My 87 190D 2.5 N/A had a problem. The A/C compressor is not engaging when the CCU Buttons, far right, upper, is engaged. I know it is not working when the button below it is pushed; I am almost sure the temp wheel position not involved in making the compressor engage with the upper button. What gives???? Thanks BTW the system pressure is normal and it DID work for a while........

Could be you're simply low on refrigerant, notwithstanding your belief that the system pressure is good (a leak HAS to start sometime). Assuming you have checked all fuses, you can look in the view glass on the filter dryer for refrigerant movement or bubbles, but you will not see them if the compressor is not running. So, find the low pressure cut off switch (usually on or near the filter - dryer). Jump the two contacts on the plug. If the compressor engages, you are likely low on refrigerant (or you have a defective low pressure cut off switch, but that is highly unlikely). Once the compressor engages, you should immediately disconnect the jumper wire (running the compressor when the system is low on refrigerant, and also refrigerant oil, is not good for the compressor). This is the quickest and easiest first step diagnostic. Try that and report back.

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After bypassing that pressure switch, verify the PBU works. Ground pin No 10 to the battery, should have 0 (zero) ohms when the PBU is set to an A/C function.

If both check out, jump the compressor. Pins 5 and 7. That will get it to stay on...

The speed sensor could be bad. The Klima relay too.

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I'm not a doctor, but I'll have a look.

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