W124 '95 E280: A/C Oddity!

JensH

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I have an odd thing going on with my DIAVIA dealer-fitted air con.

Having had a new re-gas, to 850 Kg, the system now doesn't kick in as it should, with middle 'EC' button off and the A/C default being on. Instead, in order to start the compressor, I have to depress and light up the dehumidifier button (top one of three). Once I have done this, it all works fine, and, in fact, I now get icy cold air from the vents, which I never did before; it was previously cold but not icy cold.

With the humidifier on, if I then engage the 'EC' (middle) button to turn the A/C off, it does so, turning the dehumidifier off (as it should) in the process, BUT it won't then come back on without re-engaging the dehumidifier.

The MB manual recommends that the dehumidifier is not left on, for a long time, but it looks at the moment that this is the only way the system will work.

I always gas at 850 Kg now, although the original DIAVIA spec. states 350 Kg. Some forum users have suggested 1k Kg, which is what the guy who sold me the car used to fill it to.

I have compiled a little bible of DIAVIA bumph, based on my own web research (happy to share). These include:

Original instructions:
"The quantity of gas R134a necessary for charging the system is Kg 0.350 (+/- 0.020)" - that seems very specific.
Technical Bulletin 5:
States R12 gas volume at "W124 (all models) 1,150" - I understand that you may
convert to R134a at 85-90%, so that would be range of 977.5 to 1,035.
Technical Bulletin 14:
"If the RPM option is not installed, the relevant connection (on the wiring harness) MUST NOT BE CONNECTED" - My replacement compressor has no RPM sensor and the connection has been removed from the DIAVIA CU. This was the reason that the A/C was not working when I got the car.
Technical Bulletin 26, contains some trouble shooting tips, including:
"The compressor doesn't run - Refrigerant pressure too low (P < 3 Bar), defective pressure switch."

My pressure switch is new OEM (within the last year).

My issue is that my compressor does work, but with the wrong trigger, as above. I can understand why the compressor would not work at too low pressure, but is it possible that it might not work if the system pressure is too high? The less than 3 Bar shut-off does suggest that the original instructions might be right at 0.350 Kg (if I am converting my Bar/Kg correctly!). I previously decided this was a misprint, owing to so many people telling me it was too low.

Any ideas anyone? :)

Crossed wiring somewhere?

J
 

jibcl500

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850g seems a little high. If over pressure it won't work.

jib
 

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850g seems a little high. If over pressure it won't work.

jib

Sounds about right to me. Some early mb systems took over 1kg so aftermarket is normally slightly less. Connect it to some gauges and look what the pressures are, the compressor should still kick in then cut out if too high and the fans would run.
 
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