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Old 06-13-2004, 03:17 PM
Registered User
Join Date: Mar 2003
Location: Beautiful Western NC
Posts: 20
Question air conditioning diagnostics

We got enough time off for a trip to the beach, had a great time till we got in to come home in our 89 420 SEL. No AC... It had worked fine up to that point. I have spent hours reading old posts trying to educate myself. Also have spent several hours with the repair CD.

Here is what I have checked or found so far:
Power through all fuses.
Shorting the pressure switch on the reciever dryer makes no change.
Compressor is not turning.
When defrost button is depressed, compressor still does not start
If temp select wheel is turned to red, blower blows hot; if set to blue, temp is ambiant.

Last winter I noticed that the blower would start blowing before engine was up to 33 degrees C, so I have been searching for the cold engine kill switch, but have not found a single pole sensor to test. I did however find a 4 pole sensor between the distributor and the air horn/meter body. Unable to find any reference to what this sensor is. ??????

I have located the single pole temp sensor for the gauge and the two pole sensor for the fan.

I have located the klima relay # 002 545 18 05

Can some one help with figuring out which part in this circuit needs replacing or repair

many thanks!!!!!

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Old 06-13-2004, 04:11 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The first decision to be made is whether the control is being stopped by the engine system or the AC system.

Inside the car you make a decison for AC by not hitting the EC button. Every other position has AC ON! This decision proceeds through the low/high pressure switch to the Klima relay. Electronically the decision is effected by pulling the signal from the Klima to ground. The only exception to this is when the AC gets to icing temp it is shut-off. If the problem is inside control this is the first decision to check.

So lets look at the signal. Lets do it at the low/high pressure switch, since it is just so easy. We will start with the EC position. At either terminal of the LHP switch (low/high pressure) one will see battery voltage. If the voltage is different on either side of the LHP switch the system is out of refrigerant.

Now press any other button but EC and the voltage at the LHP switch should drop to around 1v or close to zero. IF THIS DOES NOT occur, your problem is inside the car. IF THIS DOES occur then the problem is in the Klima engine control system. This system replaces dumb relays in earlier systems and allows the motor's needs to supperceed the occupants needs. To protect the engine against belt failure, overheating, low voltage, and high or low speed the request for the compressor is over ridden.

First chore is to make this decision. Inside or outside.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 06-13-2004, 09:36 PM
Registered User
Join Date: Mar 2003
Location: Beautiful Western NC
Posts: 20
Steve B

Gosh, you are so good! You know these cars inside and out.
Speaking of which, I think problem is outside, namely low on freon.....
EC pushed:voltage LHP to grd,1st termnal = 1 v
2nd terminal = 3.8v
defrost pushed: 1st terminal= 0v
2nd terminal=12.6
jumper across LHP and compressor starts......

what do I need to check next? there are no obvious leaks indicated by oily spots near line connections. I had 1 or two cans of hot shot put in over a year ago, if low now I think I would like to change to 134a. Any special considerations with r12 and hot shot?
pull vacuum, see if it holds, inject some ester oil and then charge with 134a? I've had good success with this approach in other vehicles....

thanks for the good guidance!
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Old 07-17-2004, 02:59 PM
Registered User
Join Date: Mar 2003
Location: Beautiful Western NC
Posts: 20
final repair

I had some dye injected, then put enough R-12 in to push some dye out. It came out @ manifold to compressor block.
so... removed compressor with hoses connected and then replaced the 4 o-rings under the block and the 2 o-rings under the hose manifold. Put everything back in, changed out the accumulator drier (with new o-rings at all connections), pulled a vacumn, then charged back with good old R-12.



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