190E2.6 AC Compressor

[rolfe3]

My 92 190E2.6 with 90K miles has started to drop out the compressor. At first it was intermittant but now it is often. The unit is charged and the signal for the compressor clutch is good through the low pressure switch. Applying voltage to the clutch brings the compressor on line. I am suspecting the MAS module or the feedback tach on the compressor. I have reseated all connectors and MAS module. I am planning on scopeing the tach signal later but will bypass the the MAS module temporally to allow AC to work. What is the most likely component to fail(Compressor tach or MAS)?

Thanks
Rod

[stevebfl]

The most likely failure is the belt tentioning device. If the belt flops the MAS turns off the circuit.

If the system won't reboot with an ignition cycle then this isn't the problem although close attention should be made because a loose belt will drop the compressor almost instanly.

The MAS would be my next choice although most that I've seen bad were caused by shorted clutch coils.

BTW if you want to scope the compressor speed signal; it is a very low voltage AC wave with as low as .75v peak to peak. The engine speed signal is a square wave, having been converted by the ignition comntroller.

------------------
Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

[rolfe3]

Steve,

I made a little circuit with a transistor and relay to run the compressor off the compressor enable signal off of the s31 low pressure switch. My wife now has cool air. I did notice that now that I have the MAS bypassed that if I accelerate to very fast to shift above 6000 rpm, that I can hear the belt chirp on quick decel. I change the belt at 50K and now have 90K. I did notice that the tensioner shock seems shot. How difficult is it to change? Thanks

Rod

[Benzmac]

I'd bet that the belt is slipping. You will need to replace the tensioner as well. The labor time on the 190 2.6 is a little higher. There are some specail tools required to take off the fan blades.

------------------
Benzmac:
Donnie Drummonds
1992 500E (very soon I hope
1981 280GE SWB
ASE CERTIFIED MASTER AUTO TECHNICIAN
SERVICE MANAGER FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM