300 E compressor not engaging
 

My compressor will not engage properly. I have conducted the tests concerning the Klima relay and have found the parts good.

I have another 300E which works properly and exchanged Klima relays and still have the same problem.

If I ground either wire coming to or from the presure switch the compressor engages and cools properly.

A used push button switch was aquired after instilation the same result, the compressor does not engage.

What sensors need to be checked and how is it determined if the pushbutton switch is good or not?

Thank you,

Max:cool:

I would look to the evaporator temp sensor.

New evaporator sensor installed, unfortunately the compressor still does not engage.

Any other ideas?

Max

Mine just started doing the same thing. Why does MB climate control have to suck so much?

Since there are no self-diagnostics here you are going to have to do the tests. Your goal is to decide why the PBC is not doing its job. In this case grounding the A/C control pin of the Klima relay through the low pressure switch.

It appears to not be influenced by imputs, but you must test each pin leading to a sensor for its proper signal. The other possibility is that the unit is totally dead and not controlling anything. Check other functions, such as heater water valve activity. It is a pulsed ground similar to A/C control. The water valve is powered and it should be grounded by the PBC when not needing heat. If you run the temp to MAX it should remove the ground and the valve will open (it will get hot). Any set of wiring conditions that turns the controller off (bad power or grounds) or changes its decision (open circuit after controller before low pressure switch, open or shorted sensor wiring fooling the PBC decision). If the PBC sees the evap temp sensor as cold it turns off the compressor. In reality not only does the sensor have to be the right resistance, but the controller must be biasing the circuit with a precise voltage to use it in a decision. Usually 5v (but I'm not sure in this case), the voltage is applied across the resistor to ground. The voltage drop that occurs is what the PBC is looking for internally.

Do you have the CD or the writen A/C manual for 124?

Mine is doing this also. It worked perfectly before the engine removal/replacement. I have 86psi static charge, 250psi high and 50 psi low. Blue/green wire from compressor to Klima has continuity. If I ground one pin on the pressure switch the clutch engages. Other pin does nothing. I have the CD but my inability to find what I need from it has been documented here before.:o

Just an idea
 

I was having a problem with my compressor either not engaging or cutting out intermittently on a 1987 190 2.3-16. It ended up being the connection on the compressor. There was a 3 prong connection on the back side. It has a male/female pin set up and the female connectors had become loose. I took a needle nose and squeezed them shut a little and have not had a problem since.

It’s worth a shot.

Take a dvom and check the low pressure switch for continuity. On the 103 there is a aux fan switch on the engine that will cut the compressor off. Do your high speed aux fans run when the key is on? Open makes this happen. So a disconnected switch could do this.



Joe

I printed RA 83-605 test procedure from the 124 CD. All went well until step 4, speed sensor voltage. Should have been .3 v ac, mine is .4. May not be significant so I went to next step. Resistance of the speed sensor checked out ok. Step 5 is where it got crazy. The speed sensor voltage was supposed to be 8.5 v dc. Mine is 13v. The next step says "remove interruption to the cable connector of the diagnostic socket". They lost me there. I know Steve ain't gonna want to hear this but now all of asudden I cab hear a "clicking" relay behind the PBC every time I switch the ignition on. I never noticed it before.