. Not a bad deal.

If I need to diagnose it myself, I'm sure I'll have more questions in the future. As always, I thank you all for your invaluable input. If not for this site, I maight well not have felt comfortable enough to buy this car.
Ron Brooks
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